Improvement of U.S. air quality of the past decades has slowed significantly, making it difficult for regions of the country to meet national ozone standards in the future, new research says. The study looked at the two pollutants—nitrogen oxides (NOx) and carbon monoxide—that combine with sunlight to create ground-level ozone (Proc. Natl. Acad. Sci. USA 2018, DOI: 10.1073/pnas.1801191115). For NOx, the study found emissions had been dropping at 7% per year between 2005 and 2009, but that rate slowed to 1.7% between 2011 and 2015. CO showed similar rate changes. The researchers, primarily associated with the National Center for Atmospheric Research, attribute the declining emissions reductions to increasing NOx emissions from industrial and residential sources, less-effective catalytic converters for heavy-duty diesel trucks, and a topping out of vehicle emissions reductions. The day the study published, EPA issued its national ozone attainment report that identifies regions out of compliance with air quality standards: 51 areas in 22 states and the District of Columbia still had ozone levels higher than EPA’s limit.