Greater exposure to fine particulate matter (PM) 2.5, nitrogen dioxide, and black carbon has now been linked with a higher risk of developing severe disease, admission to a hospital or an intensive care unit (ICU) and death by COVID-19.

A study, published in the journal Nature Communications, led by the Barcelona Institute for Global Health (ISGlobal) and based on a large cohort of 4,660,502 adults, found a direct association between long-term exposure to air pollution and severe COVID-19.

An increase in exposure to PM2.5 of 3.2 micrograms per cubic metre was associated with a 19 per cent increase in hospital admissions and an increase in exposure to NO2 of 16.1 micrograms per cubic metre was associated with a 42 per cent increase in ICU admissions.

Also, an increase in exposure to BC of 0.7 micrograms per cubic metre was associated with a 6 per cent increase in mortality.

"Our findings add further compelling evidence on the importance of reducing levels of air pollution to improve the health of the population in general and, in particular, to reduce the incidence of severe acute respiratory infections," said Otavio Ranzani, ISGlobal researcher and first author of the study.

Several biological mechanisms may explain why long-term exposure to air pollution increases the risk of severe COVID-19.

One hypothesis is that exposure to air pollution increases the individual's risk of developing chronic comorbidities associated with severe COVID-19, such as hypertension.

Another hypothesis is that air pollution may facilitate the infection, as there is published evidence that exposure to fine particulate matter increases the expression of the receptors the SARS-CoV-2 virus binds to, particularly in the lung.

Exposure to air pollution has also been associated in another study with changes in the immune defences key to mitigating SARS-CoV-2, such as a decrease in the "type II interferon" response to SARS-CoV-2 and the antibody response.

While many studies have been published on the effects of short- and long-term exposure to air pollution on chronic respiratory diseases, there is less data available on the effects of long-term exposure on the incidence and severity of acute respiratory infections.

Nonetheless, the authors said more research is needed to gain a better understanding of the main biological pathways involved.