Good in The Worst: COVID-19 Restrictions and Ease in Global Air Pollution

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), known to cause 2019coronavirus disease (COVID-19) pandemic is a zoonotic coronavirus and crosses species to infect human populations, where an efficient transmission of virus occurs human-tohuman. Nationwide lockdown is being adopted to stop public transport, keep people at their homes and out of their work, and maintain social distancing. In turn, large geographic areas in the world (including China, Italy, Spain, and USA) has been almost halted. This temporary halt is significantly slashing down the air pollution (air pollutants and warming gases) in most cities across the world. This paper: (i) introduces both COVID-19 and air pollution; (ii) overviews the relation of air pollution with respiratory/lung diseases; (iii) compiles and highlights major data appeared in media and journals reporting lowering of air pollution in major cities those have been highly impacted by the COVID-19; and also (iv) lists the way forward in the present context. Because COVID-19 is an ongoing pandemic and currently far from over, strong conclusions could not be drawn with very limited data at present. The temporary slashed down global air pollution as a result of COVID-19 restrictions are expected to stimulate the researchers, policy makers and governments for the judicious use of resources; thereby minimise the global emissions, and maintain their economies once the pandemic eases. On the other, lifting of the nationwide lockdown and eventual normalisation of the temporarily halted sectors may also reverse the currently COVID-19 pandemic-led significantly slashed down global air pollution that could make the future respiratory health crisis grimmer.


Introduction
The recent global outbreak of an infectious coronavirus disease  has already been considered as a global health emergency by the world health organization (WHO).
Previously known by the provisional name 2019-novel coronavirus (2019-nCoV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a strain of coronavirus is known to cause COVID-19 (Chen and Li 2020;El-Feky et al. 2020). SARS-CoV-2 is a zoonotic coronavirus that has crossed species to infect human populations, where an efficient transmission of virus occurs human-to-human. SARS-CoV-2 has already spread over about 200 countries in the world and, as of 4 th April, the WHO has reported 1,051, 635 confirmed cases and 56,985 confirmed deaths in total due to COVID-19 (WHO 2020) (Table 1; Fig. 1). Therefore, the world is trying every approach to control the rapid transmission of SAAR-CoV-2 and so the spread of COVID-19 in humans. In an effort to slow the virus' very fast pace spread, a large number of countries is adopting the strategy of practicing social distancing and telling people to stay in their homes through implementing the strict lockdown. Worldwide, the streets of the most cities are deserted; the normally bustling pubs, bars and theatres have been closed; operation of the public transport has been resricted; the most extensive travel restrictions are in place, and people are working remotely from their homes. Since mid-February 2020 and/or beginning of March 2020, about three-week nationwide lockdown has halted increasingly large geographic areas which is impacting economies across the world and also the staus of global air pollution. In the following sections, air pollution is introduced; the relation of air pollution with major respiratory/lung diseases is overviewed; major data appeared in media and journals reporting lowering of air pollution in major cities those have been highly impacted by the COVID-19 are compiled and highlighted; and also the future prospects in the present context are enlightned.

Air pollution
Owing its importance as a major part of several essential cycles on Earth, air is required to survive by most species including human beings. Air provides oxygen (O 2 ) for respiration to occur, and also carbon dioxide (CO 2 ) for photosynthesis to happen in plants. However, rapid establishment of industries aimed at making our daily life confartable, on the other hand, is adding highly toxic substances to air and making it very hard to breath. Air is called polluted when it exhibits harmful concentrations of a mix of particles and gases such as black carbon (soot), smoke, mold, pollen, methane (CH 4 ), and CO 2 . Particulate matter (PM), black carbon, ground-level ozone (O 3 ), nitrogen dioxide (NO 2 ), nitrous oxide (N 2 O), sulfur dioxide (SO 2 ) and carbon monoxide (CO) are among the major types of air pollutants (Fig. 2). Increasing levels of small size components or

WHO RISK ASSESSMENT
Global Level Very High PM (PM10; PM2.5; UFP-ultrafine particles, <0.1 mm in diameter, 20-times smaller than the width of a human hair) have been confirmed in air sampled in the major world cities.
PMs are contributed mostly as by-products of combustion from coal-fired power stations, wood and charcoal-burning stoves, vehicle engines and factories. In particular, PM10 (<10 micrometers) and PM2.5 (<2.5 micrometers) can be breathed deeply into the lungs and may cross into the bloodstream; therefore, pose higher health risks (Nunez 2019).

Fig. 2.
Types and sources of major air pollutants (Helen 2020).

Air pollution and respiratory/lung diseases
Air pollution is known since long back to affect all regions of the world. Notably, in addition to smog, soot, greenhouse gases (CO 2 ; CH 4 ), N 2 O; O 3 ), varyingly affect the planet and our health. Both indoor and ambient air pollution exposure have been argued as a major risk to respiratory health worldwide, particularly in low-and middle-income countries (Saleh et al. 2020). NO 2 at concentrations above 200 micrograms per cubic metre has beeb considered as a toxic gas which causes significant inflammation of the airways (WHO 2018a). Major health consequences of air pollutants include respiratory infections, asthma, chronic obstructive pulmonary disease, lung cancer, even in combination with stroke and heart diseases (Kim et al. 2018

COVID-19 and the status of global air pollution
In the present condition of COVID-19, experts have argued that high air pollution and smoking make people more vulnerable to this disease (Korber 2020; Perappadan 2020; TWC India Edit Team 2020). Morevoer, air pollution has been expected as a common denominator for countries with major cases of severe COVID-19 infection, China, South Korea, Iran, and northern Italy (BMJ 2020a). In particular, cities with higher air pollution were arugued as at more risk of COVID-19 (Basu 2020;Barbiroglio 2020;Korber 2020 Interestingly, halting of large geographic areas due to lockdown has also led to some unexpected consequences in terms of moderate to significant lowering of air pollution in most parts of the world including China, Italy, and California (Calma 2020a,b). In addition to pollution, greenhouse gas emissions have also fallen across continents (Henriques 2020). In context with the greenhouse gas emissions, 72% and 11% of the transport sector's greenhouse gas emissions are contributed by the driving and aviation respectively (IPCC 2018).
In China, the measures to minimize the spread of SARS-CoV-2 have resulted in reductions of 15% to 40% in output across key industrial sectors and also has temporarily reduced China's CO 2 emissions by a quarter (Table 2). In particular with CO 2 , around 800m tonnes of CO 2 (MtCO 2 ) were released in China over the same period in 2019. On the other, NO 2 , an air pollutant closely associated with fossil-fuel burning exhibited 36% lower over China (in the week after the 2020 Chinese new year holiday) than in the same period in 2019 (Myllyvirta 2020; NASA 2020) (Fig.   5). Notably, in Italy, a remarkable drop has been revealed in NO 2 pollution that comes mainly from traffic, especially diesel vehicles, which are also a major source of PM (European Public Health Alliance 2020) (Fig. 6A). PM10 levels in Lombardy have fallen dramatically after 10 days of COVID-19 social distancing measures (Fig. 6B). France also showed the measured drop in NOx as economic activity and transportation therein are at a bare minimum due to COVID-19 (Balken Green Energy News 2020) (Fig. 7).   In context with India, air pollution has been reported to plummet in the end of second week of COVID-19 lockdown. In terms of air quality index (AQI; range: 0 to 500; good:

Conclusions and prospects
Because COVID-19 is an ongoing pandemic and currently far from over, strong conclusions could not be drawn with very limited data at present. No doubt air pollution is increasing worldwide and has been widely reported to be closely related with the severe lung/respiratory diseases in human. The nationwide halting of public transports and closure of major industrial units has resulted in obvious significant reductions in emissions of a variety of gases related to energy and transport. Indeed, industrial developments and production are necessary for supporting the world population.
However, lifting of the nationwide lockdown and eventual normalization of the temporarily halted sectors may also reverse the currently COVID-19 pandemic-led slashed down global air pollution and can make the future respiratory health crisis grimmer. Hence, the indication of slowing down of public and personal trasport, and travel in slashing down the air pollution is expected to stimulate the researchers, policy makers and governments to judicious use of resources and thereby minimize the global emissions and maintain their economies once the pandemic eases.