Introduction

Coronavirus disease (COVID-19) is an ongoing devastating pandemic. As of November 05, 2021, there were more than 248,467,363 cases of coronavirus-infected persons and 5,027,183 deaths worldwide as shown in Fig. 1 (WHO 2021). COVID-19 originated from Wuhan, the capital city of Hubei Province in the People's Republic of China, in the month of December 2019 (Huang et al. 2020). Due to its highly contagious nature, it spread around the world very quickly. The Centers for Disease Control and Prevention (CDC) reported that on June 17, 2020, 216 countries and territories had confirmed COVID-19 cases (CDC 2020).

Fig. 1
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Source-WHO 2021)

Total confirmed cases throughout the world on November 05, 2021 (

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To break the chain and minimize the spread of the coronavirus, majority of the nations implemented a complete lockdown (Long 2020). As a result, most industrial activities were halted and public and personal transport was restricted during the lockdown period. As of February 18, 2021, many countries like Australia, Belgium, India, France and Spain have initiated fresh partial lockdowns and implemented new guidelines for highly populated areas. These measures have caused a massive impact on the socio-economic conditions of all nations worldwide. On the other hand, positive impacts of stringent lockdown were seen on environmental pollution, especially on water quality (Mandal and Pal 2020; Yunus et al. 2020) and air quality (Gautam 2020a, b; Saadat et al. 2020; Srivastava et al. 2020; Ankit et al. 2021). Air pollution is known to cause mass-scale health problems, particularly in densely populated developing nations (WHO 2006). The long term exposure to air contaminants including PM2.5 and ozone (O3) causes ∼ 8.8 million excess deaths yearly, while NO2 is responsible for 4 million new pediatric asthma cases every year (Burnett et al. 2018; Achakulwisut et al. 2019; Lelieveld et al. 2020). Due to lockdown measures, the reduction in air contaminants and improvement in AQI were seen in several countries like India and China (Bao and Zhang 2020; Mahato et al. 2020; Srivastava et al. 2020). The proportion of reduction in air pollutants presumably depended on the stringency of lockdown. Apart from this, many recent studies have found a positive relationship between the increase in air pollution with COVID-19 infection and mortality rates (Coccia 2020; Sarmadi et al. 2020; Shakoor et al. 2020; Wu et al. 2020; Kumar 2020; Son et al. 2020; Copat et al. 2020; Marquès and Domingo 2021). It is necessary to understand trends in the reduction of air contaminants in different countries in order to help policymakers plan and implement future strategies to combat air pollution along with handling measures for COVID-19 and similar pandemic situations. The main objective of this review is to assess the worldwide impact of the COVID-19 lockdown on air quality, particularly on the level of PM2.5, PM10, NO2, CO and SO2. The impact of the COVID-19 pandemic lockdown (CPL) on air quality was assessed in the United States of America (USA), China, India, Brazil, Italy, Spain, France, Norway, Germany, Netherland, Switzerland, Kazakhstan, Mongolia, Morocco, Iran, Peru, Australia, Malaysia, United Arab Emirates (UAE), Austria, Portugal and the Czech Republic.

Pollution assessment during COVID-19

During the current COVID-19 pandemic (CP), human activities, which are considered as major sources of various pollutants, were stopped partially to completely almost globally, resulting in reduced pollution levels (Zambrano-Monserrate et al. 2020; Muhammad et al. 2020; Saadat et al. 2020; Gautam 2020b). Community mobility reports (https://www.google.com/covid19/mobility/) depict that human mobility decreased by 90% between 23 February 2020 to 05 April 2020, specifically in European countries (Muhammad et al. 2020). During the same time, a sharp decline in air pollution was noticed worldwide. Climatologists predicted that greenhouse gases (GHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (NO) emissions declined to levels not observed since World War II (Global Carbon Project 2020; Zambrano-Monserrate et al. 2020). Some recent studies have shown that atmospheric emission of multiple pollutants such as NO2, CO2, CO, SO2, PM10, and PM2.5 significantly decreased (ESA 2020; Saadat et al. 2020; Dantas et al. 2020; Mahato et al. 2020; Quéré et al. 2020; Ju et al. 2021; Mostafa et al. 2021). Scientists from the National Aeronautics and Space Administration (NASA) revealed the reduction in NO2 pollution near Wuhan and the trend continued across China (NASA 2020; Dutheil et al. 2020).

Studies in different cities, residential areas, commercial areas, tourist spots, industrial areas, mining sites, highways, and roads have been assessed for pollution levels in various parts of the world (Chakraborty and Maity 2020; Mahato et al. 2020; Kotnala et al. 2020; Dantas et al. 2020; Nakada and Urban 2020; Sicard et al. 2020). Zambrano-Monserrate et al. (2020) predicted both indirect positive and negative effects of the current pandemic situation on different environmental components. They concluded that air quality and noise levels improved along with cleaner seashores. However, waste generation increased due to adoption of measures for health safety while recycling and management of waste reduced due to imposition of lockdown which lead to the contamination of water and soil (Zambrano-Monserrate et al. 2020). Another study assessed the data of the European Space Agency (ESA) and NASA and concluded that the pollution level in Wuhan (China), Spain, Italy, and the USA declined by 30% during the current lockdown period (Muhammad et al. 2020). Environmental impacts of COVID-19 became visible in Venice, Italy, where the water canals looked clearer than normal, which may be due to the absence of sediment resuspension compared to the past (Braga et al. 2020). The effects of motorboats on mixing of sediments and pollutants decreased, resulting in clearer water and fish could be seen again in the canals (Saadat et al. 2020). A decrease in the number of tourists in the city also led to a decrease in water pollutants. The study also reported that due to the lockdown measures, the land surface temperature (LST) was reduced by 2.74 to 7.06 °C from the pre-lockdown LST range of 31.25 to 35.11 °C and the noise level dropped to < 65 dBA from > 85 dBA after 18 days of lockdown (Mandal and Pal 2020). The same study reported that the water quality of an adjacent river improved in terms of pH, total dissolved solids (TDS), and dissolved oxygen (DO) after 30 days of lockdown. Yunus et al. (2020) also analyzed the remote sensing images of pre and post-lockdown and projected that the surface water quality of Vembanad Lake in India improved during the lockdown in terms of suspended particulate matter (SPM), which decreased by 15.9% compared to pre-lockdown periods. Other studies from different countries also reported reduction in level of environmental contaminants, especially gaseous pollutants, which are summarized in Tables 1, 2, 3, 4, 5, 6, 7 and 8.

Table 1 Impact of COVID-19 lockdown on air contaminants in India
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Table 2 Impact of COVID-19 lockdown on air contaminants in China
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Table 3 Impact of COVID-19 lockdown on air contaminants in Italy
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Table 4 Impact of COVID-19 lockdown on air contaminants in Brazil
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Table 5 Impact of COVID-19 lockdown on air contaminants in Spain
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Table 6 Impact of COVID-19 lockdown on air contaminants in France
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Table 7 Impact of COVID-19 lockdown on air contaminants in the USA
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Table 8 Impact of COVID-19 lockdown on air contaminants in other countries
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Worldwide reduction in major air contaminants

India

India has taken various preventive measures to combat the CP. However, there were over 34,333,754 confirmed COVID-19 cases, with 459,873 human deaths as of November 05, 2021 (5:11 pm CET, 05 November 2021) (WHO 2021). The Indian government implemented the first phase lockdown 1.0 for three weeks starting 24 March to 14 April and then extended it for three consecutive lockdowns until 31 May 2020. Then, the unlock phase started from 1–30 June as unlock1.0 with some relaxation that continued in unlock 2.0 (1–31 July), unlock 3.0 (1–3 August) with increasing relaxations including allowed inter and intra-state transportation, unlock 4.0 (1–30 September), unlock 5.0 (1–31 October), and unlock 6.0 (1–30 November) with partial reopening of schools. The continuous nationwide lockdown impacted the economy negatively and the environment positively. The pollution level in many Indian cities improved during the first to the fourth phase of lockdown compared to pre-lockdown periods of 2020 and the same months of the previous year (Mahato et al. 2020; Sharma et al. 2020; Singh and Chauhan 2020; Kumar et al. 2020; Srivastava et al. 2020; Jain and Sharma 2020; Kumar 2020). During the lockdown period, the air quality of Delhi, India’s capital, improved significantly (Mahato et al. 2020; Srivastava et al. 2020; CPCB 2020). Mahato et al. (2020) recorded that the level of pollutants such as NO2 and CO decreased by 52.68% and 30.35% respectively, during the lockdown period. The concentration of PM10 and PM2.5 decreased > 50% in the pandemic period compared to the pre-lockdown phase. When compared to the same period of the previous year, PM10 and PM2.5 levels declined by 60 and 39% respectively, in the city. In the year 2020, the air quality of the industrial and major transport areas of Delhi improved by 60% (Table 1) (Mahato et al. 2020). In Delhi’s busy commercial area (ITO), the concentration of PM2.5 was 124.5 μg m−3 before lockdown and declined to 46 μg m−3 during the lockdown period (Kotnala et al. 2020). Srivastava et al. (2020) studied the impact of the entire lockdown in two major cities of India; New Delhi and Lucknow. They found that there was a significant decline in the level of PM2.5 from 85 to 456 μg m−3 (before lockdown) to 47–204 μg m−3 (after lockdown). The concentrations of NO2 in Lucknow before and after the lockdown were 22–158 μg m−3 and 3–59 μg m−3, respectively. The concentration of CO decreased from 15–150 μg m−3 to 5–98 μg m−3 in New Delhi and 20–199 μg m−3 to 3–55 μg m−3 in Lucknow.

Sharma et al. (2020) investigated the impact of lockdown on 22 cities of North, South, West, East and Central regions of India and showed that AQI decreased by 44, 33, 32, 29 and 15% respectively, compared to the same period of previous year. The overall particulate matter declined by 52% throughout the nation due to restrictions in the lockdown period. However, the concentration of O3 increased by 17% compared to 2019 and was 10% higher compared to the last three year average. Another study found similar results from the riverside mining areas of eastern India, where the PM10 concentration was 189–278 μg m−3 before lockdown and 50–60 μg m−3 after 18 days of lockdown (Mandal and Pal 2020). That clearly showed that a significant portion of particulate matter attenuated during the lockdown period. Satellite data collected from Sentinel–5P on aerosol optical depth (AOD) variations indicated that the levels of aerosol/air pollution over the northern part of India recorded lowest levels in 20 years for the same period of time of the year 2020 (Gautam 2020a). Kumar et al. (2020) analyzed the concentration of PM2.5 up to fourth phase lockdown and concluded that the level of PM2.5 reduced in Mumbai, Delhi, Chennai, Hyderabad, Kolkata from 10–39, 41–53, 19–43, 26–54 and 24–36% respectively, compared to the last five years. The study also showed that the AOD variation results indicated a decreased aerosol level in Chennai, Delhi, Kolkata and Mumbai by 29, 11, 4 and 1% respectively, compared to 2019. During the four lockdown periods, the average AOD and NO2 declined to the maximum level in the Indian region by 60 and 45% compared to the average of 2017–2019 (Kumar et al. 2020).

China

China was the first country that implemented the stringent lockdown as a COVID-19 preventive measure on January 23, 2020 (Muhammad et al. 2020), which helped the country to control the spread of the virus and lighten the load on health facilities (Wilder-Smith and Freedman 2020). Due to the lockdown implementation, the pollution load in the Wuhan area decreased and the quality of air improved. Environmental pollutants such as NO2, CO, SO2, and PM2.5 were lower during the pandemic periods compared to the same period of time in previous years (Table 2) (Muhammad et al. 2020). Data from the Copernicus Atmosphere Monitoring Service (CAMS) by the European Union showed that about 20–30% of PM2.5 decreased in large areas of China in February 2020, compared to the monthly average of February 2017–2019 (Zambrano-Monserrateet al. 2020). During lockdown phase, the carbon emission rate decreased by 25% in China because factories were closed, people stayed at home to maintain social distancing, and coal use was reduced to 40% at power plants compared to the last three months of 2019 (Saadat et al. 2020; Dutheil et al. 2020). Wang and Su (2020) indicated that China’s air quality quickly improved during the pandemic and significantly helped to reduce the overall level of global carbon emissions.

Wang et al. (2020) found that the overall AQI of all the studied stations in the eastern and south eastern parts of China, reduced by 30% during the lockdown period. The concentration of air pollutants NO2, SO2, CO and PM2.5 also decreased during the lockdown period because of the reduction in emissions from industrial and transportation sectors (Table-2). However, the concentration of O3 at all of the studied stations increased from 39.0 to 59.1 μg m−3 during the lockdown period. Xu et al. (2020) found that the average concentration of NO2, SO2, CO, PM2.5 and PM10 of three central China cities (Anqing, Hefei, and Suzhou) decreased in February and March 2020 by 52.8–27.2, 52.5–41.1, 36.2–24.2, 46.5–33.0 and 48.9–25.3% respectively, compared to the same period of 2017–2019. However, the average concentration of O3 did not change significantly and showed a slight increase by 3.6%. This might be due to the limited reaction of NO and O3 in conditions of low NO2 concentration. Among the average AQI values for the 5 days in February 2020, the highest was 122.6, which was 45.1% lower compared to February 2017–2019 when the maximum AQI was 223.2.

Italy

In Italy the first case of COVID-19 was recorded on 20 February 2020 in Codogno, which is 60 km away from Milan (Collivignarelli et al. 2020; Grasselli et al. 2020; Porcheddu et al. 2020). On the very next day, 34 new cases were confirmed in Lombardy. Italy was the first country where the COVID-19 cases increased after the virus originated in China. After that, the government declared that area as a “red zone.” By 2nd March 2020, cases increased to 59,183 (WHO 2020), and the administration imposed total lockdown. The restriction measures minimized industrial activities in an unprecedented way, which resulted in improved air quality (Table 3). The pollution levels in terms of NO2, SO2, CO, PM10, and PM2.5 decreased in many cities of Italy, such as Rome, Milan and Turin (Collivignarelli et al. 2020; Sicard et al. 2020). Collivignarelli et al. (2020) found that the level of NO2 in Milan was 31.9 ± 1.9 μg m−3 during the partial lockdown and 22.1 ± 1.2 μg m−3 during the total lockdown, which was lower compared to the control/reference period (53.4 ± 2.6 μg m−3). This clearly showed that the reduction of NO2 during the lockdown period. The same study also reported that overall SO2 was reduced by 25.4% during the total lockdown period.

NO and NO2 levels reduced to a lesser extent in Turin compared to other European cities such as Nice, Rome, and Valencia, because of the continuing activities of essential food and pharmaceutical industries in Turin during the lockdown (Sicard et al. 2020). However, the levels of NO and NO2 in Turin’s high traffic areas, which were the highest contributor to NOX emission, sharply reduced by ~ 78 and ~ 65% respectively (Sicard et al. 2020). During the lockdown, the concentration of PM2.5 and PM10 decreased in Turin by 13 and 9%, respectively, but increased in Rome by 11 and 2%, respectively. This increase of particulate matter in Rome may have been due to domestic heating using fossil fuels and garden activities (e.g., burning biomass). However, the level of particulate matter declined greatly in the traffic (gasoline) station of Rome because of lower road and non-road transport and low fuel combustion in nearby commercial and institutional buildings (Sicard et al. 2020). Further, the study reported that the surface O3 level increased by 14% in Rome and 27% in Turin because of an unprecedented reduction of NOX emission (http://www-personal.umich.edu/~sillman/ozone.htm).

Brazil

Brazil recorded the first confirmed COVID-19 case on 25 February 2020. After that, the state governor of Rio de Janeiro declared a public health emergency and implemented partial lockdown measures a week later. Dantas et al. (2020) found that on the first day of the lockdown, the concentration of primary pollutants such as PM10, NO2, and CO showed a significant decrease. The study used data from the municipal Department of Environment (SMAC) automatic monitoring station to analyze the pollution level in the city. Results of different pollutants varied among different study sites. Interestingly, O3 concentration increased due to decreased NOX levels in the atmosphere. In São Paulo state’s urban road, NO and NO2 concentrations (µg m−3) declined by up to 77.3 and 54.3% respectively, while CO levels decreased by up to 64.8% in the city center during the partial lockdown in late March 2020 compared to the last five year monthly mean (Nakada and Urban 2020). The study also found that traffic-related pollutants such as CO and NOx decreased during the quarantine period (March 24, 2020 to April 20, 2020). However, in São Paulo urban areas, O3 concentration increased by approximately 30%. During the 90 days quarantine period, the concentrations of PM2.5, PM10 and NO2 in São Paulo city dropped by 46, 45 and 58% respectively, compared to the same period in 2019 (Debone et al. 2020). Marinho and Foroutan (2020) analyzed the air quality status of Rio de Janeiro and São Paulo using a compilation of satellite and ground-based data and found that the daily concentration of NO2 for May 2020 decreased by 18.8 and 13.3% respectively, compared to that of 2019. However, PM2.5 concentration of the São Paulo region showed no significant change compared to the same period in 2019 (Table 4).

Spain

The COVID-19 case was first recognized in Spain on 31 January 2020. By mid-March, the total COVID-19 confirmed cases were > 7000, with > 250 deaths. At the end of March, Spain became the third-most affected country with > 85,000 total confirmed cases and > 7000 deaths. Spanish government declared lockdown on 14 March 2020, which restricted free movement, stopped business activities, and reduced public transportation (Tobías et al. 2020). This lockdown helped in controlling the coronavirus and in reducing urban air pollution in many cities (Table 5). The concentration of major air pollutants such as NO, NO2, PM10, and PM2.5 were reduced in Barcelona, Madrid, Valencia, and Seville during the lockdown period (Sicard et al. 2020; Muhammad et al. 2020; Tobías et al. 2020; Zambrano-Monserrate et al. 2020). In Barcelona, the PM10 average concentration decreased in urban and traffic stations by 28 and 31%, respectively during the 14-day lockdown compared to before lockdown (Tobías et al. 2020). During the lockdown period the average NO2 loads detected by the tropospheric monitoring instrument (TROPOMI) over the Barcelona metropolitan area decreased by 57% compared to before lockdown. O3 concentration increased by 29% at Barcelona urban stations. Sicard et al. (2020) stated that the concentration of PM10 and PM2.5 significantly decreased at traffic stations in Valencia by 51.3 and 29.3% respectively. The concentrations of NO, NO2, CO and PM10 were reduced significantly; however, the level of ozone increased in Rio de Janeiro (Siciliano et al. 2020).

France

France detected its first COVID-19 case on 24 January 2020 and cluster of cases were identified first in Haute Savoie on 8 February 2020 and then in Oise, and Brittany (Stoecklin et al. 2020; Roux et al. 2020). In Europe, France was the most COVID-19 impacted country after Spain, Italy and Germany. By November 05, 2021, more than 6,966,140 confirmed cases of COVID-19 were recorded in the country. To lower the impact of the virus, the French government declared full lockdown on 17 March 2020 (Roux et al. 2020; Salje et al. 2020). After the beginning of the lockdown, the air quality of cities improved during the restriction period (Table 6). In France, NO, NO2, and particulate matter concentration decreased significantly in Paris, Nice, and other major cities during the lockdown period compared to before lockdown (Muhammad et al. 2020; Sicard et al. 2020; Zambrano-Monserrate et al. 2020). PM2.5 and PM10 concentrations declined at traffic stations in Nice by 8 and 7.6%, respectively due to reduction of road and non-road transport during the lockdown period (Sicard et al. 2020). These findings indicated that there was an increase in surface ozone levels by 24% in Nice during the lockdown period compared to before lockdown, because of the unprecedented reduction of NOX emission.

United States

The United States (US) reported its first confirmed case of COVID-19 on 20 January 2020 (Holshve 2020). With ensuing increase in COVID-19 cases, most of the States declared some level of social distancing by mid of March 2020 and compulsory quarantine for COVID-19 diagnosed people or those showing similar symptoms (Archer et al. 2020). By the first week of April, all States imposed lockdown (Mervosh and Swales 2020). Despite the strict measures taken by the government, the US has the highest number of COVID-19 cases in comparison to any other country in the world. As of November 5, 2021, there have been 45,968,940 confirmed cases and 744,398 deaths. During the lockdown and social distancing period, there was a drastic reduction in personal mobility and transportation (Gao et al. 2020). Nationwide, the number of personal vehicles and flight movement decreased by approximately 46 and 13%, respectively (Pishue 2020). Road transport vehicles and airports are the major sources of NO2 pollution (Archer et al. 2020). Several studies reported decline of various pollutants for this period (Table 7). The concentration of NO2 declined by 25.5%, with an absolute decrease of 4.8 ppb during the lockdown period (Berman and Ebisu 2020). The concentration of PM2.5 also declined during the same period compared to previous years (2017–2019). Another study from New York City recorded decreases in PM2.5 and NO2 levels by 36 and 51%, respectively, immediately after the shutdown. However, analysis through linear time lag model showed no significant difference comparing the same period between 2015 and 2019 (Zangari et al. 2020). Archer et al. (2020) found that more than 65% of the monitoring sites in the US recorded a significant decline in NO2 concentration compared to the previous five year means, with an average drop of 2 ppb.

Other countries (Germany, Netherlands, Kazakhstan, Mongolia, Morocco, Peru, Australia, Iran, UAE, Norway, Austria, Portugal, Czech Republic and Switzerland)

The coronavirus has spread quickly; therefore, many countries implemented preventive lockdown measures to reduce its impact. Menut et al. (2020) reported that the concentration of NO2 reduced by 15 to 30%, respectively for Germany and the Netherlands, while in Austria, Portugal and the Czech Republic, concentration declined by an average of 35–45% during the lockdown period as compared to the previous year. In Sydney (Australia) and Abu Dhabi (UAE), the level of PM2.5 decreased by 35 and 32.4% respectively, during the lockdown period compared to similar months in previous years (Shrestha et al. 2020). The same study recorded that the level of NO2 declined in Oslo (Norway) and Bern (Switzerland) by 39.7 and 19.1% respectively, during the lockdown period. Tehran (capital of Iran) recorded a lower concentration of PM10, NO2, SO2, and CO during the COVID-19 restriction period compared to the previous year (Broomandi et al. 2020). Kanniah et al. (2020) reported from Malaysia that the lockdown contributed to a 27–30% decline of tropospheric NO2, a noticeable decrease in AOD over the Southeast Asia region, and decreased pollution outflow over the oceanic areas in Malaysia. During the lockdown period the concentration of PM10, PM2.5, NO2, SO2, and CO were reduced by 26–31, 23–32, 63–64, 9–20, and 25–31% respectively, in the urban area of Malaysia compared to the same period in 2019 and 2018. In Almaty, Kazakhstan, the concentrations of PM2.5, NO2 and CO declined substantially; however, the concentration of O3 increased by 15% during the lockdown period (Kerimray et al. 2020). Otmani et al. (2020) reported that the concentrations of NO2, SO2, and PM2.5 in the city of Sale, Morocco declined during the 10 day lockdown period. Industrial and vehicle emissions declined significantly during this period and thus helped in lowering of those pollutants. Shrestha et al. (2020) found that NO2 and CO concentrations significantly declined after the lockdown in Amsterdam, Bogota, Sao Paulo, and San Francisco. The levels of PMs, NO2, and CO declined in major world cities due to the implementation of lockdown almost at the same time period (Table 8).

Future prospects and recommendations

A global health emergency/crisis was witnessed with the onset of the CP and that is still continuing. The CP has severely impacted the economy. However, it has led to positive impacts on the environment, especially air and water pollution that can be taken as an inspiration and example for behavioral changes required, which can be helpful to handle future environmental pollution-related problems. The increased concentration of environmental pollutants correlate with the increasingly high rates of COVID-19 infection, levels of transmission, and mortality rate. Fifty-five capital provinces of Italy recorded associated changes in air pollution levels with increase in symptomatic COVID-19 cases (Coccia 2020). In the US, with an increase of 1 μg m−3 in the long-term average PM2.5, 11% increase in COVID-19 mortality rate occurred (Wu et al. 2020). Debone et al. (2020) predicted that the reduction of air contaminants and the improvement of air quality prevented or avoided 78,377 COVID-19 cases and 387 premature deaths in Brazil. Air quality should therefore be considered as an integral part of the protection and prevention of public health that breaks the spread of similar diseases. The COVID-19 pandemic related lockdown highlighted and showed that the levels of contaminants have a direct relationship with the major economic activities that include industrial activities, energy generation sectors, transportation, and other small and large activities in cities. From this, it is confirmed that moving to more environmentally favorable energy options can lead to lower pollution loads. Governments, policymakers, and individuals should work collaboratively to introduce stringent policies to maintain a healthy and sustainable environment for the future so as to reduce the impacts of such pandemics.

Conclusion

Implementation of rigorous lockdown in response to the CP resulted in the ban of public and personal transport and industrial activities. During lockdown, global environmental pollution levels significantly improved. The level of major air contaminants like NO2, SO2, CO, PM (PM2.5 and PM10) declined sharply during lockdown in major countries of the world. The lockdown not only effectively restricted the mass scale spread of coronavirus but also significantly reduced major air contaminants and thus prevented severe COVID-19 cases due to decreased lung and breathing complications. In many countries, this type of reduction in air pollution was unprecedented.