Air pollution is a major risk to both pregnant mothers and their unborn babies. But what pollutants are the most dangerous?
Since the air is such a complicated mixture, it can be hard to know which combinations of chemicals pose the greatest threat. That is the challenge Brenna Kelly, a researcher at the University of Utah, set out to solve.
In a recently published study, Kelly investigated how complex mixtures of pollutants affect preterm birth. To do so, she analyzed enormous data sets that mapped tens of thousands of births across Utah over a span of three years.
By combining the map of births with records of local air pollution, Kelly could visualize which combinations of pollutants were most associated with preterm birth.
“We identified 12 mixtures of pretty common pollutants: fine particulate matter, ozone, and nitrogen dioxide," Kelly said. "The only one that had a significant effect on preterm birth was a mixture of PM2.5 and ozone.”
PM2.5 is a set of airborne particulates that are smaller than 2.5 microns. For comparison, a strand of human hair is about 50 microns wide. PM2.5 are especially dangerous for us to breathe in because they can pass through our lungs and enter the bloodstream.
Kelly was surprised to see that levels of PM2.5 and ozone not considered individually dangerous by the Environmental Protection Agency in the Air Quality Index seemed to work synergistically to increase the risk of preterm birth.
“The EPA would say this is a good air quality day," she said. "But actually, the fact that it's elevated across multiple pollutants might mean it's unsafe. But the AQI, the tools we have for public health, don't take into account that combination.”
Kelly also found that the timing and length of exposure to this mixture of pollutants could further affect the risk of preterm birth.
“This is a common mixture," she explained, "and it's not uncommon to be exposed more than once to high ozone and high PM2.5. If you experience the same exposure over time, that can compound the effect of an individual time point.”
Kelly found that certain critical periods of gestation seemed to demonstrate the most vulnerability to disruption by elevated PM2.5 and ozone.
"We found that exposure to this mixture in weeks 9 through 14, which is the late first trimester and early second trimester, was important," she said. "If you were exposed multiple times, that's where it had the strongest effect."
This analysis showed that the preterm birth risk increased by a factor of 2.8 with repeated exposure to elevated ozone and PM2.5 levels during the late first trimester and early second trimester of pregnancy.
And for Kelly, this demonstrates the need to continually refine public health messaging with special regard to pollution co-exposure.
