Pollutants that are discharged or emitted from discrete "point" sources, such as pipes and smokestacks. Both the Clean Water Act and the Clean Air Act focus control requirements on point sources and both require permits for major sources of discharges from point sources. While much agricultural pollution is nonpoint source, some agricultural activities are affected: for example, feedlots of over 1000 animal units are considered point sources requiring permits under the Clean Water Act. However, irrigation return flows, although considered point sources, are expressly exempted from the permit requirement.