Air pollution exacerbates lung conditions, including asthma. Researchers have now added to the evidence that pollution exposure is correlated with onset of the disease in children—and that tighter regulations on one pollutant in particular, nitrogen dioxide, could reduce the incidence of childhood asthma. University of Southern California environmental health researchers led by Kiros Berhane and Erika Garcia drew on 20 years of data from the Southern California Children’s Health Study. In May, the team reported that falling NO2 emissions from 1993 to 2014 were correlated with a 20% decline in the incidence of childhood asthma (JAMA, J. Am. Med. Assoc. 2019, DOI: 10.1001/jama.2019.5357). Garcia says they wanted to provide policy makers with actionable information on the effects of further regulation of emissions, so they used modeling to predict what would have happened if NO2 emissions had dropped even more. The US Environmental Protection Agency limits annual average NO2 levels to 53 ppb; emissions of the gas, which plays a role in ozone formation, are regulated at vehicle tailpipes and from industrial facilities. No child in the study had been exposed to levels greater than 40 ppb, Garcia says. About 22% of the children in the study got asthma; if emissions had been reduced by 20% from those measured in the study, the asthma incidence would have dropped to about 17% (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1815678116).