The health effects of inhaling particulate matter has been widely studied in humans and animals and include asthma, lung cancer, and premature death. Particulate matter pollution is estimated to cause thousands of deaths per year in the United States (200 000 deaths per year in Europe). For this reason, the US Environmental Protection Agency (EPA) sets standards for PM10 and PM2.5 concentrations in urban air, and provides data so that the public can check the air quality index. EPA regulates primary particulate emissions and precursors to secondary emissions (NOx, sulfur, and ammonia). Many urban areas in the US and Europe still frequently violate the particulate standards, though urban air has gotten cleaner, on average, with respect to particulates over the last quarter of the 20th century.