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COVID-19 and environment: An ecological study of four metropolitan cities

 Department of Community Medicine, Armed Forces Medical College, Pune, Maharashtra, India

Date of Submission25-Mar-2021
Date of Decision25-Jun-2021
Date of Acceptance05-Jul-2021

Correspondence Address:
Arun Kumar Yadav,
Department of Community Medicine, Armed Forces Medical College, Pune - 411 040, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mjdrdypu.mjdrdypu_223_21


Aim: The aim of the study was to correlate environmental changes with coronavirus disease 2019 (COVID-19) cases in four metropolitan cities. Methodology: Data on monthly cumulative new cases, monthly average temperatures, humidity, precipitation, and average monthly air quality index (AQI) levels for the four Indian cities – Chennai, New Delhi, Mumbai, and Kolkata, and biomedical waste (BMW) quantity at national level were abstracted for the period from March 2020 to December 2020. Results: An inverse correlation was observed between temperature and transmission of the disease in New Delhi and Kolkata which was of fair degree for New Delhi (r = 0.299, P = 0.402) and poor for Kolkata (r = 0.200, P value = 0.579). Positive correlation of fair and poor degrees was observed between temperature and transmission of the disease in Mumbai (r = 0.272, P = 0.448) and Chennai (r = 0.196, P = 0.588), respectively. A positive correlation was observed between the transmission of the disease and humidity in New Delhi, Mumbai, and Kolkata, which was of moderate-to-good degree for Mumbai (r = 0.729, P = 0.017) and poor for New Delhi (r = 0.242, P = 0.501) and Kolkata (r = 0.123, P = 0.735). A significant positive correlation was observed between the rise in COVID-19 cases and the quantity of BMW generated (r = 0.88, P = 0.009). Furthermore, significant improvement in air quality AQI in four metros and water quality biological oxygen demand/chemical oxygen demand of Yamuna river was observed. Conclusions: Understanding of the interplay of environmental meteorological factors on COVID-19 disease transmission and vice versa is necessary for better informed policy framing and future research.

Keywords: Biomedical waste, humidity, pollution, temperature

How to cite this URL:
Khajuria A, Kaushik S K, Anand V, Yadav AK, Bobdey S. COVID-19 and environment: An ecological study of four metropolitan cities. Med J DY Patil Vidyapeeth [Epub ahead of print] [cited 2022 Dec 7]. Available from: https://www.mjdrdypv.org/preprintarticle.asp?id=337213

  Introduction Top

The coronavirus disease 2019 (COVID-19) pandemic continues, causing considerable morbidity and mortality worldwide. The severity of COVID-19 ranges from asymptomatic to fatal pneumonitis, with asymptomatic, presymptomatic, and mildly symptomatic patients accounting for approximately 80% of all cases according to current understanding.[1] The pandemic has caused 107,179,301 cases worldwide, with 2,341,521 deaths and 79,032,605 recoveries as of February 9, 2021.[2] India has recorded 10.85 million cases and 155 thousand deaths as on February 9, 2021.[3]

COVID-19 has taken an unprecedented toll on human life globally and exposed lacunae in the health infrastructure. Being a novel disease with an alarming impact, it warranted novel measures to control the sudden surge of cases such as government-imposed nationwide lockdowns, restrictions on peoples' movement, vehicles, and suspended industrial activities.[4] There have been few reports which say that such interventions can have a significant impact on the environment. The positive consequences of such lockdowns can be reduction in pollution levels in the form of greenhouse gas emissions, nitrogen dioxide, and water pollution. There can also be other indirect effects such as cleaner beaches and environmental noise reduction. On the other hand, there can also be adverse effects on the environment such as the increase in the quantity of per capita waste generated, especially of biomedical waste (BMW) generated from the hospitals (e.g. personal protective equipment [PPE], needles, syringes, bandage, mask, gloves, used tissue, and discarded medicines).[4]

While there can be beneficial and harmful effects of COVID-19 on the environment, the environmental factors also have a significant influence on COVID-19 transmission and mortality, as seen in other communicable diseases. Several studies have reported a correlation between meteorological factors such as temperature, humidity, rainfall, and COVID-19 transmissions and fatality.[5],[6] However, most studies have focused only on one side of the coin and not many studies have explored the bidirectional relation between COVID-19 and the environment in India. The objectives of the present study were to quantify and analyze the effects of meteorological factors including temperature, humidity, and rainfall, on COVID-19 transmission, and to describe the positive and negative impact of COVID-19 on the environment. Being an emerging study domain, a review of the relation between the COVID-19 and the environment can formulate a current state of knowledge that can guide the policymakers for better planning and directives during such pandemic and provide directions to future research.

  Methodology Top

This record-based descriptive study was done from the time period of March 2020 to December 2020. Four metropolitan cities of India – New Delhi, Mumbai, Chennai and Kolkata, were chosen to analyze better geographical and meteorological representation.

The four cities' monthly average temperature, humidity, and rainfall data were collected from Indian Meteorological Department database and correlated with the monthly cumulative new cases detected for the respective months.[7] The data on air quality index (AQI) levels and quantity of BMW generated were collected from central pollution control board (CPCB) official website, and the same was correlated with the rise in cases of COVID-19.[8],[9] Data on biological oxygen demand/chemical oxygen demand (BOD/COD) were obtained from CPCB official website for river Yamuna at three important monitoring stations in Delhi.[10] The monthly record of BMW generated was available only from June to December 2020 for the whole country instead of the above four cities, which has been analyzed accordingly. The cumulative monthly incidence of confirmed COVID-19 cases for the four metropolitan cities was collected from Ministry of Health and Family Welfare and respective state health department websites.[11],[12],[13],[14]

The institutional ethics committee approved the study vide IEC/2020. Spearman's rank correlation coefficient was calculated to compute the correlation. The data were compiled using Microsoft Excel software and IBM Corp. Released 2011. IBM SPSS Statistics for Windows, Version 20.0. (Armonk, NY: IBM Corp.) was used to analyze the statistics.

  Results Top

Effect of environment on coronavirus disease 2019

Coronavirus disease 2019and climate/meteorological factors

There was an increase in the number of COVID-19 cases from May onward, peaking in September–November 2020 in all the four metro cities except Chennai, which had maximum cases in June 2020. [Graph 1] shows the month-wise trend of COVID-19 cases in the four cities. [Graph 2] shows the trends of monthly cumulative confirmed new cases and the changes in monthly average temperature, humidity, and precipitation in the four metro cities for the study period.

[Table 1] shows the result of Spearman's rank correlation test analyzing the correlation between meteorological factors and the number of cumulative monthly new cases detected in the respective cities.
Table 1: Correlation between meteorological factors and cumulative monthly new cases detected in the four metropolitan cities

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Effect of coronavirus disease 2019 on environment

Increased biomedical waste quantity generated

[Table 2] summarizes the monthly cumulative new COVID-19 cases recorded and the month-wise total quantity of BMW generated in India from June 2020 to December 2020 and during the corresponding months.
Table 2: Monthly quantity of biomedical waste generated in India and the new coronavirus disease-2019 cases detected in respective months

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A significant positive correlation was observed between the rise in COVID-19 cases and the quantity of BMW generated (Spearman's rank correlation coefficient: 0.88, P value significant at 0.009).

Improvement in air quality and pollution

AQI is a standard composite index of pollution which categorizes air quality into six categories from good (with AQI ranging from 0 to 50) to severe (with AQI more than 401).[8] The complete nationwide lockdown was imposed in India in March 2020 which was applied in four phases –Phase I (25 March–14 April), Phase II (15 April–3 May), Phase III (4 May–17 May), and Phase IV (8 May–31 May). [Graph 3] demonstrates the trend of AQI levels in the four cities from January to December 2020. It shows how the air quality has significantly improved in all four metro cities during lockdown from April onward and how this trend has started to reverse after lifting of the lockdown.

Improvement in water quality

A considerable improvement in water quality of river Yamuna (with respect to BOD and COD when compared pre and post lockdown) at three monitored locations has been observed as shown in [Table 3].[10]
Table 3: Prelockdown and during lockdown comparison of biological oxygen demand and chemical oxygen demand at three key monitoring locations in Yamuna

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  Discussion Top

The environment has an inseparable relation with disease transmission, being one of the important factors in the agent-host-environment model. Importantly, this relation between environment and COVID-19 is bidirectional, with a pandemic like COVID-19 also causing a significant impact on environment as well. The effects on environment have been both beneficial and harmful as there is reduction in air, water, and noise pollution and increased waste generation and hampering of recycling activities on the other hand. Understanding the effect of environmental meteorological factors on COVID-19 transmission will definitely help in better understanding of mechanism of disease transmission. On the other hand, comprehending the direct and indirect impact of a global pandemic on environment will aid the policymakers to make informed decision in planning for mitigation of the long-term harmful effects and ways to restore the disrupted ecological balance.

In the present study, we found that temperature had an inverse relation with the rise in COVID-19 cases in two out of four metro cities and positive correlation in the remaining two cities. Furthermore, a positive relation was observed between the transmission of the disease and humidity for three out of four metro cities, varying from weak to good degree of correlations and was statistically significant for one of the cities. There was no relation observed with precipitation. Elevated temperatures through evaporation can prevent the spread of the infection due to disruption of the lipid layer of coronavirus, as seen in case of other viruses of similar nature. On the other hand, humidity can enhance the survival time of the virus in the atmosphere and increase the transmission. In our study, however, statistical significance could not be demonstrated in most of the above correlations which may be because of the fact that a month-wise analysis was done rather than a day-wise analysis due to nonavailability of day-wise uniform meteorological records. There is also a role of the level of social distancing measures in affecting the disease transmission which may confound the impact of meteorological attributes. A recent Indian study which analyzed effect of temperature in four major cities showed a negative correlation of temperature with rise in COVID-19 cases for two cities and a positive correlation for the rest two. The strength of these correlations varied from strong to weak relations.[15] Similar results were observed in one study done in four cities in China and five cities in Italy which observed that, with increasing temperature in most of the studied cities, the prevalence of COVID-19 had decreased.[16] An inverse association was identified between the COVID-19 incidence and temperature in a study which analyzed data of 190 countries globally.[17] Wang et al. in their study of 429 cities found that humidity had a positive effect on transmission of COVID-19.[18] A recent study in Malaysia demonstrated a positive association between temperature, humidity, and daily new COVID-19 infections, particularly above 29.7°C temperature and below 22.6 g/m3 humidity.[19] Results of another global analysis showed higher COVID-19 cases through April 2020 at absolute humidity of 5–10 g/m3, which may be suggestive of a “sweet point” for viral transmission, however, controlled laboratory experiments would be required to prove it.[20] There is no contesting that environmental factors do affect disease transmission, however, to what extent is yet to be answered conclusively due to large number of confounding factors which vary from region to region. Nonetheless, the present level of scientific understanding shows that the substantial effect of environmental meteorological factors on COVID-19 transmission and their role cannot be negated.

Second, our study depicted that the air quality had temporarily improved in all four metro cities in India from April 2020 onward, as evident by the AQI levels which came to “double digits” during the nationwide lockdown period. This is mostly an indirect effect due to movement restriction and a significant slowdown of social and economic activities, popularly termed now as “Anthropause.” A study done in capital city Rio de Janerio, Brazil, also found similar results.[21] Another study showed improved AQI in ten states of India after the lockdown.[22] Satellite data captured by European Space Agency also showed a significant reduction in the averaged nitrogen dioxide concentrations in major cities across India during lockdown compared to the same time-frame as last year. Mumbai and Delhi saw drops of around 40%–50% compared to the same time in 2019.[23] Based on a critical review of 57 studies, a recent global study also concluded that the COVID-19 pandemic has led to sharp reductions in carbon emissions and air pollution particularly, in countries with severe COVID-19 transmission such as China, USA, Italy, and Spain. These reductions were due to lockdown and were persistent within the lockdown period.[24] Furthermore, it is well known that high-pollution level in atmosphere causes trapping of heat and increase in surface temperature thereafter. Thus, it is possible that the temporary reduction in pollution levels during lockdown period in India, as shown in the present study, might have caused a reduction in the surface temperature. Suggestive findings were observed in a recent study carried out in the four metropolitan cities of India which showed that there was a significant fall in average temperature in Delhi, Kolkata, Mumbai, and Chennai close to 3°C, 2.5°C, 2°C, and 2°C, respectively, during the lockdown period.[25] It may be a strong pointer in the direction that implementation of stricter pollution control measures may indirectly lead to favorable temperatures in environment.

Third, The present study showed that there has been a positive indirect impact of pandemic on river water quality during the lockdown period. Improvement in water quality during lockdown can be attributed to significantly lesser industrial effluent discharged due to industrial slowdown during lockdown. Furthermore, reduced water-related human activities such as throwing of pooja materials in rivers, solid waste disposal, bathing, and washing of clothes during lockdown could have played a considerable role in improving quality. A similar study demonstrated a declining trend in nitrate concentration observed in most of the locations of Ganga river during the lockdown.[26] A recent CPCB report showed that seven major rivers of the country including Brahmani, Brahmaputra, Cauvery, Godavari, Krishna, Tapi, and Yamuna showed a significant improvement in overall water quality during the same period.[27]

Fourth, our study gave evidence that the rise in COVID-19 cases nationwide has a strong positive correlation with the quantity of BMW generated. This is due to increased use of PPE (e.g. face mask and hand gloves), their haphazard disposal, and generation of a huge amount of hospital waste, which is a major public health threat in the long run. Similar findings were elucidated in other studies which showed that activities such as sample collection of the suspected, diagnosis, and treatment of huge number of COVID-19 patients are generating exaggerated amounts of BMW from hospitals.[4],[28] Another point of worry with regard to waste management is that due to the current pandemic, recycling activities have been postponed by many countries, notably the USA, as the officials are worried about the danger that recycling hubs will catch the virus.[29]


Since it is an ecological study, the things which are true at group level may not be so at individual level due to multiple confounders. Moreover, all the four metro cities were having their own distinct epidemics due to individual population demographics, varied compliance to social distancing norms, the effect of state-wise lockdowns, and subsequent graded unlocking. Further studies with adjustments for these confounders need to be done to accurately understand the complex effects of environment on COVID-19 transmission. Second, effect on other environmental parameters such as noise and water pollution could not be analyzed in the present study due to nonavailability of uniform reliable data.

  Conclusions Top

The present study explored the bidirectional relation between environment and COVID-19. Environmental factors such as temperature and humidity have a significant role to play in COVID-19 disease transmission which also have to be taken into account while trying to understand the impact caused by the disease. Undoubtedly, COVID-19 has brought a fearful devastating scourge for human beings, but it has also emerged as a blessing for natural environment by providing it a temporary “recovery time.” It is evident that the environmental degradation caused by humans is not totally irreversible. This is a signal for us to react and the policymakers should take necessary steps so that this healing process does not become a temporary thing.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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