COVID-19 mortality and contemporaneous air pollution

This paper examines the relationship between contemporaneous exposure to fine particulate matter and COVID-19 morbidity and mortality. Harnessing daily changes in county-level wind direction (within the United States), they show that fluctuations in local air quality can almost immediately impact the rate of confirmed cases and deaths from COVID-19.

The authors find that a small increase in fine particulate matter – a one microgram (µg) per cubic metre increase in PM_2.5 – increases the number of confirmed cases by approximately 2 per cent from the mean case rate in a county, and the same-day death rate by 3 per cent from the mean.

The results suggest that contemporaneous exposure to air pollution plays an important role in mediating the severity of respiratory syndromes such as COVID-19, for which progressive respiratory failure is the primary cause of death.  

The findings have important policy implications: it follows that keeping current pollution at low levels may have an immediate payoff in that it may allow for fewer additional cases and deaths when reopening the economy. Therefore, policy levers to improve air quality may lead to improvements in COVID-19 outcomes.

Key points for decision-makers

• Fine particulate matter, or PM_2.5, is a combination of particles with a diameter of 2.5 micrometres or less, such as nitrates, sulphates, ammonium and carbon. Wind speed and direction are among the factors influencing transported PM_2.5. The authors’ approach uses plausibly random daily changes in wind direction to predict levels of fine particulate matter pollution.
• The paper is distinct from previous studies analysing exposure to pollution before the pandemic, upon which policymakers cannot act to improve COVID-19 outcomes. Changes in wind direction are also key to isolating the causal effect of pollution from other socioeconomic characteristics that correlate with pollution.
• The study controls for policies and practices aimed at limiting spread of the virus (such as limits on mass gatherings, social distancing controls, quarantine orders and school closures).
• The authors find a 2 per cent increase in confirmed cases resulting from a 1 µg/m³ increase in PM_2.5 – where confirmed cases are likely to be a measure for severe cases, given that many infected people do not show symptoms. In absolute terms, this represents 0.7 additional confirmed cases in a county on any given day.
• The effects tend to increase in magnitude over longer time horizons, being twice as large over a three-day period.
• Meanwhile, a 1 µg/m³ increase in PM_2.5 increases the same-day death rate by 3 per cent from the mean, or by between 0.05 and 0.06 additional deaths per 100,000 individuals in a county. These estimates are robust to a host of sensitivity tests.
• These results are in line with the medical literature, which points to progressive respiratory failure as the primary cause of death from COVID-19, as well as an older literature showing the immediacy of the relationship between exposure to pollution and potential death.
• Since the trade-off between reopening the economy and COVID-19 is already known, the study brings some good news in that it shows that it is possible to partly delink the relationship between reopening and COVID-19 cases and deaths by keeping pollution low. In this case, the quest for two conflicting desirable goals, jobs and health, can be helped by controlling a ‘bad’ factor – i.e. pollution.