COVID-19 spreads through the air, PM, and aerosols (from the coughs, sneezes, conversations, or singing of infected patients), all of which have been recognized as the potential mediators of patient mortality (Paital et al., 2020). Air pollution accelerates COVID-19 transmission because PM acts as a carrier of viable virus particles and spreads the virus beyond a 2.0-m distance (Sharma et al., 2021). The increased levels of air pollutants such as PM, CO, SO2, and NO2, along with the surging number of COVID-19-related deaths, have been reported in several studies worldwide (Gupta et al., 2020). Inhalation of pollutants such as PM, CO, SO2, and NO2 can considerably harm the human respiratory system, and respiratory organs are one of the primary targets for the novel coronavirus. Environmental exposure to pollutants can make a person susceptible to COVID-induced respiratory damage (Thind et al., 2021).
Additionally, a strong relationship exists between percentage mortality per unit reported COVID-19 cases and PM2.5, which has been responsible for most air pollution-related deaths worldwide and had a profound impact compared with PM10 (Gupta et al., 2020). The level of PM pollution in India was recorded to be the highest globally (Balakrishnan et al., 2019). Moreover, epidemiological literature has consistently demonstrated the adverse health effects of extended exposure to PM toxins (Patra et al., 2021). NO2 emissions are linked to industrial activities (mainly gasoline, coal, and diesel) and road equipment and are considered anthropogenic sources in the troposphere (Suthar et al., 2020; USEPA 2011a, b). The probability of novel coronavirus infection was higher in the Indian cities where high atmospheric NO2 levels frequently contaminate the atmosphere than in the country's rural areas. As a consequence of NO2 exposure, inflammatory cells accumulate in the lungs' alveoli (Chakraborty et al., 2020). COVID-19 could spread rapidly throughout a city if NO2 levels or the ratio of NO2/PM2.5 in the atmosphere reaches high levels. Over 1.67 million deaths related to stroke, heart attack, diabetes, lung cancer, chronic lung disease, and neonatal deaths were attributed to long-term exposure to outdoor and household air pollution. Although many researchers reported a decrease in the pollution level, some highly polluted Indian cities witnessed a large number of deaths and COVID-19 cases (Neel 2020; Singh 2020b; Mukherjee et al. 2021, Suthar et al. 2020). Lockdowns were imposed to stop the spread of COVID-19, which led to dramatic improvements in air quality and reduced pollution levels on average by 30% in the COVID-19 hotspots (Jain et al., 2020). During lockdown Phase 1, relatively high reductions in PM10, PM2.5, and O3 levels by 61%, 48%, and 42%, respectively, were observed, whereas the highest reduction in the NO2 level (49%) was observed during lockdown Phase 2 (Maji et al., 2021). After the lockdown in Delhi, the level of PM10 has decreased dramatically throughout the city (Sikarwar et al., 2020).
3.2 Association of air pollution (Acute and Chronic diseases) with COVID-19
Nearly 2.3 billion people live in areas where air pollution levels are high, which mainly include the Asia-Pacific region, particularly the countries such as China and India (Pawankar, 2019). Prolonged exposure to air pollution adversely affects humans and the environment at large (Patra et al., 2021). This issue has been acknowledged as one of the main factors contributing to global mortality. During the COVID-19 pandemic, severe lockdowns were imposed to curb the spread of the disease, which caused both positive and negative repercussions. Several studies have been conducted to examine changes in the atmosphere and their impact on human health. Air pollution has been predicted to be the leading cause of premature deaths globally (Balakrishnan et al.2019; Nair et al. 2020). It can affect people’s health by causing various diseases such as respiratory infections, lung cancer, strokes, and chronic obstructive pulmonary disease (ref. ). Many reports suggest a drastic decline in the level of pollutants, particularly PM, by 50% in Delhi and Mumbai over the past few years, leading to low incidences of respiratory diseases (Mahato et al.2020, Sharma et al. 2020; Shehzad et al.2020).
Air pollution can affect both indoor and outdoor air quality, which can affect children and adults. A study in India suggested that biomass-based cooking fuel could contribute to indoor air pollution and COVID-19 (Saha J et al.; Saha & Chouhan 2020; Mandal et al., 2020). Anthropogenic activities were the leading cause of air pollution during the pandemic. Gaur et al. (2020) reported a positive link between non-communicable diseases and COVID-19-associated deaths, including heart attack (0.22–0.33) and hypertension (0.28–0.16). According to symptoms among the infected persons, high fever, cough and cold, and respiratory problems were identified to be the basic characteristics of COVID-19 (Singhal et al. 2020). A study conducted in Ghaziabad (Uttar Pradesh) reported a reduction in mortality by 8.01 per 100,000 people during Phase 1 compared with the pre-lockdown period (Ray et al. 2020).
Several studies have suggested that the high levels of some major air pollutants such as O3, VOCs, CO, NO2, PM10, PM2.5, and SO2 are linked to adverse health outcomes in different states of India; the increased risk of having COVID-19 was reported majorly in destitute people, powerless individuals, roadside vendors, and other people who are habitually subjected to vehicle emanations (Chakraborty et al. 2020; Naseem & Kulshrestha, 2021). Studies have found that these major air pollutants have acute and chronic effects on human health depending upon exposure duration. Another study in Northwest India provided evidence of a direct relationship between chronic health problems caused due to air pollution and the threat of the SARS-CoV-2 virus. The people of this area already faced high pollutants levels (particularly PM) during the winter season due to the burning of rice crop residue, which led to a rapid increase in COVID-19 cases, encouraging people to avoid an open agriculture system (McDonald et al. 2020).
The introduction to NO2 can cause irreversible obsessive changes within the lungs, such as alveolar hyperplasia and expanded fibrin within the alveoli, leading to the progression of bronchiolitis, pneumonic edema, and focal pneumonitis, all of which can increase the vulnerability of a person to COVID-19 by many folds (Chakraborty et al.2020). Supporting evidence regarding the delayed presentation of PM toxins as deleterious to human well-being is well established in the epidemiological literature (Patra et al., 2021). Studies have also found that the short-term PM2.5 and O3 exposure is related to the increase in untimely death among adults aged ≥ 25 years from nonaccidental causes such as cardiovascular disease (CVD), ischemic heart disease, respiratory disease (RD), and stroke (Bherwani et al. 2020; Maji et al. 2021). As stated by the WHO that “Propagation of the virus in the human host is likely to be carried out with the transmittance of air pollution” (WHO, 2020 a,b) it can be inferred that the widespread COVID-19 is transmitted through air, PM, and particles supporting air pollution or aerosols, which are generated through infected patients' coughs, sneezes, talks, or singing and are more likely to cause complications or death in COVID-19 patients (Paital b 2020). Nearly a half of COVID-19-related deaths in nine Asian cities were related to long-term exposure to PM2.5 (Gupta A et al. 2020). Bherwani et al. (2020) reported that the reduction in the PM2.5 level during lockdown period 1 (LDP-1) alone is mindful of a 35.5% reduction of long-term all-cause add-up to premature mortality in 2019. Considering the high mortality rates associated with PM2.5 pollution in India, it was conceivable that the number of lives spared due to the reduction in pollutant levels amidst COVID-19 lockdown might also reduce the number of deaths due to COVID-19 (Patra et al. 2021).
3.3 Health issues and the association of PM2.5 and PM10 with COVID-19
SARS-CoV-2, which originated in Wuhan, was named by the International Committee on Taxonomy of Viruses, and it exhibited resemblance with 2003-SARS. The newly originated virus has affected nearly 210 countries across the world (Gautam 2020a; Adhikari et al. 2020 Shereen et al., 2020). Many studies have found a link between the coronavirus pandemic with air pollution and its consequences in people who witnessed the disaster (Khan and Hussan, 2020; Gupta et al., 2020; Girdhar et al. 2020; Bhat et al. 2021; Bherwani et al. 2021). Because of a rapid surge in the number of positive cases across the globe, considerable attention has been paid to the effects of environmental conditions and their health consequences (Jain and Sharma 2020). The adverse impact of air pollutants, particularly fine PM, on health is well known; these pollutants cause respiratory tract diseases, eye irritation, asthma, and CVDs, which increases the susceptibility of an individual to contract coronavirus infection (Yadav et al. 2020; Balakrishnan et al. 2019; Nair et al. 2020; Singh et al. 2020; Pani et al. 2020). According to several studies, air pollution causes early death in patients; in particular, PM10 and PM2.5 were linked to the deaths of approximately 29 lakh people across the world, and the Indian northern plain witnessed a high death rate due to air pollutants and COVID-19 (Rajak and Chattopadhyay, 2019; Saxena and Srivastava, 2020; Saxena and Sonwani, 2019; Saxena et al., 2020).
Fine PM has been reported to be the major source of COVID-19 that persist in the ambient atmosphere for a long time and facilitates the spread of the virus (Shamsipour et al. 2019; Bherwani et al. 2020a; Dutheil et al. 2020; Bherwani et al.2020; Gautam 2020b; Arora et al. 2020). Gupta et al. (2020) reported that air pollution is directly proportional to the number of COVID-19 cases; deaths due to PM2.5 and PM10 were reported to be < 0.05 and 0.118, respectively. Another research at the end of 2019 and 2020 across many Asian cities indicated that the COVID-19 incidence was affected by long-term and short-term exposure to several deadly pollutants; by using harvest plots, the authors revealed that in many areas, 52% of the total assessed patients were pretentious by short-term exposure to pollutants, whereas 48% cases were affected by long-term exposure to pollutants (Katao et al. 2020). Scientists have revealed that the exponential rise in COVID-19 cases exhibits a significant positive relationship between physical contact with an infected person and air pollution (Mitra et al.2020; Jain and Sharma, 2020; Mohd Nazdir et al. 2020; Babu et al. 2020). Moreover, studies have claimed that various cities of northern India exhibited a reduction in the PM level (40–80%), which led to a significant decline in the number of COVID-19 cases and mortality rate (Navinya et al. 2020; Kumar et al., 2020; Singh et al., 2020; Naseem and kuleshrestha 2021; Dutta et al. 2020; Kotnala et al. 2020; Navinya et al. 2020; Srivastava et al. 2020; Goel et al. 2020).
A study on 40 patients with COVID-19 demonstrated the inverse pleural insertion in computed tomography scan, which clearly indicated the effect of the virus along with pollutants on the lungs of those patients (Mahapatra et al. 2020). Long-term exposure to harmful particles exerts toxic effects on the immune and respiratory systems, increasing the chances of a person contracting coronavirus infection and increasing the associated mortality rate (Khan et al. 2020; Pani et al. 2020).
A study conducted in Delhi revealed a positive relationship between reductions in the atmospheric concentrations of PM2.5 (93%) and PM10 (83%) and the number of COVID-19 cases (Sikarwar and Rani 2020). A study in Sakchi, Jamshedpur, in 2020 by Applied Advanced Air Mass Back Trajectory showed the impact of PM2.5 on health between the first and the fourth lockdown period. According to many researchers, a strong link exists between the fine PM and meteorological factor, which provided a large surface area favorable for SARS-CoV-2 for attachment and long-distance transmission, leading to the more rapid spread of the virus (Mangi and Devnath 2020; Pawar et al. 2020; Sahoo 2020; Hashim 2020; Alluri and Nazim 2020; Sethwala et al., 2020). Kulkarni et al, (2020) claimed that meteorological and climatic factors were independently associated with the transmissibility of COVI-19. Several studies supported the statistically significant correlation between daily temperature and daily count of COVI-19 cases in India (Das and Chatterjee, 2020; Gupta and Pradhan, 2020; Singh and Agarwal, 2020).
A study conducted across various states of India focused on the decline in the COVID-19-associated mortality rate due to the decrease in PM2.5 concentrations by using data from the concentration-response function (Shrangi and Pillai 2020; Purohtit et al. 2019; Patra et al. 2021). A study conducted in Uttar Pradesh and National Capital Region (NCR) region showed a marked decline in the death rate (29.85%) due to sequential reductions in the PM2.5 level (Goel et al. 2019). Furthermore, the lockdown led to a decrease in the mortality rate due to road accidents (Naqvi et al. 2020). A study categorized the association between the health risk and pollution in different states based on the risk score; Mizoram, Meghalaya, Uttaranchal, West Bengal, Uttar Pradesh, Jammu and Kashmir, Odisha, Madhya Pradesh, Jharkhand, Bihar, Maharashtra exhibited a high-risk score (1.27 to 0.12), whereas Delhi, Assam, Rajasthan, Goa, Manipur, Chandigarh, and Haryana exhibited a moderate risk score (− 0.12 to 0.27). States with a low-risk score included Daman and Diu, Sikkim, Andaman, and Nicobar Island, Kerala, Dadra and Nagar Haveli, Arunachal Pradesh, Karnataka, and Nagaland (− 1.18 to − 0.36) (Saha and Chauhan 2020). Many studies have revealed that mortality and severity of COVID-19 cases are affected by both high population density and PM2.5 levels, which play a vital role in deteriorating health conditions by providing a surface to the virus for attachment (Purohit et al. 2020; Setti et al. 2020). Mishra et al. (2020) revealed that PM reduces the functioning of the immune system; therefore, the chances of a person contracting the infection from flu viruses increase, resulting in rapid death due to lung and heart failure. Although the COVID-19 pandemic interfered with human functioning, affecting the health of individuals, its positive effect was observed in terms of clean and better air quality, which helped overcome the disease burden to some extent (Garg et al.2020; Lal et al. 2020; Muhammad et al. 2020; Patel et al. 2020).
Anthropogenic actions such as the burning of crop residues were the main reason for high air pollutant concentrations during the COVID-19 pandemic (Shyamsundar et al. 2019; Gautam and Hens 2020a; Tripathi 2020). In addition, indoor air pollution has been a problem due to improper ventilation and old fuel techniques associated with health risks (Gautam and Trivedi 2020; Nair et al. 2020; Gupta et al. 2020; Mandal et al al. al. 2020).
Another similar study was conducted in Uttar Pradesh, including the NCR. However, the study claimed that although the pollution level reduced during the lockdown period, the increased engagement of people after lockdown again led to ground-level pollution load, mainly in terms of PM concentration (Kumar et al., 2021). Many studies have shown the positive consequences of lockdown on environmental pollution levels (Mahato et al.2020a; Muhammad et al.2020a). A study was conducted in western cities of India, namely Ankleshwar and Vapi, Gujarat, during the four phases of lockdown. These cities also showed a declining trend of air pollution as the other parts of India. Results also depicted that the areas located far away from populated areas receive the cleanest air. On the other hand, PM pollution increased during the fourth lockdown phase due to ignorance of the rules (Nigam et al., 2021).
3.4 Health issues and gaseous pollutants associated with COVID-19
On January 30, 2020, the WHO proclaimed COVID-19 a global health emergency. Unlike other viruses, SARS-CoV-2 has no genetic marker (Paital 2020; Ramesh, 2020). Nevertheless, people and society were infected by SARS-CoV-2 more severely than by any other infectious disease that emerged previously, according to several researchers (Das et al. 2020a, b; She et al. 2020; Shereena et al. 2020; WHO 2020; Saha et al., 2020; Singhal, 2020; Sohrabi et al., 2020). Air pollution affects humans mainly by causing cardiovascular, respiratory, and skin problems. In addition to cancer with a hazard quotient > 10− 6, cadmium (Cd) was found to be responsible for a number of other health problems, including human respiratory ailments connected with various pollutants (Gupta et al. 2020). Contagious bronchiolitis and other disorders can result from exposure to 50–100 ppm of NO2 for 30 min (Chakraborty et el.2020; Jayachandran et al.,2020). Several studies have claimed that the reduction in the concentrations of gaseous pollutants is linked to a decrease in the incidence of aerosol-induced lung disease (Paital et al. 2020; Mishra et al. 2019; Sarmadi et al. 2020), and its direct effects were observed in the reduction in the mortality rate during COVID-19 pandemic (Girdhar et al. 2020; Mele 2020).
A study reported that chronic exposure to high NO2 levels in the environment causes lung inflammation, which could significantly contribute to the high COVID-19 fatality rates. Bashir et al. (2020) evaluated the link between COVID 19 severity, NO2 emissions, and ACE-2 expression levels; the results showed a correlation between the infection rate in humans and the severity of COVID-19 caused by increased levels of air pollutants such as NO2 and NO2-mediated ACE2 expression (WHO, 2020; Das and Paital et al. 2020). Several Indian scholars have claimed that the concentration of the gaseous pollutant NO2 in the most polluted city of Delhi was reduced by 70% during the lockdown period, which served as a boon for the health of the population residing in the city (Sikarwar and Chattopadhyay 2020; Dutheil et al. 2020; Sharma et al. 2020; Muhammad et al.2020; Rani et 2020). A similar study in Delhi, India, found a link between air pollution and COVID-19-associated mortality V. In Delhi, reduced NO2 levels in the air were found to have a strong negative relationship with new COVID-19 cases (Dutheil et al. 2020). The average SO2 level in the air was somewhat higher (24.05 ppb), which exhibited a significant negative association (–0.402) with new COVID-19 cases in Delhi. The total number of COVID-19-associated deaths (r = 0.79, p = 0.05) and case fatality rate (r = 0.74, p = 0.05) were found to have positive relationships with the atmospheric NO2 level; thus, a decrease in morbidity rates was noted (Jayachandran et al. 2020; Singh et al. 2020). The conclusion is that air pollutant levels were lower during the lockdown period than on regular days, which reduced morbidity rates (Ambade et al.2020).
Several researchers have claimed that gaseous pollutants and COVID-19 transmission are intimately linked, particularly at low temperatures and humidity (Shahin et al.2020; Babu et al. 2020; Rafiq and Suhail, 2020; Lakhsmi and Suresh, 2020; Jain et al., 2021; Babu et al. 2020; Dutta and Jinsart 2020). An association between the mean reduction in AQI and health improvement was established, and mortality rates were shown to be 72 times lower than a corresponded year (Shehzad et al., 2020). Morbidity rates and poor air quality, including indoor air, have been associated with RDs and COVID-19 in children under five age groups. SARS-CoV-2 infection and mortality rates were found to decrease during the lockdown period due to a marked decline in air pollutant levels (Padhi and Padhy, 2008; Bruce et al., 2015; Mandal et al., 2020; Afshari, 2020; Ahmed et al. 2020; Naqvi et al. 2020). A study conducted in Sakchi, Jamshedpur, India, found that lockdown had a positive impact in terms of reduction in polycyclic aromatic hydrocarbons and black carbon in the Asian atmosphere, thereby reducing adverse health effects. Co-factors such as age, comorbidities, and cross-immunity can also influence COVID-19-related mortality (Chakrabarti et al., 2020; Koshy, 2020).