In the United States, the Clean Air Act (CAA) is the umbrella. The National Ambient Air Quality Standards (NAAQS) set limits for six criteria pollutants: CO, NO₂, O₃, particulate matter, SO₂, and lead. New Source Performance Standards (NSPS) regulate stationary combustion turbines and compression-ignition engines for NOₓ, PM, CO, and VOCs, and NESHAP limits hazardous air pollutants such as formaldehyde and benzene from the same equipment. Sites in nonattainment areas often face stricter permit conditions, and some states add their own rules for backup generator runtime.