A Bibliometric Study and a Narrative Review of the Effects of Particulate Matter (PM) on Human Health and Mitigation Strategies in Urban India

1. Introduction

1.1. A Brief Overview of Air Pollution Sources and Pollutant Types in Indian Cities

Air pollution significantly affects the environment and human health. According to the United Nations Environment Programme, 1.1 billion people worldwide breathe in poor air [1]. In Indian cities, air pollution has been dangerously increasing and threatening human health [2]. It is the second leading risk factor for disease after malnutrition [3]. The negative health effects of air pollution include morbidity, mortality, heart disease, diabetes, lung cancer, and respiratory infections. Particulate matter (PM) exposure caused over 1.67 million premature deaths in India [4]. Air pollution causes approximately 8.1 million deaths worldwide annually [5,6]. The United Nations’ Sustainable Development Goals (SDGs) call for reducing disease and mortality (SDG target 3.9.1) from air pollution and improving air quality in cities (SDG target 11.6.2) [7].

Air pollution is broadly classified as indoor and outdoor, and the respective sources prevalent in Indian cities are summarized in Figure 1. Developmental activities in the industrial, roadworks and construction, transportation, and household sectors consume energy. These activities significantly contribute to air pollution, particularly in megacities [8,9]. Particulate matter, a major air pollutant in Indian cities, occurs in various sizes (ranging from>10 μm (PM₁₀) to <1.0 μm (PM_1.0)) and exhibits physical and chemical properties. PM is composed primarily of carbonaceous materials [10].These are borne from the incomplete combustion of organic matter, such as diesel and petroleum, other fossil fuels, wood, biodiesel, elemental carbon, polycyclic aromatic hydrocarbons, brake and tire wear, rubber wear, and metallic dust from machining [11]. Other sources include iron and sulfur dust from steel factories, mining sites, and brick kilns; cement dust from manufacturing factories; and construction-site dust. PMs also originate from natural sources, which include desert dust, ash from volcanic eruptions, forest fires, dried oceanic salts, river sands, aerosolized soil, pollen grains from plants and trees, grass and weeds, spores from fungi, and dead microbes (bacteria and viruses) [12,13,14]. In northern India, influxes of desert dust from western arid regions cause significant air pollution [15]. Similarly, household activities, such as cooking, use of spray products to eliminate pests and insects, cleaning surfaces, and freshening indoor air; conventional stoves using wood chips for cooking and heating; smoking; fungal spores from stale food waste; sweeping, dusting, and other cleaning activities, cause pollution[16,17,18,19]. PMs travel from one location to another via air and wind, and their concentrations vary over hours, months, seasons, and years [20,21,22]. Approximately 40% of PM₁₀ is composed of inorganic matter. The organic part is carbonaceous, borne from dioxins, alkanes, polycyclic aromatic hydrocarbons, etc., and contributes ~20% to the PM composition. Soil dust and sea salt constitute ~14% and ~8%, respectively, of the PM composition, whereas ~4% of the composition is elemental carbon borne from shoots caused by the burning of fossil fuels [23,24].

The 2019 Council on Energy, Environment and Water (CEEW) compilation report indicates that, in Indian cities, the transport sector is the leading source of PM_2.5 emissions, accounting for 17.9%-39.2%. Industrial contributions vary from 2.3% to 28.9%. Additional sources of PM_2.5 include waste burning, dust, domestic cooking and heating, and diesel generators. Notably, road dust is the primary contributor to PM₁₀ levels, accounting for 35.6%-65.9% of the pollutant load. Additional sources of PM₁₀ include waste burning, transport, industries, domestic cooking, and heating [24].

The sources of pollution vary in Indian cities. Dust from construction and roads is the primary source of pollution in Delhi, Mumbai, Pune, and Bengaluru (formerly Bangalore), while transportation is a major source in Chennai. In Kanpur, industries and transport are the main sources of pollution [25]. Transportation is reported to be the second most important source of pollution in most cities. The contributions of sources vary across cities. The average percentages of major sources of pollution, as per the CPCB inventory, in major metropolitan cities, are shown in Figure 2.

The effects of air pollution can range from minor effects to long-term health issues, including eye irritation, coughing, hair and skin damage, and even cancer. The three major pathways that have been identified to cause severe damage to human health are (a) oxidative stress and inflammation, which cause biological “wear and tear” inside the human body, (b) autonomic imbalance, which disrupts the person’s nervous system and consequently the involuntary functions, and (c) direct translocations of the PMs into the blood stream and deposition into the soft tissues. These adversely affect the cardiovascular and respiratory functions and result in serious consequences in terms of diseases, such as ischemic heart disease (IHD) and stroke, cerebrovascular diseases, type-2 diabetes, chronic obstructive pulmonary disease, lower respiratory infection, tracheal bronchus and lung cancer, infertility, and premature birth [26,27]. A summary of the adverse effects of air pollution on human health is displayed in Figure 3.

PM₁₀ and PM_2.5 have become major concerns in Indian cities, particularly during the winter season, when concentrations are higher, and health effects are more pronounced. In the Delhi-NCR region, PM_2.5 concentrations during winter (296 ± 45 μg/m³) were found to be higher than summer (114 ± 48 μg/m³) [28]. The major sources of these pollutants were vehicular emissions, road dust, coal combustion, open burning, and industrial activities. The adverse effects on health are lung and heart diseases, and cancer [16]. In Ghaziabad city, the annual mean PM₁₀ concentrations were 260 ± 150 μg/m³ while the PM_2.5 concentrations were 140 ± 90 μg/m³ in the year 2018-2019 [29]. In the holy city of Varanasi, the seasonal mean concentration of PM_2.5 was observed to be 244 μg/m³ (Winter: November-February) and 65 and 55 μg/m³ (Monsoon: August-September) [30]. This is causing premature death, acute lower respiratory infection, and even COPD (Chronic Obstructive Pulmonary Disease). SI-Table S2 presents the PM concentrations across various cities and highlights their health effects.

1.2. Existing Research and Knowledge Gaps

Numerous reviews on air pollution and health impacts in India have been published, making it important to examine them to understand research progress and identify knowledge gaps. Previous literature reviews have focused on air pollution trends [31], the effect of household solid fuel combustion on human health [32], the impact of transportation projects on health [33], the exposure to particulate matter (PM) in India and its health implications [34,35] and the economic burden of air pollution in India [36]. Nevertheless, bibliometric analyses of air pollution and human health in Indian cities, as well as complementary narrative reviews on mitigation strategies, remain scarce.

1.3. Specific Objectives and Research Questions

To fill this knowledge gap, the present study is structured to address three primary research questions (RQ):

• RQ1: What is the current state of research on the impact of air pollution on human health in India, including the evolution of research topics, research trends, research clusters, pollution sources, and health impacts?
• RQ2: How does the study on air pollution and health vary across spatial scales (at the city level)?
• RQ3: What mitigation strategies can be implemented to address the problem of air pollution?

The study is organized around a review and analysis of 172 scientific studies from the past four decades (refer to SI-Table S1), covering geographic distribution and trends in studies of air pollution sources, types, and health impacts, using bibliometric methods to address RQ 1 and 2. Because there are few articles on mitigation strategies (only 34 studies have contributed to the discourse on air pollution mitigation out of the 172 studies), it was necessary to consult additional sources. Additional research articles, reports, and policy documents were reviewed to address RQ 3, leading to a complementary narrative review of air pollution mitigation strategies.

2. Data Acquisition and Analytical Methods

2.1. Data Acquisition and Processing

For this review on Air Pollution and health in India, a detailed literature search was performed in the Web of Science (WoS) and Scopus databases. With no time restriction on publication, the initial search was conducted on May 25, 2023, and the revised search on March 20, 2024. To identify the relevant literature, the search fields were narrowed to include ‘Title, Abstract, and Keywords’ in the Scopus database and ‘Topic Search’ (includes Title, Abstract, and Keywords) in the WoS database. As the work aims to investigate the relationship between air pollution and particulate matter and human health in India, journal articles were identified using multiple keywords, relevant Boolean operators (“AND”, “OR”), and Truncation (*) to obtain more focused results (Table 1). This entire process resulted in the retrieval of 3307 articles.

Additionally, a complementary literature search was conducted to identify articles on air pollution mitigation strategies.

2.2. Screening and Extraction of Relevant Literature

Duplicate articles from the initial search were removed from the retrieved data prior to screening using the exclusion criteria (Table 2), resulting in 2770 articles. Additionally, 509 studies with irrelevant titles, abstracts, and keywords were removed, yielding 2261 articles.

This was followed by a full paper screening process, during which 1361 articles on air pollution that did not address health and 261 articles addressing only health, not air pollution, were excluded. Furthermore, 410 articles that did not cover India or Indian cities were excluded, reducing the sample to 239 relevant articles. Finally, 67 additional articles were removed due to unavailability. Therefore, a total of 172 studies were selected following the exclusion criteria. The detailed workflow of the literature review process is shown in Figure 4.

2.3. Bibliometric Analysis

The scientific literature was examined through bibliometric analysis. ‘VOS viewer’, ‘Biblioshiny’ (‘R’ package) [37,38], and Microsoft Excel were used for data analysis and visualisation via performance analysis (research contribution), scientific mapping (for relationship assessment), and network analysis (enrichment of bibliometric analysis). This was done to identify the primary trends and geographic distribution of the scientific literature, research focus, temporal trends, clustering, and influential research in the field [39]. The bibliometric indicators examined in this work include annual scientific publications, most relevant sources, most-cited articles, thematic evolution, trending topics, keyword analysis, and co-occurrence networks (through clustering). A brief description of each is provided in Table 3:

2.4. Mapping and Schematics

Mapping and schematics are valuable tools for visualising the patterns and processes of a phenomenon. ‘QGIS’ and ‘Canva’ were used for mapping and graphic representations, respectively.

3. Results

3.1. Trends of Scientific Literature

Pollution-health-related publications have experienced rapid and sustained growth over the past four decades. Figure 5a-d show the number of studies on air pollution and health in Indian cities (before and after the screening process). From 1987 to 2000, only 6 articles were published; from 2001 to 2010, 21 articles were published. Between 2011 and 2020, there was a marked increase in the number of publications (89 studies), reflecting the importance of the theme. The number of papers published between 2021 and 2024 (up to the search date) was 56 (Figure 5c-d).

3.2. Geographic Distribution of Publications: A Regional Gap

Air pollution-health-related research in India exhibits high spatial variation in study site selection, as observed among the 172 records. A city-name search across all identified literature was conducted to determine which Indian cities were most frequently studied. New Delhi (has been the focus of study since 1997), with the highest number of studies, leads with 52, followed by Kolkata with 15, Mumbai and Lucknow with 12 each, Agra with 8, and Chennai with 6. This map on geographic locations was created using data from 156 publications that reported investigations focused on a single city (Figure 6).

An overview of the state-wise and regional distribution of investigations shows a primary focus on cities in Northern India, particularly the NCR and Uttar Pradesh. The remaining studies, although distributed across India (in Punjab, Maharashtra, Gujarat, Rajasthan, West Bengal, Odisha, Assam, Bihar, Jharkhand, Chhattisgarh, Karnataka, Tamil Nadu, etc.), are most densely concentrated in Western and Eastern India. Reports are not available for cities in the Northern and North-Eastern Himalayan regions (with the exception of Guwahati) and Central India. Similarly, in Southern India, reports are available for only a few cities, namely Bengaluru and Mysore in Karnataka, and Chennai and Coimbatore in Tamil Nadu.

These locational disparities suggest geographic disparities in the country-wide distribution of air pollution-health studies, with medium- and small-sized cities often underrepresented in data collection and pollution-health research. There are several reasons for regional variations and the overrepresentation of megacities in studies. Megacities face rising air pollution from heavy traffic operations, industrial activities, and high energy generation and consumption. The availability of data on air pollution and health impacts, improved institutional capacity, better funding, and research have facilitated research in these cities. Consequently, these cities can develop more effective strategies to address air pollution and reduce health costs. On the other hand, medium- and small-sized cities have received limited attention in scholarly articles due to data scarcity and institutional gaps. This “metro-centricity” results in a notable knowledge gap, since the health risks in small cities can be just as serious but often go unnoticed. Thus, further studies on these cities are needed.

3.3. Most Relevant Sources and Cited Articles

A journal plays a very important role in disseminating knowledge and serves as a platform for knowledge sharing. The top 25 most relevant sources of 172 articles on air pollution and health in Indian cities from 1987 to 2024 are listed in SI-Figure S1. The most prominent sources are ‘Environmental Monitoring and Assessment’ and ‘Air Quality, Atmosphere and Health.’ Other significant sources include ‘Environmental Science and Pollution Research’, ‘Science of the Total Environment’, ‘Atmospheric Environment’, and ‘Human and Ecological Risk Assessment’. These journals reflect their significant role in air pollution and health research. These journals have published more articles because they have consistently maintained a focus on the atmospheric impacts of human activities and their implications for human health. Furthermore, in addition to advancing knowledge, these journals promote methodological innovations in health risk assessment.

Citation counts indicate an article’s influence and recognition in academia. Data on articles within a journal offers a metric for assessing the journal’s overall impact. The top eight most-cited articles on air pollution and health impacts in urban India are presented in SI-Figure S2. Agarwal et al.(2002) [40] is the most-cited article, with 237 citations, followed by Guttikunda et al. (2013) [41] with 171 citations, while Pandey et al. (2005) [42], received 125 citations. The citations for the remaining articles range from 66 to 94. Other articles with a good number of citations have focused on metropolitan and large cities. These top-cited articles contribute to a journal’s impact in disseminating scholarly work.

3.4. Research Focus

A Sankey diagram visualizes changes in research focus and shows how thematic areas intersect over time. The flow of the diagram, shown as grey lines, indicates the evolving focus of research topics, with line thickness proportional to each theme’s significance over the years, and the nodes represent the key themes.

The diagram illustrating the thematic evolution in this field from 1987 to 2024 shows how the research focus shifted from broader themes to more targeted and specific topics (Figure 7). In the first period, 1987-2001, the research communities mainly focused on ‘air pollution’, ‘air pollutants’, and ‘adolescent’. From 2002 to 2010, researchers began to investigate ‘atmospheric pollution’, ‘health assessment and ‘risk assessment’ in India. During the years 2011 to 2020, the research expanded by investigating ‘health risks’, ‘environmental health’, ‘seasonal variations’, particularly in ‘urban areas’. From 2021 to 2024, ‘environmental monitoring’, ‘health impact’, ‘air quality’, and ‘mortality’ in ‘India’. This emphasizes that health risk assessment of air pollution has attracted increasing attention.

3.5. Research Trends

Trend topics reflect the use of different keywords across time frames in the literature and indicate the temporal evolution of core keywords. SI-Figure S3 shows that ‘indoor air quality’, ‘lung function’, ‘morbidity’, and ‘children’ were trending topics in research published between 2004 and 2012. During 2012 – 2019, air pollution, ‘PM10’, ‘PM2.5’, ‘mortality’, ‘source apportionment’, ‘health risk’, and ‘Delhi’ were the trending topics. After 2020, the trending topics were ‘health risk assessment’, ‘PM2.5’, ‘particulate matter’, and ‘COVID-19’. ‘PM_2.5’, and ‘health risk’, which appeared twice, are important trending topics in the research field. A notable observation is the limited number of publications on COVID-19, attributable to the 2020 outbreak. This scarcity may have stemmed from the difficulty in establishing a clear link between COVID-19, air pollution, and health outcomes. The nationwide lockdown limited health data collection and clinical investigations. Nevertheless, the COVID-19 outbreak in 2020 created valuable opportunities to examine air pollution in India, particularly through satellite observations, as traffic and industrial activities decreased during the nationwide lockdown. Studies reported a significant decline in PM exposure during the COVID-19 lockdown. However, one study also found that poor air quality in Delhi was associated with excess mortality during the COVID-19 period [43].

3.6. Keyword Analysis Through Word Cloud

The purpose of using a word cloud is to identify the most common terms, indicating that the main analysis focuses on those areas. The frequency of the keywords indicates their relevance in the literature database, and their relative importance is shown by their size in the diagram. From the total database, the top 50 keywords were selected and plotted through word cloud maps. The word cloud (shown in SI-Figure S4) generated from the databases demonstrates the content of the keywords and helps visualise the dominance of certain keywords (e.g., air pollution, particulate matter, PM_2.5, health risk, PM₁₀, health risk assessment, and Delhi) in the literature. Keywords such as cost of illness, cancer risk, black carbon, and aerosols are less common.

3.7. Co-Occurrence Analysis Through Clustering

Co-occurrence analysis is a technique for identifying the structure of a field by tracking how frequently two keywords or concepts co-occur in the same document or appear together in one article. Each circle in the diagram represents a keyword extracted from the article’s title and abstract and indicates the number of occurrences in the publications. The size of the node indicates the frequency of the keywords in the article; the edge represents the co-occurrence between the keywords. In this process, four thematic clusters were identified and are shown in Figure 8a. In the co-occurrence map, the most dominant keywords are “India”, “human”, “particulate matter”, “health risk”, and “environmental monitoring”, indicating that these have been central topics in the field. The blue cluster comprises “air pollution”, “particulate matter”, “India”, “atmospheric pollution”, “air quality”, “mortality”, and “city”, which are more widely studied. The red cluster represents “human”, “environmental exposure”, “urban health”, and different population groups such as “adult”, “male”, “female”, and “child”, along with “indoor air quality”, indicating the health impact of air pollution on these groups, with “lung function” and “asthma” as major health issues. The green cluster comprises “environmental monitoring”, “health risks”, “PM2.5”, “dust”, and various pollutant types. A small yellow cluster comprises “Asia”, the region. The time-period overlay (Figure 8b) indicates that “cancer risk”, “cities”, “health risks”, “ozone”, “aerosol”, “winter”, and “particulate matter” have been more widely studied in recent years.

3.8. Co-Occurrence Analysis of Source and Types of Air Pollution

The co-occurrence network link (1987–2024) for sources of air pollution is shown in Figure 9a. The five clusters of air pollution sources are identified as follows: The green cluster comprises “exhaust gases”, “vehicle emissions”, “automobiles”, “traffic”, and “smoking”. The red cluster centres around the main “industrial” area node, which is associated with “road dust”, “metallurgy”, and “construction activities”. The yellow cluster emphasizes indoor sources such as “cooking”, “combustion”, and “coal combustion”. The purple cluster encompasses “traffic emissions” and “industrial emissions”. The blue cluster includes “electric power”, “coal”, “fuel”, and “car”. Additionally, a small pink cluster is linked to “human activities”, “industries”, “power plants”, and “exhaust emissions”. Time period overlay indicates that “road dust”, “industrial emissions”, “fungal spores”, “hazardous waste”, “agricultural land”, and “cremation” have caught attention in recent times (Figure 9b and Figure 9c). Overall, the findings suggest that the main sources identified by the research articles are “industrial emissions”, “vehicular emissions”, “road dust”, and “indoor activities”.

Figure 10 shows the co-occurrence network for air pollutant types (studied between 1987 and 2024). Keywords such as “particulate matter”, “PM_2.5”, and “dust” are the dominant themes and received sustained interest (Figure 10a). In recent times, some other types of pollutants have received more attention, such as “carbon”, “surface ozone”, “trace metals”, “heavy metals”, “anthracene”, “Polycyclic Aromatic Hydrocarbons (PAHs)”, and “acenaphthene” (Figure 10b). The surge in interest since 2020 in ‘greenhouse gases’, ‘nitrogen oxides’, ‘aerosols,’ and ‘ozone’ highlights that air pollution is studied not only for its health risks but also for its role in driving global warming and climate change. The findings indicate that addressing air pollution is closely linked to mitigating health and climate change challenges. The co-occurrence networks reveal a shifting research focus on the sources and types of air pollution, transitioning from traditional to modern pollution in urban India. Cities that emit diverse chemical and particulate matter pose exposure risks to urban populations.

Road and construction dust have become a nuisance in cities. Growing vehicular and development activities also expose urban residents to BTEX (benzene, toluene, ethylbenzene, and xylene) pollution. Biomass burning severely affects air quality and public health. Indoor air quality studies in urban homes are also affected by the presence of heavy metals in indoor PM, which is primarily generated by combustion processes, dust, and cooking.

3.9. Co-Occurrence Analysis of Impacts of Air Pollution on Human Health

Figure 11 depicts co-occurrence links regarding the health impacts of air pollution studied from 1987 to 2024. The red cluster signifies the association of air pollution with “metabolic disorders”, “abortion”, “neoplasms”, and “dermatitis”. The blue cluster indicates “respiratory diseases”. The green cluster represents “cardiovascular and respiratory diseases” as well as “mortality”. The yellow cluster connects air pollution with “malnutrition” and “headaches”, while the pink cluster highlights “Covid-19”. Time overlay highlights that recent research has focused on malnutrition, chronic obstructive pulmonary disease, ischemic heart disease, insulin resistance, and Covid-19. Respiratory diseases, cardiovascular diseases, and cancer (as the dominant node) have been consistently associated with air pollution.

The following section discusses how, over the past four decades, numerous illnesses linked to air pollution have been documented. The most affected health conditions, in terms of case numbers, include asthma, bronchitis, and respiratory diseases [44,45,46,47]; cardiovascular diseases, and cancer [48]; pulmonary/lung diseases [48], blood abnormalities, and altered immune profiles [49,50,51]; nervous system and metabolic disorders [52]; high blood pressure, diabetes, and tuberculosis [53,54]. Seasonal variations in (PM) raised further concerns. PM_2.5 levels were higher in winter than in summer, and consequently, respiratory deposition was also higher in winter [28,55,56]. The risk of cancer induced by air pollution was estimated to increase markedly during the winter season [57]. Health effects of fungal spores [58,59] are a major cause of allergic reactions that can trigger asthma attacks [60].

Respiratory and cardiovascular mortality are on the increase because of air pollution [61,62], especially in transport corridors [8,63]. Children are seriously affected because of prolonged exposure to high [PMs] [64,65], leading to mortality, neonatal mortality, preterm birth, acute respiratory infection (ARI) symptoms, and low birth weight [3,42,66,67]. Studies have shown that the low-income group faces significantly greater health risks than the upper-income group [47,68] and that women face relatively greater exposure to indoor air pollution and related health vulnerabilities than men [69]. Additionally, both young and adult populations are experiencing issues such as bone and skin damage, hair fall problems, and skin cancer [70,71]. Burning mosquito coils indoors has also been linked to respiratory and heart diseases [72]. Metal concentrations, such as Nickel in the air [73] and presence of PAHs and benzo(a)pyrene in PMs also causes health risks [74]. Investigation into the role of bacterial endotoxin in PMs and associated inflammatory responses has also been conducted [75].

A visual illustration of the apparent adverse effects of air pollution on human health is shown in Figure 12. Ultrafine dust or PMs cause eye irritation and trigger ocular hypertension [76]. Pollens and dust particles (1.0–100 μm) passing through the nasal passage cause allergic rhinitis [77]. PM_2.5 or particles of this size can easily enter the lungs and alveolar tissues. These particles can then enter the bloodstream and reach various organs in the body. It has been discovered that PMs can cause liver diseases and increase the risk of non-Hodgkin lymphoma. According to the Multiple Path Particle Dosimetry (MPPD) model, fine particles accumulate more in the lungs, while coarse particles accumulate more in the head [78].

Accumulation of PMs in the brain can cause Alzheimer’s disease, dementia, and strokes among all age groups [79,80,81,82,83,84,85]. In addition to PMs, gaseous-phase pollutants, such as O₃, SO_x, and NO_x, can cause short- and long-term adverse effects on human health. These gaseous species possess oxidative properties and may induce oxidative stress (OS) and inflammation in the body [86,87,88]. An interplay of inflammation and oxidative stress contributes to initiating pathophysiological effects in multiple organs and aggravates other mechanisms, which can cause cardiovascular dysfunction [89].

PMs and NO_x have been reported to be responsible for premature aging of blood vessels and excessive calcium accumulation in the coronary arteries. This causes endothelial dysfunction and restricted blood flow to the heart and other major blood vessels, resulting in cardiovascular abnormalities, heart attacks, and strokes. Potential pathophysiological mechanisms induced by air pollution in humans include autonomic nervous system disorders, leading to overactivity of the sympathetic nervous system and the adrenal glands. The effect is generally observed as anxiety and hypertension. At this stage, the release of pro-inflammatory mediators and modified lipids, including phospholipids, occurs, leading to leukocyte and platelet activation and hypercoagulability. The interplay of inflammation and oxidative stress leads to dysfunction of endothelial tissues, ultimately activating prothrombotic pathways, altering cardiac function and rhythm, and causing myocardial ischemia [90]. Oxidative stress also contributes to neurodegenerative diseases and is characterized by excessive neuronal loss. This results in dementia, cognitive impairment, disrupted motor control, and eventually death. Cigarette smoke has been established as a risk factor for neurodegenerative diseases and prominent neurological disorders, such as stroke, Alzheimer’s disease, and multiple sclerosis (MS) [91]. Microglia, a type of glial cell located throughout the brain and spinal cord, are macrophages that participate in immune defence in the central nervous system (CNS) by acting as complex, dynamic mediators of neuroinflammation. They are often referred to as “brain-resident macrophages.” These cells form a structured, heterogeneous network that maintains brain homeostasis, responds to damage or pathogen-generated stimuli, and mediates both protective and deleterious responses. Activation of microglia induces overproduction of cytokines, creating a pro-inflammatory environment in the nervous system. The latter contributes to neuronal cell damage [92,93,94,95,96]. Prolonged exposure to PM_2.5 causes metabolic disorders, including modulation of hepatic pathways and glucose homeostasis, and induces systemic and pulmonary inflammation that promotes a series of maladaptive signalling pathways, ultimately leading to insulin resistance [97]. Polymorphonuclear cells (PMCs) in the lungs produce excessive ROS during exposure to particulate matter (PM_2.5), causing oxidative damage to alveolar tissue and inflammation-induced IL-8. Reactive PMCs and damaged alveolar cells induce cytokine production, leading to the release of immature PMCs from the bone marrow and thereby worsening health conditions. Air pollutants can impact brain tissue directly via the nose, reaching the olfactory bulb. Indirectly, they enter the lungs, pass into the bloodstream, cross the Blood-Brain Barrier (BBB), and circulate throughout the blood.

This process impacts cells by generating excessive ROS and oxidative stress (OS), producing toxins that harm microglia. This leads to microglial activation and cytokine release, causing neuroinflammation and damage to proteins, lipids, and DNA. Consequently, there is a reduction in astrogliosis (the formation of new astrocytes) and an increase in alpha-synuclein and amyloid beta aggregation in brain tissue. These changes may contribute to neurodegenerative diseases (NDD) such as Parkinson’s, Alzheimer’s, and Multiple Sclerosis.

3.10. Mitigation Strategies

This section provides a narrative review of the mitigation strategies proposed by the included studies. Of the 172 works collected for review, only 34 contributed to the discourse on air pollution mitigation strategies, and of these, only 14 articles were published in recent years (2020 onward). A limited number of studies addressing mitigation strategies constrained the bibliometric analysis in this section. However, the growing concern about air pollution requires a thorough analysis of the country’s current mitigation strategies to inform a recommendation. Therefore, an additional literature search on air pollution mitigation strategies was conducted via Google to identify implementable strategies.

The literature highlights the effectiveness of remedial measures, such as urban zoning, in managing air pollution in Indian cities [98,99]. Action-oriented strategies such as urban land zoning, eco-design, dust control, and city expansion in accordance with land-use plans are particularly effective [42,47,100,101,102,103,104]. Additionally, the effectiveness of stricter monitoring and regulatory standards during the pandemic reestablished their significance as mitigation measures [42,47,100,101,102,103,104]. To address indoor air pollution, ventilation and the use of cleaner fuels have emerged as the primary mitigation measures [67,105,106]. However, monitoring strategies remain unevenly used due to limited data availability and slow adoption of monitoring technologies. The direct and indirect monetary burden of health on low-income households (via self-medication, loss of wages, health care deprivation, out-of-pocket expenditures, and increased vulnerability to future poor health episodes) remains underrepresented despite its relative significance for urban policy frameworks [107,108,109,110,111,112,113]. Given the susceptibility of low-income households to pollution-induced health impacts, this warrants more focused research [111,112,113,114].

The following sections discuss various air pollution mitigation strategies based on the included studies, including structural, technological, and behavioural interventions [3]. These strategies are broadly categorized as action-oriented, monitoring-oriented, and policy-oriented [41].

3.10.1. Monitoring-Oriented Strategies

These strategies aim to improve the coverage and scope of air quality measurement systems. The literature highlights the significance of monitoring secondary pollutants, their precursor emissions, and fine toxic particulates, in addition to primary pollutants [72,104,115]. To assess the spatial and temporal variability of air pollution, vertical and seasonal monitoring of particulate matter has also been suggested [116]. Some studies also recommend adherence to CPCB guidelines to maintain a minimum number of monitoring stations for SPCBs. For example, A million-plus city must have at least 25 air pollution monitoring stations according to CPCB guidelines [117]. Furthermore, the use of emerging technologies, such as biomagnetic monitoring (used to assess PM mineral composition), may facilitate effective emission assessment and, therefore, provide relevant data for developing targeted action plans [118].

The Air (Prevention and Control of Pollution) Act of 1981, as amended in 1987, established a framework for controlling and reducing air pollution. To assess air quality, the GoI introduced the National Air Quality Monitoring Programme (NAMP) across 419 cities and towns, emphasizing the reduction of fossil fuel dependence, the promotion of clean energy, and the adoption of electric vehicles. Key initiatives include the National Electric Mobility Mission Plan 2020 (launched in 2013) and the Faster Adoption and Manufacturing of Hybrid and Electric Vehicles India scheme (with Phase 1 in 2015 and Phase 2 in 2019). The National Clean Air Programme (NCAP) 2019 was introduced with an initial target of a 20-30% reduction in PM by 2024 across 122 non-attainment cities. This target was later revised to a 40% reduction in PM₁₀ concentrations by 2025-26. Since the programme’s launch, PM₁₀ concentrations in Mumbai have decreased by 34% [119].

3.10.2. Policy and Plan-Oriented Strategies

This aims to regulate and plan for structural and technological change through plans and policies. Several studies have called for stricter air quality standards and their effective enforcement, including the imposition of fines for violations of the National Ambient Air Quality Standards (NAAQS) [51,115,120,121]. Improvements to public transportation systems (buses, metro, canal networks), the creation of dedicated bus corridors, and subsidies for non-motorized vehicles have been proposed as key policy provisions [63,67,109,122]. To address industrial pollution, mandatory pre-establishment Environmental Impact Assessment (EIA) and periodic post-establishment environmental audits of industries are also recommended [123]. Measures to reduce suspended dust, such as using vacuum-equipped vehicles for road cleaning instead of sweeping, mandating covers for construction sites, increasing urban tree cover, and creating greenbelts, have been reaffirmed [65,67,114,123,124,125,126,127]. Additionally, periodic interventions based on need—such as penalties for firecracker burning, vehicle restrictions, or the limitation of industrial activities on certain days—are recommended to address seasonal spikes in pollution [61,114,128]. Governments are also advised to leverage social and mass media to raise awareness and to incorporate courses, summer schools, and training programmes on air pollution into school curricula [1,3].

Urban planning strategies should include establishing pedestrian walkways and bicycle lanes, minimizing parking zones, and raising parking fees to help reduce pollution [41,122,129]. Ultimately, implementing a holistic policy that combines direct pollution-control measures with improvements in energy and transport efficiency is essential, with higher-income groups—who tend to consume the most energy-intensive goods—shouldering most of the costs [68].

Indian cities are developing comprehensive clean air action plans (CAAPs) and Climate Action Plans (CAPs) [130]. Reducing indoor and outdoor air pollution is now a national priority for ‘Swasthya Nagrik Abhiyan’ under the National Health Policy [131]. Effectively tackling air pollution necessitates a comprehensive approach. This approach should combine structural reforms, advanced pollution-control technologies, climate-focused policies, improved urban planning, and energy-efficiency projects to support sustainable growth [115].

Given the transboundary nature of air pollution, several studies emphasize interstate and interagency cooperation, including active collaboration between governments and NGOs [115,132,133]. Stakeholder consultation is also essential for eco-designing cities and for developing effective action plans that account for the local context [67,128].

3.10.3. Action-Oriented Strategies

This focuses on reducing emissions at the source and across various scales [123]. At the household level, studies recommend a complete phasing out of solid fuels and kerosene, replacing them with cleaner fuels such as LPG [114]. To reduce indoor air pollution, changes in housing design, such as ensuring adequate ventilation in homes and kitchens, are recommended [105,106]. At the city level, some authors recommend strict control and planned reduction of anthropogenic activities that cause outdoor pollution, such as vehicular and industrial emissions [103,134]. These actions should be supported by the use of non-motorised vehicles, renewable energy sources, alternative fuels, and the adoption of green technologies [41,65,67,104,115,122,135]. Additionally, the use of fly ash in brick kilns and the recycling of industrial and domestic waste can be effective in controlling air pollution [114].

India’s transition to BS-VI emission standards in 2019 was a significant step toward reducing vehicular emissions. The GoI’s concerted efforts to incentivize electric vehicles in Indian cities complement these measures. With these policies in place, India aims to achieve a 30% share of electric vehicles in total vehicle sales by 2030 [136]. Indian cities are witnessing a significant shift in motorized transportation. Auto-rickshaws, which account for 5-25% of motorized trips, are rapidly transitioning to emission-free electric alternatives (e-autos) [137,138].

Strategies to control air pollution in India also include upgrading (a) the health care sector, (b) policies, (c) the transportation sector, and (d) urban design. Outdoor air pollution can be controlled by checking industrial and vehicle emissions [42,120,139,140]; expansion of public transport networks, increasing green cover, and implementation of strategic long-term air quality management strategies [67]. The use of cleaner fuels and efficient kitchen ventilation can reduce indoor pollution [66,105,141]. Advancement in public transportation [142], integration of road-rail-metro services, promoting non-motorized mobility systems (cycling and walking), separating transportation corridors (motorized and non-motorized, signal-free roads), enforcing strict parking controls and parking charges with taxes, introducing cleaner and low-fuel-consuming vehicular technologies, establishing waste-to-energy plants, implementation of PAT scheme, carbon sequestration, etc., are suggested strategies. Public policies focusing on better cities (e.g. zonation, eco-designing, urban greening, etc.) and housing designs (e.g., ventilation, cooking stoves, etc.), increased greenery in the surroundings [125], and the formulation of effective air quality management plans are suggested. Sustainable pyre burning practices [43], adoption of the Clean Air Action Plan is suggested to achieve the UN SDG target of reducing air pollution-borne health burden by 2030 [52]. Furthermore, climate change mitigation policies are recommended to cause substantial health benefits to the poor [143]. Scholars suggest adopting robust policies through technical, institutional, and behavioral interventions across all sectors to meet air quality targets and reduce the future health burden from air pollution [41]. Figure 13 highlights the need for integrated actions at different levels.

4. Discussion

This work contributes to the discourse on urban environment in India by synthesising four decades of research on air pollution and human health. 172 papers from the years 1987 to 2024 were retrieved from WoS and Scopus databases using PRISMA guidelines. The evaluation of research themes indicates that PM10 and PM2.5 are the most significant pollutants with respect to health risks in India. Additionally, it explores trends in this subject area, sources and types of air pollution, and emerging health concerns in India. It thoroughly discusses strategies to reduce air pollution, providing valuable insights for policymakers and urban planners.

4.1. Key Findings of This Study

The findings of this study indicate substantial progress in research on air pollution and health in urban India. Over the last forty years, studies have greatly improved our understanding of pollution sources, types, pathways, and health effects. These developments have provided essential insights into the development of effective air pollution mitigation strategies. The number of research articles on the chosen themes has been increasing, with a marked rise since 2010, indicating greater interest in the research area. This may be attributed to higher air pollution levels, particularly in India’s megacities, improved monitoring mechanisms, and increased multidisciplinary efforts on the topic. The National Air Quality Monitoring Programme of the Government of India, which expanded air pollution monitoring capabilities in Indian cities, has been instrumental in providing data for research on urban air pollution. The results of the bibliometric analysis indicate that PM_2.5 and PM₁₀ are the primary drivers of the rise in air pollution and the resulting health impacts. Additionally, a bias in the spatial focus of the research, particularly in favour of megacities like Delhi, Mumbai, Kolkata, and Chennai, has been observed.

4.2. Evolution of Research Focus

These developments indicate not only an increase in the quantity of research but also a narrowing of its focus. The co-occurrence analysis results indicate that air pollution has consistently been considered a potential cause of worsening health conditions in Indian cities, from the earliest studies to the most recent. The scope of research has broadened to include studies of indoor and outdoor environments related to air pollution and its health effects. The initial focus on air pollution components broadened to include their impacts based on occupational status, health economics, transboundary flows, temperature-pollution interactions, hotspots, and ultimately, the pandemic. These shifts can be further detailed, ranging from an initial focus on air pollution in general to the impacts of specific pollutants such as PM_2.5, PM₁₀, PM_1.0, and heavy metals. The methods and tools used in these works have also evolved from monitoring-station-based data to include detailed bloodwork-based exposure analysis, PCA, GIS-based interpolation, and modelling [144,145]. Additionally, the studies focused on epidemiological investigations of various diseases attributable to air pollution. The health risks have also expanded to include diseases such as cancer, cardiovascular disease, immunological disease, and renal disease, as well as long-term genetic modifications [48,50,106,146]. The study of linkages between the COVID-19 lockdown and air quality has also emerged as a key focus of air pollution research in recent years [103,124,147]. At the same time, understanding different vulnerabilities based on age, gender, socio-economic status (children, women, slum residents), occupation (traffic policemen, roadside vendors), and seasonality of air pollution (annual, seasonal, daily mean) has become a part of these works for gaining additional insights and comparisons [42,49,99,106,124,148,149]. Such studies consistently emphasize the need for structural, policy, and action-oriented strategies to mitigate air pollution. Ongoing research on air pollution and its health effects increasingly focuses on genetic and molecular studies of how particulate matter contributes to various diseases in humans. This research employs a bibliographic analysis to assess the health impacts of air pollution in Indian cities, complemented by a narrative review of mitigation strategies. Consequently, our work enhances understanding of both environmental geography and the field of environment and health.

4.3. Spatial Bias

The study reveals that megacities are overrepresented at both regional and city levels, while medium- and small-sized cities are underrepresented. This highlights the concern about the lack of data at smaller scales and its role in exacerbating spatial bias in reporting [150]. Small and medium-sized cities are undergoing the highest degree of urban transformation in India and face pollution pressures comparable to those of mega-cities [106,142,150,151,152]. This imbalance could result in insufficient evidence and fewer interventions targeting small cities in India. This study suggests focusing on medium- and small-sized cities, where monitoring stations remain scarce. The lack of available information is a major concern that amplifies health risks, making it imperative to monitor and plan for air pollution in advance.

4.4. Implementing Mitigation Strategies

4.4.1. Abolishing the Burning of Waste; Remediation of Solid Waste

The exhaust gases produced during the production of clay bricks give rise to greenhouse gases [153]. The study indicated that using agroforestry and construction and demolition waste-derived fly ash to produce bricks can potentially cut CO₂ emissions by 0.5-1.5 metric tons when replacing 1-2% of traditional burnt clay bricks. Large-scale brick manufacturing can effectively lower overall carbon footprints [154,155,156]. Furthermore, studies have shown that plastic waste can be used in constructing roads, lightweight bricks for flooring, and panels for external wall insulation [157,158].

Since 2006, India has built over 60,000 miles (~half the width of the US) of plastic roads. These roads outperform traditional roads in durability, high-temperature tolerance, water resistance, and crack resistance. Overall, they require less maintenance and repairs [157]. Using discarded plastic water bottles as an insulating material in buildings can help reduce landfill waste and minimize heat loss during winter, which in turn decreases wood usage [159,160,161]. Delhi and the National Capital Region (NCR) have been experiencing severely polluted air due to stubble burning during the previous winter. This can be addressed by utilising the entire stubble/straw in diverse applications.

4.4.2. Blending Ethanol with Petroleum and Promotion of Electrical and Hydrogen Fuel-Powered Vehicles

The ethanol molecule contains an oxygen atom that facilitates complete combustion, reducing emissions and environmental pollution. As a result, pollution can be significantly decreased. Ethanol, a renewable biofuel made from sugarcane molasses, is also a relatively inexpensive alternative to petroleum. As of August 31^st, 2024, ethanol blending in petroleum fuel rose from 38 crore litres in the Ethanol Supply Year (ESY) 2013-2014 to 545 crore litres in the ESY 2023-2024. India achieved 15% ethanol blending in petroleum fuel for vehicles in 2024 and aims to reach 20% blending by 2025. A surge in ethanol blending in petrol from 1.54% in 2014 to 15% in 2024 is commendable progress in view of the commitment to curbing pollution. This not only saves foreign exchange on petroleum imports but also substitutes crude oil, and ultimately reduces CO₂ emissions by 544 lakh metric tons.

Blending of 20% ethanol in petrol fuel is estimated to reduce the fuel import bill by 4 billion USD (INR 30,000 crores) annually[162,163]. A study on petrol fuelled motorbikes using 10% ethanol blend saw a reduction in total hydrocarbon (THC) and CO by 26%-45% and 63%-73% respectively, while observing an increase in NO_x emissions by 36%-54% compared to emissions from motorbikes using pure gasoline [164]. This is attributed to the increased oxygen content in the ethanol-blended petroleum. Another study on compression ignition (CI) engine using a blend of 10% ethanol in petrol with 1.5% of emulsified hydrogen peroxide (H₂O₂) resulted in a reduction in all emissions, viz., CO by 80%, HC by 43%, and NO_x by 17% compared to unblended regular gasoline [165].

The National Green Hydrogen Mission, approved on January 4, 2023, with an allocation of INR 19,744 crores, aims to position the nation as a global hub for the production, use, and export of green hydrogen and its derivatives, and to contribute to achieving clean energy targets. The mission targets the installation of electrolysers with 60-100 GW capacity to support production. It aims to achieve a cumulative carbon abatement of 50 million metric tons and to reduce road traffic load [166]. Indian Railways has reportedly achieved 97% electrification of the broad-gauge network and is aiming for a 100% green rail network [147,167]. The electrification of railways relies heavily on thermal power stations that generate electricity from coke and coal. However, the resulting carbon emissions remain localised rather than being dispersed over a large area as trains pass by. This approach to fully electrified railways continues to offer significant benefits in pursuing emission-free transportation.

4.4.3. Restructuring Roads, Housing Infrastructures, and Implementing Solar-Power-Based Energy Systems

Restructuring roads by constructing pedestrian pavements and separate routes for two-wheelers and cyclists can help create a clean and secure city. In India, a large population commutes by two-wheelers, and this innovation will improve traffic management, prevent road congestion, improve fuel efficiency and electric power savings in both two- and four-wheelers, and, most importantly, provide commuter safety by preventing road accidents while reducing PM emissions (Figure 14). Harnessing solar power was recognized several decades ago as a way to reduce pollution. Electricity generated by solar panels to meet household energy needs significantly reduces dependence on fossil-fuel-powered thermal power stations (Figure 15). Although eliminating PMs from the atmosphere remains challenging, minimising PM concentrations is always possible. For instance, Pradhan Mantri Ujjwala Yojana (PMUY), 2016, has benefited 10 million people with liquefied petroleum gas (LPG) connections by the year 2024, replacing conventional cooking methods and significantly reducing indoor air pollution (PIB, 2024) [162]. In all Indian cities, the construction of buildings and roads is a major source of atmospheric PMs. Covering these sites and construction materials can prevent dust, asbestos, and cement particles from being carried by moving winds by ~90-99% (Figure 16). Rain acts as a natural cleaning agent, washing away atmospheric dust and gaseous components and leaving the atmosphere clean. As shown in Figure 17, modern cities face additional problems from waterlogging on roads and in residential areas. After the water level drops to normal and the roads become dry again, only a thick cover of dust remains on the surface of vehicles and the roads themselves. The winds lift dried mud and carry it everywhere, increasing PM levels in the atmosphere. Installing proper drainage systems along roads, parks, and residential areas helps prevent mud dust. Figure 18 and Figure 19 highlight the importance of covering soil with grass and planting shady trees and bushes to prevent ground moisture from evaporating. Residential towers and high-rise buildings house a large population within a compact area, and this dense population breathes the air in that limited space. Figure 20 shows the proposed layout of the premises surrounding the residential zones. High walls separating busy roads from residential buildings and towers, along with a green canopy of trees and bushes in between, help prevent PMs and dust from entering residential houses carried by wind. The tree canopy traps dust. Rain washes dust deposited on trees and bushes into the soil. Thus, residential areas remain free of PMs for most of the year, and the population residing in these areas is protected from inhaling polluted air.

5. Limitations of the Current Study

This study acknowledges several limitations. The bibliometric analysis relies solely on two databases, Web of Science and Scopus, which may have resulted in the omission of important studies on the topic. Additionally, the inclusion criteria were limited to research articles and review papers, excluding other publication types. The unavailability of some research papers led to their exclusion (inherent risk of the PRISMA guideline in the inclusion process), potentially reducing the depth of insights into the research theme. This study reported state- and region-wise gaps in the literature. While ensuring methodological rigour, the inclusion criteria likely excluded grey literature and local reports that could have provided greater insight into state- and city-level perspectives. Similarly, the search results depended on the keywords used in this study, and the outcome may change if the keywords are different. Including additional databases and articles, along with adjusting the Boolean search string, could yield deeper insights in this field. Future research may utilise other databases for literature searches and analyses. In recent years, Artificial Intelligence (AI) has been increasingly incorporated into bibliometric analysis to enhance data retrieval and visualisation. Future studies may adopt advanced AI-based methods. Most studies were conducted in large Indian cities, where the population is large, the problem is severe, and adequate resources are available for research; limited information is available at the pan-India level due to insufficient funding to support investigations in smaller administrative units.

6. Conclusions

This research employs bibliographic analysis to assess the health impacts of air pollution in Indian cities, complemented by a narrative review of mitigation strategies. It explores trends in this subject area, sources and types of air pollution, and emerging health concerns in India. The research highlights the alarming levels of air pollution in Indian cities, particularly in the megacities. The evaluation of research themes indicates that PM₁₀ and PM_2.5 are the most significant pollutants with respect to health risks in India. Air pollution-health studies published between 1987 and 2024 strongly indicate that long-term exposure to particulate matter causes serious health issues.

The study shows that megacities are overrepresented both regionally and at the city level, whereas medium- and small-sized cities are underrepresented. This imbalance may lead to limited data and fewer targeted interventions for small cities in India. The study recommends paying attention to these smaller cities as well, which also lack sufficient monitoring stations. As these cities undergo transformation, proactive planning can improve residents’ health outcomes.

Some prior work on mitigation has suggested broader policies rather than actionable changes. The assessment of existing policies, therefore, needs more evaluation. Therefore, future research should aim to transform knowledge into actionable interventions. The study advocates for enhanced interdisciplinary and multidisciplinary research collaborations to address air pollution and associated health impacts. The mitigation strategies suggested can help shape effective measures. Nonetheless, the study recommends implementing city-specific strategies and targeted interventions to improve outcomes. It also recommends incorporating air-pollution mitigation measures into comprehensive urban development and planning frameworks.

The study suggests that initiatives to reduce pollution will not only improve public health but also strengthen climate mitigation efforts. By implementing multi-factor control policies and targeting air pollution, the total disease burden attributable to air pollution can be reduced by 25%. Although policy implementation was initiated systematically, the overall rollout has faced several hurdles for multiple reasons, viz., (a) only 25% of the proposed clean air action plans could be implemented by the year 2020; (b) challenges because of COVID-19, which rendered the spread of microbial levels high in the atmosphere; (c) the WHO gap attributed to the cities failing to meet national standards despite implementing mitigation strategies (indicated by exceeding the WHO guideline limits of 5 µg m^-3).

SGD 3.4 has been adopted by the Indian administration to reduce deaths from non-communicable diseases (NCDs) by 33% by 2030 (using 2015 levels as the baseline) [27,168,169]. Achieving clean air will help the government of India to achieve the targets of Sustainable Development Goals (SDGs), specifically SDG 3-Good Health and Well-being and SDG 11-Sustainable Cities and Communities, which depend on addressing air pollution in India.