Image: Shutterstock.comFine particulate matter (PM 2.5 ) – pollution particles measuring 2.5 micrometers or less – is released by human activity but also by natural sources like wildfires. “At the same time, the contribution of wildfire smoke to prenatal PM 2.5 exposure has more than doubled. By 2019, wildfire smoke had become the dominant contributor to high-pollution days experienced during pregnancy in the US.” The first run included human and natural sources of PM 2.5 pollution; including cars, industry output, and fires. However, when PM 2.5 pollution caused by wildfires was excluded, the decline was steeper – around 37% – suggesting that wildfires offset some gains.