Air quality laws govern the emission of air pollutants into the atmosphere. A specialized subset of air quality laws regulate the quality of air inside buildings. Air quality laws are often designed specifically to protect human health by limiting or eliminating airborne pollutant concentrations. Other initiatives are designed to address broader ecological problems, such as limitations on chemicals that affect the ozone layer, and emissions trading programs to address acid rain or climate change. Regulatory efforts include identifying and categorising air pollutants, setting limits on acceptable emissions levels, and dictating necessary or appropriate mitigation technologies. Air pollutant classification Air quality regulation must identify the substances and energies which qualify as "pollution" for purposes of further control. While specific labels vary from jurisdiction to jurisdiction, there is broad consensus among many governments regarding what constitutes air pollution. For example, the United States Clean Air Act identifies ozone, particulate matter, carbon monoxide, nitrogen oxides (NOx), sulfur dioxide (SO2), and lead (Pb) as "criteria" pollutants requiring nationwide regulation. EPA has also identified over 180 compounds it has classified as "hazardous" pollutants requiring strict control. Other compounds have been identified as air pollutants due to their adverse impact on the environment (e.g., CFCs as agents of ozone depletion), and on human health (e.g., asbestos in indoor air). A broader conception of air pollution may also incorporate noise, light, and radiation. The United States has recently seen controversy over whether carbon dioxide (CO2) and other greenhouse gases should be classified as air pollutants. Air quality standards Air quality standards are legal standards or requirements governing concentrations of air pollutants in breathed air, both outdoors and indoors. Such standards generally are expressed as levels of specific air pollutants that are deemed acceptable in ambient air, and are most often designed to reduce or eliminate the human health effects of air pollution, although secondary effects such as crop and building damage may also be considered. Determining appropriate air quality standards generally requires up-to-date scientific data on the health effects of the pollutant under review, with specific information on exposure times and sensitive populations. It also generally requires periodic or continuous monitoring of air quality. As an example, the United States Environmental Protection Agency has developed the National Ambient Air Quality Standards (NAAQS) NAAQS set attainment thresholds for sulfur dioxide, particulate matter (PM10 and PM2.5), carbon monoxide, ozone, nitrogen oxides NOx, and lead (Pb) in outdoor air throughout