COVID-19 Pandemic as an Environmental Health Indicator: Lessons Learned from Air Pollution Changes and Climate Implications

1. Introduction

There is no doubt that the most important environmental problem threatening the whole world today and in the next century is global climate change in particular caused by the increase in the concentrations of greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), nitrous oxides (NO_X), other fluorinated gases released into the atmosphere as a result of the burning of fossil fuels especially during energy production. Not only fuel-fired thermal power plants and industries that cause GHG emissions are responsible for this global environmental phenomenon, but also urbanization, deforestation, land degradation, and agricultural and mining activities are of great importance by being GHG sources. Global climate change has numerous negative effects on human beings, and undoubtedly its effects on public health have gained great attention (Akan et al., 2020; Agache et al., 2022; Akan et al., 2023). Considering the increased exposure to air pollutants including carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ground-level or tropospheric ozone (O₃), particulate matters (PM₁₀: aerodynamic diameter smaller than 10 µm and PM_2.5: aerodynamic diameter smaller than 2.5 µm) due to global climate change, some important diseases, such as asthma, lung inflammation, heart ischemic, vector-borne infectious diseases affecting the cardiovascular and respiratory system and immune system were observed (Agache et al., 2022; Franchini and Mannucci, 2015).

In recent years, increasing attention has been given to the interactions between environmental changes and public health risks, particularly after the COVID-19 pandemic demonstrated the vulnerability of societies to environmental and biological stressors. The pandemic has also provided a unique opportunity to evaluate the connection between anthropogenic activities, environmental changes, and public health outcomes, highlighting the importance of integrated environmental health strategies for future global challenges. In this context, many previous studies indicated that outdoor air pollutant concentrations, such as CO, NO₂, SO₂, O₃, PM_2.5, and PM₁₀, as well as climatological parameters including ambient temperature, atmospheric pressure, relative humidity, wind speed, gust, cloud conditions, were associated with the transmission of COVID-19 incidence (Akan, 2022; Li et al., 2020; Zoran et al., 2020; Taniş and Karakaya, 2021; Wu et al., 2020; Tang et al., 2020). To prevent the spread of this virus, which caused significant global health impacts, many strict lockdown measures restricting human mobility, such as public, airport, and ship transport limitations, were implemented on regional or national scales by decision-makers. These measures, although primarily introduced as public health interventions, also provided an unprecedented opportunity to evaluate the relationship between anthropogenic activities and environmental quality. Previous studies examined the effects of COVID-19-related lockdown measures on regional air pollution by comparing air pollutant concentrations obtained from monitoring stations or remotely sensed data with previous years’ observations. Most of these studies demonstrated improvements in air quality during quarantine periods, highlighting the strong influence of transportation and industrial activities on atmospheric pollution (Akan and Coccia, 2022; Bray et al., 2021; Filonchyk et al., 2021; Sannino et al., 2021; Cucciniello et al., 2022). More recent systematic reviews and global-scale assessments have further confirmed that COVID-19-related restrictions resulted in significant reductions in major pollutants, particularly NO₂, PM_2.5, PM₁₀, CO, and NO_x, while also emphasizing the role of mobility reduction and industrial activity changes in improving air quality (Navaratnam et al., 2024; Zheng et al., 2025). Correspondingly, this review paper critically evaluates the associations among climate parameters, air pollutants, and SARS-CoV-2 transmission, as well as the impacts of COVID-19-related lockdown restrictions on outdoor air quality. Furthermore, this review highlights the lessons learned from the pandemic by examining the relationships between human activities, environmental quality, and climate change mitigation strategies. The current state of knowledge is discussed from three main perspectives: (i) the influence of air pollutants and climatological conditions on COVID-19 transmission, (ii) the impacts of COVID-19-related lockdown measures on outdoor air quality, and (iii) the implications of pandemic-related environmental changes for future environmental health and climate policies. The conceptual framework illustrating the scope and main components of this review is presented in Figure 1.

2. Associations Among Air Pollution, Climatological Parameters, and the COVID-19 Outbreak

COVID-19 has been one of the most influential global health challenges in recent years, affecting various aspects of human life due to the substantial changes in social, economic, and environmental systems following the emergence of the SARS-CoV-2 virus. Since its first identification in December 2019 and subsequent rapid global spread, the COVID-19 pandemic has not only placed significant pressure on healthcare systems but also caused considerable socioeconomic impacts, particularly on trade, production, manufacturing, air and land transportation, shipping, and service sectors associated with social activities (Skirienė and Stasiškienė, 2021). Investigations into the factors influencing SARS-CoV-2 transmission have demonstrated that environmental conditions, including meteorological parameters and air pollution levels, together with demographic, socioeconomic, behavioral characteristics, and non-pharmaceutical interventions, played important roles in the spread and severity of the pandemic across different regions (Aboura et al., 2022; Weaver et al., 2022). Many previous studies have revealed that climatic parameters, such as temperature, relative humidity, wind speed, precipitation, atmospheric pressure, gust, cloud, sun hour, and air pollutants including CO, NO₂, SO₂, O₃, PM_2.5, and PM₁₀ have a significant contribution to the spread of respiratory infectious diseases (Bolaño-Ortiz et al., 2020; Somayaji et al., 2020). Faruk and co-workers (2022) evaluated the associations between meteorological factors and COVID-19 transmission considering previously published manuscripts. They stressed that the most significant associations with the spread of COVID-19 are mostly based on two environmental factors: temperature and humidity. Moreover, air pollution leads to an increase in the number of COVID-19 confirmed cases depending on PM₁₀, PM_2.5, and O₃ concentrations. Han and co-workers (2022) explored globally the factors affecting the transmission of COVID-19 incidence examining data from 188 countries and related to meteorological parameters, outdoor air pollutants, and socioeconomic aspects for the period between March and November 2020. They found that the spread of the SARS-CoV-2 virus was negatively correlated with meteorological variables and positively correlated with air pollutants and socioeconomic characteristics. In another study, Aboura (2022) evaluated the relationship between climate parameters including ambient temperature, relative humidity, precipitation, and wind speed, and the number of cases and deaths in 134 countries related to the SARS-CoV-2 virus for the period 1 January 2020 to 23 March 2021, considering control parameters, such as government interventions, total population, and population density. The results obtained on a global scale have exhibited that relative humidity causes a decrease in case and death rates in both high and low regimes, while temperature and wind parameters decrease mortality rates. Another remarkable result of the study is that relative humidity is more effective in confirmed cases and wind on deaths. Moreover, another study conducted by Sun and coworkers (2022) evaluated the effects of meteorological factors including air temperature, relative humidity, wind speed, and surface pressure on the epidemic using regression models. They stated that ambient temperature is both significantly and positively correlated with the transmission of the COVID-19 outbreak. It is clear that from multiple studies exploring the associations between air pollutant concentrations, meteorological parameters, and the spread rate of COVID-19, people who are exposed to air pollutants in high concentrations and to diverse climatological conditions, such as low wind speed preventing the dispersion of air pollutants that remain stagnant in the air with the content of bacteria and viruses, are under the risk for capturing to infectious diseases, such as SARS-CoV-2 (Coccia, 2020, Coccia, 2021; Akan, 2022; Huang et al., 2022). In addition to the studies summarized here so far, some of the studies expressing the effects of meteorological parameters and air pollutant concentrations on the transmission of the SARS-CoV-2 virus considering different periods in different countries are presented in Table 1.

The studies summarized in Table 1 demonstrate that the association between environmental parameters and COVID-19 transmission is complex and influenced by multiple interacting factors. Although several studies have reported significant relationships between air pollutants, particularly particulate matter and NO₂, and COVID-19 incidence, the magnitude and direction of these associations differ depending on geographical location, meteorological conditions, population characteristics, and implemented control measures. Recent investigations have further emphasized the importance of considering nonlinear relationships, delayed effects, and potential causal mechanisms when evaluating the influence of environmental factors on infectious disease dynamics. Overall, these findings highlight the close interaction between atmospheric conditions, human activities, and public health risks, supporting the consideration of COVID-19 as an important environmental health indicator.

3. Impacts of Lockdown Measures Based on COVID-19 Outbreak on Outdoor Air Quality

To prevent the transmission of the COVID-19 virus from person to person on a national scale, countries implemented various measures to reduce human activities in society across different regions and periods while controlling the health impacts of the epidemic and improving urban air quality (Anil and Alagha, 2021; Islam et al., 2021; Munir et al., 2021; Mor et al., 2021). While strict lockdown measures imposed by governments to combat the transmission of COVID-19 impact the countries’ economies adversely, they can also improve air quality by reducing anthropogenic emissions from sources such as diesel-fueled vehicles, power plants, cement and iron-steel factories (Ray et al., 2022). COVID-19-based restrictions have shown that reductions in human activities can temporarily decrease greenhouse gas emissions and atmospheric pollution, providing valuable insights into the relationship between societal activities and environmental quality (Siddique et al., 2021).

Akan and Coccia (2022) explored globally how lockdown measures taken by governments to prevent the spread of the COVID-19 outbreak influenced air quality levels by evaluating average changes in air pollutant concentrations across 300 cities in 19 countries of five geoeconomic regions. They concluded that air pollutant concentrations generally decreased during the lockdown periods, but the variation in each country was different from one another, especially due to their differences in geographical, economic, and industrial characteristics. Another study performed by Skirienė and Stasiškienė (2021) investigated how lockdown strategies influenced air quality changes during three periods: i) pre-lockdown from Jan to Feb 2020, ii) I period from Mar to Apr 2020 including lockdown measures applied nationally, iii) II period covering only May 2020, when countries started to relax measures by comparing the data with the same periods from 2018–2019 years for five European countries: the United Kingdom, Spain, France, and Sweden, as well as the Northern Italy region. Results showed that NO₂, PM_2.5, and PM₁₀ emissions were reduced by 20–40% in 2020 because of the decline in industrial and mobility actions. A study carried out in the Kolkata Metropolitan Area, India, to determine the possible influence of the reductions in mobility activities based on lockdown restrictions on air pollutants concentration indicated that the concentrations of air pollutants showed significant reductions contributing to improvements in air quality index (AQI) by 54.94% during the lockdown months (Chowdhuri et al., 2022).

In another study carried out in cities of India, including Delhi, Mumbai, Kolkata, and Chennai, enforced lockdown measures from March 24, 2020, to May 17, 2020, to comprehend the influence of restrictions on air quality improvements, it was found that air pollutants levels and aerosol concentration were reduced by the range of -37.13% and -54.94% for all examined cities compared to previous years. Moreover, the variation in PM_2.5 concentration in these megacities demonstrated the highest decline, directly related to restricted vehicular movement (Pal et al., 2022). In the study conducted in Wuhan during the COVID-19 pandemic lockdown and the post-pandemic period, it was observed that PM_2.5 concentrations were reduced by 9.57–18.67% from a momentum effect and by 7.07–27.60% from a spillover effect through the spatiotemporal model (Gao et al., 2022). On the other hand, another study conducted by Jakob et al. (2022) evaluated the variations in the concentration of air pollutants based on lockdown restrictions in Jakarta, Indonesia. Since the results indicated that the concentrations of PM₁₀ and PM_2.5 did not change significantly because of the lockdown measures, meteorological factors impacting air quality should also be considered. Furthermore, in the study conducted in downtown Toronto, Canada, it was found that even if a significant reduction in traffic density was observed, the reduction in the concentration of PM_2.5 remained negligibly low (Jeong et al., 2022). Moreover, Das et al. (2022) compared the effects of lockdown measures applied on a regional and national scale on air quality. They indicated that regional lockdown in 2021 improved air quality by 32% more than the national lockdown in 2020. In another study focusing on the impact of the lockdown imposed in India nationally from March 25, 2020, to prevent the spread of COVID-19 disease on the anthropogenic emission sources by analyzing a multi-scale (national-regional-city), it was found that a significant reduction in surface measurements of NO₂ (46–61%) and fine particulate matter (42–60%) during the lockdown period. Other species, namely coarse particulate matter (24–62%) and ozone (22–56%), also showed a substantial reduction, whereas CO exhibited a moderate decline in the range of 16-46%. However, SO₂ levels did not exhibit any defined reduction trend but increased in Mumbai, Bengaluru, and Kolkata (Sathe et al., 2021).

Lockdown measures generally improved air quality by reducing transportation and industrial emissions; however, the magnitude of these improvements varied significantly among regions depending on emission sources, meteorological conditions, and local characteristics. To stress the influence of lockdown measures applied regionally or nationally by counties on air quality improvements, Table 2 demonstrates significant results of the previous studies from different countries.

Overall, the studies summarized in Table 2 demonstrate that COVID-19-related lockdown measures generally resulted in short-term improvements in outdoor air quality, particularly through reductions in transportation, industrial activities, and other anthropogenic emissions. However, the magnitude and direction of these changes varied considerably among regions and pollutants, depending on local emission sources, geographical characteristics, meteorological conditions, and the intensity and duration of lockdown measures. Recent studies have further demonstrated that these effects should be interpreted cautiously, as observed changes in pollutant concentrations may also reflect seasonal variability, atmospheric processes, and natural emission sources. Therefore, the environmental improvements observed during the pandemic should not be interpreted as evidence of a permanent reduction in air pollution, but rather as an important environmental health lesson demonstrating the sensitivity of air quality to changes in human activities.

Although many studies have reported that lockdown measures improved air quality, Jakob et al. (2022), on the other hand, stated that the decrease in pollutant concentrations observed during the quarantine period was not necessarily related to the restrictive measures, as seasonal changes may also have contributed to the observed variations. In this context, considering that quarantine restrictions may provide insights for future policy actions, such as the preparation of national strategies and the development of new technologies to combat air pollution and climate change, future studies should not only evaluate changes in pollutant concentrations using empirical measurements, various models, and remote sensing data, but also examine seasonal variations in pollutant concentrations over longer periods.

4. The Impacts of the COVID-19 Pandemic on Climate Change

To reduce human-to-human transmission of COVID-19, temporary lockdowns at various scales were imposed during different periods of the pandemic to control the spread of the virus. The considerable reductions in economic production and transportation activities contributed to a sharp decline in major GHG emissions, particularly CO₂. In China, CO₂ emissions were reduced by 23% due to lockdown measures applied nationally limiting thanks industrial, commercial, and transportation activities by February 2020. According to a report published by the International Energy Agency in 2020, in the first quarter of 2020, CO₂ emissions were reduced by 5% depending on the decrease in energy demand compared to the previous year’s data. Considering the reductions in CO₂ emissions based on several areas, they were in the range of 1.4-19.3%, depending on the countries and regions in the power sector as well as they were recorded as 5.5% in heavy industry and manufacturing sources (Nguyen et al. 2021; Liu et al., 2020). In the transportation area, by the end of March 2020, due to the reduction of flights by 85%, global aviation emissions recorded remarkable drops compared with 2019 (Quéré et al. 2020; Nguyen et al. 2021).

In another study conducted by Kumar et al. (2022) investigating how international air transportation and industrialization restrictions based on the COVID-19 pandemic impact CO₂ emissions, comparisons of daily CO₂ emission data between 2019 and 2020 for 69 countries indicated a 17% reduction. The studies conducted by Liu et al. 2020 explored how the diminishing of human activities due to COVID-19 lockdown measures affects energy demand and CO₂ emissions. They found that the COVID-related decreases in CO₂ emissions (as a total of 938 Mt CO₂) were as follows: 15.5%, 6.4%, 4.4%, 28.9%, 2.7%, and 15% caused by road transportation, power, industry, aviation, residential buildings, and international shipping, respectively. A study conducted by Gillingham et al. (2020) explored the importance of limitations based on COVID-19 incidence associated with less traffic- and industrial-based emissions on the air quality of the USA. Results pointed out that CO₂, NO_x, and PM emissions were reduced by 15, 12, and 1%, respectively.

Monteiro (2022) aimed to determine the changes in GHG emissions including CO₂, CH_4, and air pollutant (CO) concentration comparing lockdown-period impact with the data from previous years for North American cities. Results indicated that CO₂ and CO emissions showed much more reductions because of decreasing in traffic density rather than CH₄ emissions associated with characteristics of cities. The mitigation in the concentration of air pollutants is significantly attributed to the changes in traffic trends compared to changes in meteorology and biology. Ray et al. (2022) investigated how COVID-19 lockdown measures affect carbon emissions by evaluating data from different countries on a global scale. Following their results, the strict lockdown measures have been playing a significant role in the mitigation of carbon emissions.

Hickman et al. (2023) investigated how the atmospheric composition and climate impacts of COVID-19 pandemic responses depend on background emissions, climate conditions, and the season in which lockdowns occurred. Using the ModelE Earth system model, they evaluated atmospheric and climate responses to lockdown-related emission perturbations occurring in different decades and seasons. The results showed that global COVID-19-related anomalies in aerosols and trace gases differed by up to an order of magnitude or more when lockdowns occurring in 1980, 2008, 2020, and 2051 were compared. Regional atmospheric composition anomalies were generally largest when emissions were close to their historical peak, with the largest responses occurring in different periods depending on the region. Regional aerosol direct-effect anomalies were almost always below 0.1 W/m² during the first pandemic year, whereas larger anomalies were obtained in some historical scenarios. The study also demonstrated that shifting the onset of COVID-19 by 3, 6, or 9 months could alter global atmospheric composition by approximately 2–25%. However, changes in surface temperature and precipitation were not systematically related to local changes in atmospheric composition. These findings indicate that the atmospheric and climatic consequences of temporary emission reductions are strongly dependent on the background emission conditions, timing, season, and regional characteristics.

In summary, strict measures, including lockdowns and reductions in industrial activities and traffic density, resulted in substantial short-term reductions in CO₂ and other greenhouse gas emissions at regional, national, and global scales. However, these reductions were primarily associated with temporary decreases in economic and transportation activities and therefore should not be interpreted as evidence of a sustained reversal in global climate change trends.

Liang et al. (2023) examined the spatiotemporal characteristics and co-effects of changes in air quality and CO₂ emissions during the COVID-19 lockdown in China. The study analyzed 324 prefecture-level cities during the period from January 24 to April 30, 2020, using a regression discontinuity in time approach. During the major lockdown period, SO₂, NO₂, and CO₂ concentrations decreased by 5.6%, 16.6%, and 25.1%, respectively, at the national scale. The reductions in CO₂ and NO₂ were particularly pronounced in southern regions, indicating considerable spatial heterogeneity in the response to lockdown measures. However, from March onward, the improvement effects on air quality and CO₂ emissions weakened, and pollutant concentrations began to rebound. These findings demonstrate that lockdown measures can produce substantial short-term reductions in CO₂ emissions, although such effects may diminish as economic and human activities resume.

Wang et al. (2024) reviewed the responses of short-lived atmospheric pollutants and climate forcers to the large-scale emission reductions associated with COVID-19 lockdowns. The review focused particularly on aerosols, ozone (O₃), nitrogen oxides (NO_x), and methane (CH₄), providing an assessment of their atmospheric and climatic responses at regional and global scales. The authors reported that reductions in transportation-related emissions generally decreased near-surface particulate matter and ozone concentrations, although the magnitude and even the direction of these changes varied among regions because of atmospheric oxidation capacity and anomalous meteorological conditions.

Zeppini and van den Bergh (2025) investigated the long-term climate implications of the COVID-19 pandemic by developing a global energy competition model that jointly considered lockdown measures, economic stimulus programs, climate policies, and CO₂ emissions. Unlike studies focusing primarily on the short-term emission reductions observed during lockdown periods, their analysis examined how the pandemic could influence long-term emission pathways. The results indicated that, in the absence of countervailing climate policies, the COVID-19 pandemic could increase long-term emissions. In contrast, when a carbon tax was already in place, the pandemic scenario resulted in lower emissions than scenarios without the pandemic or without climate policy. The combination of economic stimulus and carbon taxation produced the strongest long-term emission reductions, indicating that the climatic consequences of a pandemic depend not only on temporary changes in human activities but also on the policy framework implemented during the recovery period.

Taken together, the evidence indicates that the COVID-19 pandemic provided a unique opportunity to observe how rapid changes in human activities can influence greenhouse gas emissions and atmospheric composition. Although lockdown measures resulted in substantial short-term reductions in CO₂ and other emissions, these reductions were insufficient to produce a sustained reversal in long-term climate change trends. The subsequent recovery of economic, industrial, and transportation activities further demonstrated the limitations of temporary restrictions as a climate mitigation strategy. Therefore, one of the most important lessons from the pandemic is that lasting climate benefits require structural changes in energy systems, transportation, industrial production, and climate policy rather than temporary reductions in human mobility and economic activity.

5. Concluding Remarks

The inevitable fact is that climate change is the most serious environmental problem threatening public health in the 21st century. While there is a need to understand how and to what extent climate change is affecting public health, from respiratory diseases, heart diseases, and mental illnesses to infectious diseases, it is necessary to urgently identify and take the necessary measures at the individual and societal level, as well as to put into effect the laws and policies to be implemented by states. The positive effects of partial and full quarantine measures applied during the pandemic on air quality, which were explained in detail in this review paper by presenting an extensive and comprehensive literature review, provide valuable evidence for understanding how changes in human activities can influence environmental quality and may inform the scope of countries’ commitments to combat global climate change. Correspondingly, the dissemination of post-combustion CO₂ capture technologies that can capture the CO₂ emissions released into the atmosphere from the flue gas of coal-fired thermal power plants used for electrical energy production, as well as the implementation of government policies that will support the preference of electric vehicles and motorcycles instead of the use of fossil-fuel combustion-powered vehicles, are just a few of the steps that will make significant contributions to the fight against climate change not only on a national scale but also on a global scale.

This review has several limitations. First, considerable heterogeneity was observed among the reviewed studies in terms of study periods, geographical coverage, environmental parameters, data sources, and statistical or modeling approaches, which limit direct quantitative comparisons among their findings. Second, many studies conducted during the early stages of the pandemic relied on short observation periods and were therefore potentially influenced by seasonal variations, meteorological conditions, and other confounding factors. Third, the reviewed literature predominantly focused on the initial lockdown periods, whereas fewer studies evaluated the longer-term environmental consequences following the relaxation of restrictions. Finally, because this review did not perform a formal meta-analysis, the magnitude of associations reported across individual studies should be interpreted in the context of their respective methodological approaches and study settings.

Overall, the evidence reviewed in this study highlights that the COVID-19 pandemic should be regarded not only as a global public health crisis but also as an important environmental health indicator, providing valuable lessons for developing integrated and long-term strategies to address air pollution, climate change, and future environmental health challenges.