Air pollution may worsen COVID-19 outbreak

Even just small spikes in air pollution could have huge consequences in the coronavirus disease 2019 (COVID-19) death rate, according to a new US study.

“This is the first nationwide study in the United States that estimates the relationship between long-term exposure to PM2.5 and COVID-19 death rates,” researchers said. “Our results are adjusted for a large set of socioeconomic, demographic, weather, behavioural and healthcare-related confounders and demonstrated robustness across a wide range of sensitivity analyses.”

With data from more than 3,000 counties in the United States, capturing around 98 percent of the population, the research team constructed zero-inflated negative binomial mixed models looking at the effect of county-level long-term average PM2.5 on COVID-19 deaths. At the time of the analysis, 77.8 percent (n=2,395) of the counties had not reported COVID-19-related deaths. [medRxiv 2020;doi:10.1101/2020.04.05.20054502]

Counties with missing confounder values were excluded, such that 1,783 remained for the main analysis. These counties accounted for 90 percent of the COVID-19 deaths that had been reported at the time of the analysis.

The model reported a statistically significant mortality rate ratio of 1.15 (95 percent confidence interval [CI], 1.05–1.25). That is, each 1-µg/m³ increase in PM2.5 concentration inflated the death rate from COVID-19 by 15 percent.

In absolute terms, Manhattan, New York, which had reported 1,905 COVID-19 deaths, would have seen 248 fewer mortalities if the long-term average PM2.5 exposure were to be lowered by 1 µg/m³.

“This analysis provides a timely characterization of the relationship between historical exposure to air pollution COVID-19 deaths in the United States. Research on how modifiable factors may exacerbate COVID-19 symptoms and increase mortality risk is essential to guide policies and behaviours to minimize fatality related to the outbreak,” the researchers said.

Secondary analyses―each omitting confounders such as smoking rate, the number of tested cases, the number of hospital beds, seasons and body mass index, among other factors―generally produced the same results as the main analysis.

The only exception was the model where hospital beds were omitted, which weakened the point estimate and attenuated the relationship between air pollution and COVID-19 mortality. “This suggests that number of hospital beds is a strong confounder,” the researchers said.

An important caveat is that testing is currently limited in the US, which precluded the adjustment for the true size of the outbreak. Moreover, the COVID-19 outbreak is ongoing and evolving, which may further confound the results.

“The results of this paper suggest that long-term exposure to air pollution increases vulnerability to experiencing the most severe COVID-19 outcomes,” the researchers said. “These findings align with the known relationship between PM2.5 exposure and many of the cardiovascular and respiratory comorbidities that dramatically increase the risk of death in COVID-19 patients.”

“The results of this study also underscore the importance of continuing to enforce existing air pollution regulations during the COVID-19 crisis. Based on our result, we anticipate a failure to do so can potentially increase the COVID-19 death toll and hospitalizations, further burdening our healthcare system and drawing resources away from COVID-19 patients,” they added.

This study is a preprint and has not been peer-reviewed.