In the United States, most air pollution policy is undertaken under the authority of the Clean Air Act (CAA), first passed by the US Congress in 1963 with amendments in 1970 and 1990 that significantly expanded its purview. The CAA sets out two types of NAAQS: primary standards that provide public health protection and secondary ones that provide public welfare protection against decreased visibility and damage to animals, crops and buildings. In the power sector, the hugely influential Large Combustion Plant Directive required member states to limit emissions of particulate matter, S02 and NOx from combustion plants having thermal capacity of 50 MW or more.