Can the Air in Our Cities Contain the SARS-CoV-2 Virus?

This article has been written by Jose Luis Jimenez, Professor at Colorado University-Boulder; Jordi Sunyer, Head of the Childhood and Environment programme at ISGlobal; and Xavier Querol, Research Professor at IDAEA-CSIC.

There is some debate in the scientific community about the relationship between air pollution and the COVID-19 pandemic. Some scientists have advanced the hypothesis that airborne particulate matter can act as a vehicle for the transmission of viruses—in particular SARS-CoV-2.

In fact, a recent study published by Harvard University suggests that people living in areas with high levels of air pollution are more likely to die from COVID-19 than those living in areas with cleaner air. The findings of that study indicate that even a slight increase in levels of fine particulate matter (PM_2.5) is associated with a 15% increase in the death rate. The data analysed was collected from 3,080 counties in the United States and it is impressive that the authors were able to obtain all this data and construct sophisticated mathematical models in just a few days. The researchers tried to adjust for environmental confounding factors in their models, but the final results of this study should, nevertheless, be interpreted with caution owing to limitations related to the dynamics of the epidemic (the correlation between temporal increase and geographic spread). Nevertheless, the hypothesis is of great interest and warrants further study.

A recent study published by Harvard University suggests that people living in areas with high levels of air pollution are more likely to die from COVID-19 than those living in areas with cleaner air

The implications of these findings, if they are correct, are worrying: 10% of deaths per 1 μg/m³ of PM_2.5 could have been avoided if air pollution levels over the past 20 years had been lower.

The findings of an Italian study indicate that the impact of the epidemic may have been greater in the most polluted Italian cities, which would explain why the north of the country—the region with the highest pollution levels—was the area most affected. However, another explanation for the north-south difference could be that the areas most affected are those that receive more international travellers and are characterised by greater local mobility—two factors that would increase the likelihood of contagion.

We know that airborne particles can carry viruses, bacteria and fungi, and another study in Italy has found coronavirus RNA on particulate matter, although the concentrations observed were low. To date, the contribution of airborne transmission to viral spread was thought to be low, while contact with contaminated objects (or through droplets from coughs or sneezes from people nearby) was considered the most likely route of transmission . However, in light of evidence from the most recent studies published, this is no longer so clear and the scientific community is now trying to ascertain the individual contribution of each one of the potential modes of transmission as quickly as possible.

As explained in this recent study, airborne transmission, caused by breathing and talking while close to other people, is a likely occurrence. However, the differences in the impact of airborne transmission indoors and outdoors are quite different. The authors of a Japanese study concluded that people are almost 20 times more likely to be infected indoors than outdoors. We must take into account that an infected person coughing, sneezing, talking or simply breathing emits microdroplets. If they are in a poorly ventilated indoor space, such as a normal house with the windows closed, some of these microdroplets (those measuring less than five micrometres in diameter) will float in the air for several hours and larger droplets will settle on surfaces. This means that a person can be infected by the viral load transported in the microdroplets when they inhale the air containing viral particles or touch surfaces where the droplets have settled. Adequate ventilation helps to eliminate this viral load and is particularly important in homes where one person is sick and the others are not.

The differences in the impact of airborne transmission indoors and outdoors are quite different. The authors of a Japanese study concluded that people are almost 20 times more likely to be infected indoors than outdoors

In a recently reported case, 45 of the 60 people who attended a choir rehearsal in the United States were infected despite taking all the recommended precautions to avoid infection via the routes of transmission currently accepted as being the most plausible. Health officials in Washington State believe the explanation is airborne transmission of the virus. In the 1970s, it was discovered that the seasonal flu virus was transmitted in the same way among the passengers on an airplane that was parked on a runway for four hours without ventilation, allowing the virus to accumulate in the air.

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The situation is different in outdoor settings. It is hard to believe that the small amounts of airborne virus in the outdoor air could play a significant role in the spread of the pandemic, unless we stand very close to infected people.

It is hard to believe that the small amounts of airborne virus in the outdoor air could play a significant role in the spread of the pandemic, unless we stand very close to infected people

Other scientists have advanced the hypothesis that the relationship between air pollution and the COVID-19 infection rate could be due to the fact that exposure to air pollution prior to the pandemic may have damaged the defences of the patient’s respiratory system, thereby facilitating the infection process. One hypothesis that should be studied, therefore, is that people who have been exposed to higher levels of air pollution are more vulnerable to lung damage—as has been suggested in the case of people with allergies—because of the inflammation caused by the pollutants in the air.

In China, SARS-CoV-2 mortality was much higher among men than women. Although the reasons for this difference are not yet clear, researchers suspect that it may be due to the higher percentage of smokers in the male population compared to the very low percentage among women. Pre-existing lung damage among smokers would contribute to more severe disease in that population. It is possible that air pollution could act in the same way.

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