Nitrogen dioxide can cause respiratory problems by itself, besides also reacting with "ammonia, moisture, and other compounds to form small particles," or particulates, that can worsen emphysema and other lung ailments, the EPA noted. The gas also contributes to the formation of ozone, which is an irritant and pollutant at ground level. NASA credits the improvement in air quality to improved fuel efficiency in cars and "technology to reduce emissions of nitrogen dioxide and sulfur dioxide from coal-fired power plants." Generally, levels of nitrogen (and sulfur) dioxide began declining soon after enactment of the 1990 Clean Air Act Amendments, and have continued to fall. Overall, levels of nitrogen dioxide have declined by more than 50 percent since 2000, according to the EPA.