The 1970 CAA initiated four major regulatory programs measuring and limiting the emission of both stationary source and mobile devices. Among the four very sets,
National Ambient Air Quality Standards, known widely as NAAQS, has played a significant role in regulating emission and setting air quality index standards. Six common pollutants – particle matter, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides and lead - were called as “criteria pollutants” due to their potential harmfulness to human health and therein introduce EPA to set permissible levels on their emission. Among the six pollutants, particulate matter (PM), along with Ozone, is the major factor that causes health problems.
    The standards set by EPA for particles are made up of a quite complex measurement. On one hand, the CAA classified two types of national ambient air quality standards, namely primary standards and secondary standards. Primary standards are designed to provide health protection to “sensitive” population such as asthmatics, children and the elderly. While secondary standards focus on public welfare protection against decreased invisibility, damage to animals, plants and buildings.
    On the other hand, there are two forms of standards – a 24-hour standard and the annual standards - for either primary or secondary standards. Thus, there are totally four standards measuring the amount of PM in the air. Since our subject is fine particle (PM2.5), standards for the inhalable corpse particle (PM10) will not be discussed in the following context. An area will meet the 24-hour standard if the 98th percentile of 24-hour PM2.5 concentrations in a year, averaged over three years, is less than or equal to the level of the current 24-hour standard. The annual standard is reached when the three-year average of the annual average PM2.5 concentration is less than or equal to the current annual standard. 
   



​