Human health may be at risk from long-term exposure to air pollution

Long-term exposure to air pollution appears to still be linked to higher mortality despite the existence of air quality standards that restrict levels of pollution, according to a new study published in The BMJ.

For the study, researchers investigated if there was an association between low levels of air pollution concentrations and natural and cause-specific deaths. Low level air pollution was defined as concentrations below current limit values as set by the European Union, the Environmental Protection Agency in the United States, and the World Health Organization (WHO) air quality guidelines.

The researchers analyzed data on eight groups of people within six European countries, including Sweden, Denmark, France, the Netherlands, Germany, and Austria, totaling 325,367 adults collectively. The study recruited participants in the 1990s or 2000s.

Of the 325,367 participants who were followed up over an almost 20-year period, around 14.5% (47,131 people) died during the study period. Analysis of the results showed that people who had higher exposure to particulate matter (PM2.5), nitrogen dioxide, and black carbon were more likely to die.

An increase of 5 µg/m3, a concentration measure of particulate matter, in PM2.5 was associated with a 13 percent increase in natural deaths while the corresponding figure for a 10 µg/m3 increase in nitrogen dioxide was 8.6 percent. Associations with PM2.5 and nitrogen dioxide were largely independent of each other. Moreover, associations with PM2.5, nitrogen dioxide, and black carbon remained significant at low to very low concentrations.

For people who were exposed to pollution levels below the US standard of 12 µg/m3, an increase of 5 µg/m3 in PM2.5 was associated with a 29.6 percent increase in natural deaths. People exposed to nitrogen dioxide at less than half the current EU standard of 40 µg/m3, a 10 µg/m3 increase in nitrogen dioxide was associated with a 9.9 percent increase in natural deaths.

This is an observational study, and as such, can’t establish cause, the researchers said. The study also has some limitations, they said, such as the fact that it focused on exposure in 2010 which was towards the end of the follow-up period for most participants and, given the downward trend in air pollution, this measure might not exactly reflect the concentrations experienced during follow-up. However, this was a large study from multiple European groups of people with detailed information provided.

“Our study contributes to the evidence that outdoor air pollution is associated with mortality even at levels below the current European and North American standards and WHO guideline values,” the authors said in a statement. “These findings are therefore an important contribution to the debate about revision of air quality limits, guidelines and standards, and future assessments by the Global Burden of Disease [study].”