Long-term exposure to even small increases in PM2.5 linked to more deaths in study

A study from Harvard T.H. Chan School of Public Health suggested that Covid-19 patients living in areas of the United States with more air pollution face an increased risk for death from the disease.

Researchers examined levels of fine particulate air pollution (PM2.5) from 3,080 counties across the country, and they compared those levels with coronavirus death counts.

Adjusting for multiple variables, the analysis suggested that even a small increase in long-term exposure to PM2.5 may be linked to large increases in the death rate from Covid-19.

Study findings were published prior to peer review in the pre-print server medRxiv, and the research has been submitted for publication in a peer-reviewed medical journal.

An increase of just 1 ug/m3 in PM2.5 was associated with a statistically significant 15% increase in the Covid-19 death rate (95% CI, 5%-25%), according to the analysis.

“The results of this paper suggest that long-term exposure to air pollution increases vulnerability to experiencing the most severe Covid-19 outcomes,” wrote Harvard researcher Francesca Dominici, PhD, and colleagues. “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.”

Air pollution exposure was found to be associated with a dramatic increase in death risk during the 2003 Severe Acute Respiratory Syndrome (SARS) outbreak.

And, the Global Burden of Disease Study identified air pollution as a major contributor to total and cardiovascular disease mortality, attributing approximately 5.5 million premature deaths worldwide to air pollution exposure.

Commenting on the Harvard research group’s findings, John Balmes, MD, said while they are consistent with what is known about the impact of air quality on lower respiratory infection risk and poor outcomes, other factors could explain the observed association.

Although the researchers attempted to control for socioeconomic status, he cited recent reports finding African American and Latinos living in densely populated urban areas to have an increased risk for death from Covid-19.

Balmes is a professor of medicine at the University of California, San Francisco, and a professor of environmental health sciences at the University of California, Berkeley.

“It is important to remember that communities with high air pollution exposure across the country are the same communities that tend to be of color and low socioeconomic status,” he said, adding that these populations also tend to face multiple challenges that increase vulnerability to Covid-19, including insecure housing, crowded housing, crime, and residing in food deserts.

Balmes said the study also had several significant limitations, including the inability to assess the impact of air pollution on Covid-19 case rates due to the very limited testing that has been conducted to-date nationwide.

The researchers instead examined Covid-19 deaths, but 2,395 of the 3,080 counties included had reported no Covid-19 deaths at the time the analysis was completed.

And, he said, relying on countywide PM2.5 measurements may not accurately convey risk in homogeneous areas like LA County, which has a mix of very affluent and very poor populations and very different PM2.5 exposures by location.

Despite the limitations, Balmes said he believes exposure to air pollution will prove to be an independent risk factor for Covid-19 severity and death, just as was in the SARS 2003 outbreak and in other lower respiratory infections.

The study was the first to look at long-term exposure to fine particulate air pollution — which is generated from car exhaust and certain industrial emissions — and the risk of death from Covid-19.

The researchers obtained temporally averaged PM2.5 values (years, 2000-2016) at the county level by averaging estimated PM2.5 values within a given county. They averaged 2016 PM2.5 exposure analogously for each county to use in sensitivity analyses.

Sixteen county-level variables and one state-level variable were potential confounders: population density, percent of the population ≥65, percent living in poverty, median household income, percent black, percent Hispanic, percent of the adult population with less than a high school education, median house value, percent of owner-occupied housing, population mean BMI (an indicator of obesity), percent ever-smokers, number of hospital beds, and average daily temperature and relative humidity for summer (June-September) and winter (December-February) for each county, and state level number of COVID-19 tests performed.

Several sensitivity analyses were also performed to assess the robustness of the initial results.

The original analysis suggested that “spatial patterns in Covid-19 death rates generally mimic patterns in both high population density and high PM2.5 exposure areas,” and the sensitivity analyses findings were largely consistent with the main analysis.

The researchers concluded that their results “underscore the importance of continuing to enforce existing air pollution regulations during the Covid-19 crisis.”

“Based on our results, 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 wrote.

  1. Be aware that this study was published online prior to peer-review, thus the findings should be regarded cautiously.

  2. Be aware that these findings had several limitations, including the inability to assess the impact of air pollution on Covid-19 case rates due to the very limited testing that has been conducted to date nationwide.

Salynn Boyles, Contributing Writer, BreakingMED™

This research was funded by grants from the NIH and the USEPA.

The researchers declared no relevant competing interests.

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