Practice question

Methane is an important greenhouse gas and a precursor in atmospheric chemistry, but it is not one of the conventional criteria air pollutants regulated through ambient concentration standards. Lead, carbon monoxide, and ground-level ozone are criteria pollutants because widespread exposure and documented health or environmental effects justify routine monitoring and legally defined limits. Sound classification links the source, atmospheric fate, exposure route, and mechanism of toxicity rather than relying only on the pollutant's name. Effective management combines cleaner fuels, emission standards, monitoring, urban planning, and protection of populations with high exposure or physiological vulnerability. The distinction is ecologically important because emission control must target either the directly released precursor or the product formed after atmospheric transport and reaction. Exposure depends on concentration, duration, ventilation, weather, and individual susceptibility, so a pollutant's presence alone does not fully predict biological harm. Atmospheric inversions can trap contaminants near the surface, while wind, turbulence, precipitation, and chemical transformation determine their eventual distribution and removal.