Air pollution associated with greater virulence of COVID-19

A new study published on the preprint server medRxiv* in April 2020 provides convincing evidence that air pollution contributes to greater severity of COVID-19. The researchers say, “This suggests the detrimental impact climate change will have on the trajectory of future respiratory epidemics.”

Linking air pollution and respiratory disease

The first outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), in Wuhan, China, led to a global pandemic, involving almost every country of the world. It has been difficult to contain the virus in most situations. However, some regions show a more significant impact than others, with higher mortality and infection rates.

The problem with air pollution is both momentary and chronic. Polluted air causes respiratory disease. The longer the exposure, the higher is the risk, when the individual is exposed to foul air for more than several days a year. Some studies show that diesel and coal emissions can cause inflammation and hyperactive immune responses in the lungs, not necessarily at high concentrations.

Air pollution also contributes to the impact of respiratory viruses. The presence of fine particulate matter PM 10 and PM 2.5 is especially associated with a higher rate of respiratory disease, and of hospitalization for pneumonia and chronic lung disease.

Image Credit: Shyjo / Shutterstock
Image Credit: Shyjo / Shutterstock

Microbes in the air can infect the mucosal membranes, spread through the air, or lodge on surfaces to provide another route of spread. PMs and nitrogen oxide particles can act as vectors on which viruses and bacterial aerosols can survive for a long time and hitch a free ride. One piece of evidence in this direction comes from a Northern Italian study.

Empirical Proofs

Notwithstanding the prompt and decisive containment actions adopted by the Chinese government, limiting the spread of the virus to polluted and nonpolluted regions along natural routes, it is noteworthy that Wuhan’s air is among the most heavily polluted cities in the world, and the virus claimed a relatively heavy toll of life here.

If, as in Italy, the virus spread silently for weeks before manifesting as the first outbreak, viral outbreaks may worsen in association with air pollution.

Male gender, smoking, and high population density have all been discussed as risk factors for higher morbidity and mortality of COVID-19. Though a higher density of population does predict a faster and greater spread, it should not affect the morbidity and mortality rates, since it does not determine the virulence.

Transportation facilities should also be transmission hubs, for instance, around airports and large train stations. However, air pollution also plays a significant role, being linked to a high rate of new respiratory infections and higher mortality rates.

How was the study done?

The current study looks at the evidence for a link between air pollution and airborne transmission of COVID-19 in China, Italy, Iran, Spain. France, the UK, and the US. The questions they asked included:

  • Does poor air quality increase COVID-19 incidence?
  • Is more significant air pollution linked to a higher death rate from COVID-19?

The researchers looked at eight countries severely affected by the virus and assessed the link between the measures of air quality and the number of infections. They used data from 6/8 countries, collecting the number of infections and deaths from COVID-19 per 100,000 residents.

Satellite observations, supplemented by ground data in the case of the US, China, and Italy, served as the basis for air quality data. Satellite data provide regular and constant acquisition of benefits, widespread coverage, and consistent methods of measurement at all locations. It can be affected adversely by clouds. However, they are less prone to changes derived from wind and temperature inversion, which is also an effect of air pollution.

Ground-level data relate to actual measures of single pollutants in the air rather than derived spectral-based data, as with satellite data. On the other hand, ground-level data also needs to be filled out using interpolated and other arbitrary estimations.

The researchers then analyzed the data to generate air pollution distributions in each country.

What does the study show?

The study reveals that air quality measures correlate positively with infections in China, the US, Italy, Iran, France, and the UK. However, Germany showed a weak negative correlation between the two. Italy showed the strongest correlations in terms of both infections and deaths. The variation in incidence could not be explained by population size or by population density.

In China, infections and deaths were correlated positively with air pollution as well as population density, but in the US and the UK, population density showed a stronger relationship. Spanish infection patterns and fatalities were not explained by air pollution, but the population size and density were negatively correlated. Similarly, inexplicable negative correlations with COVID-19 were present with ozone and sulfur dioxide levels measured at ground stations in the US and China.

The implications of the Study

The study shows for the first time that air pollution is often a risk factor for COVID-19, whatever the population density. In badly polluted areas, the virus fatality rates are also higher than elsewhere.

The study is necessarily interim, as virus transmission is active in most of the countries included. Many confounding factors may be identified, such as the criteria for diagnosis in different countries. The impact of these factors is mitigated by the large size of the regions included in the study.

Nonetheless, the researchers found that statistically significant positive correlations are present between air pollution and COVID-19 infections. In almost all these countries, infected people had a higher fatality rate. In Spain, there was no correlation, perhaps because PM 2.5 and nitrogen dioxide levels were uniform over most of the country.

Newer time analyses show that high NO2 is related to delayed viral outbreaks after 12 days in China, and high PM 2.5 in the US is related to a 20-day delayed rise in mortality due to COVID-19 by 20 times. In Italy, air quality is definitely correlated to the appearance of large numbers of deaths and infections. Mathematical models predict this.

An interesting observation is the number of infections is negatively related to population density, perhaps because of the large-scale movement of people from big cities to the countryside, taking the virus with them.

The Italian strain could be the result of antigenic drift in the strain crisscrossing Europe before it was positively selected in the massively polluted region of northern Italy.

The researchers say that with preliminary evidence that the virus jumped from animals to humans long before 2019, “we can speculate that air pollution could have played a role in gradually exacerbating morbidity and mortality, mutating the virus from an initial evolutionary stage not causing any more serious morbidity than a cold, to becoming so threatening to humans.”

As such, the study concludes, the findings will help epidemiologists to choose the right measures to prevent such outbreaks in the future, by restricting air pollution and climate change. Looking after the earth, preventing the loss of biodiversity, ending wars and battles, and dealing with poverty are all steps to help reduce such epidemics.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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