Despite significant gains in controlling ground-level ozone pollution, some residents of California, Arizona, Colorado, Texas, the Midwest and the Mid-Atlantic spent more than 15 days each year between 2010 and 2014 breathing unhealthy levels of pollution, according to information from a new global database developed with NOAA support.

Ground-level ozone in the form of smog, shown here in Denver, Colorado, is a growing problem in the western United States and elsewhere around the world. (Image credit: National Renewable Energy Laboratory, Colorado State University )