After exposure, the amount of time spent exercising before the onset of chest pain decreased by 4.2%, and the amount of time spent exercising before this specific electrocardiogram signal emerged decreased by 5.1%.  * An EPA primary clean air standard for carbon monoxide is an 8-hour average of 9 parts per million (ppm), not to be exceeded more than once per year.  From 1980 through 2014, the average U. ambient carbon monoxide level decreased by 85% as measured by this standard: * All of the U. population live in counties that meet the EPA’s clean air standards for carbon monoxide.  Per the EPA, a “large proportion” of monitoring sites have CO levels that are below the limit that conventional instruments can detect (1 ppm). * The EPA’s primary and secondary clean air standard for ozone is 0.075 parts per million (ppm) as measured by a 3-year average of the fourth-highest daily maximum 8-hour concentration per year.  From 1980 through 2014, the average U. ambient ozone level decreased by 33% as measured by this standard: * Lead (Pb) is a metallic element that can be released as particles into the air.