Air quality can be considered “ambient” when it is measured outside, away from buildings and structures. Ambient air quality is commonly measured in urban areas or in industrial areas where there is a potential for pollution.
Ambient air quality standards are set by the U.S. Environmental Protection Agency (EPA) under the authority of the Clean Air Act (CAA). The EPA sets National Ambient Air Quality Standards (NAAQS) for six pollutants referred to as “criteria” air pollutants: carbon monoxide, lead, nitrogen oxides, ozone, particulate matter, and sulfur dioxide.
What is Ambient Air Pollution?
Ambient air pollution is defined as exposure to elevated levels of pollutants in outdoor air. The main sources of ambient air pollution are industrial emissions, vehicle exhausts, biomass-burning emissions such as those from the open burning of agricultural waste or wood, and ash from coal-fired power plants. In addition to these local sources, ambient air pollution may also result from large-scale transport of pollutants across regions or continents (transboundary pollution).
Ambient air pollution differs from household (or indoor) air pollution. Household air pollution results from burning solid fuels for cooking and heating in enclosed spaces. Household solid fuels include coal, charcoal, animal dung, and crop residues.
The main causes of ambient air pollution are emissions from motor vehicles, fossil fuels, and power plants as well as biomass burning. Out of these, motor vehicle emissions contribute the highest share globally.
Motor Vehicle Emissions: Motor vehicles are the number one cause of ambient air pollution. It is estimated that about 50% of all mortality due to ambient air pollution can be attributed to motor vehicle emissions.
Fossil Fuel and Power Plants: Fossil fuel combustion in power plants and industries (e.g. cement production) is a major source of air pollution globally.
Biomass Burning: Biomass burning (e.g. agricultural waste or forest waste burning) is a common practice in many parts of the world, causing high levels of air pollution locally and regionally, especially during the dry season.
Ambient Air Quality Standards
The ambient air quality standards are the most important measure for improvement in air quality. Some of the standards are national and others are prescribed by the state governments. The pollution levels in Indian cities are worse than what is permitted by ambient air quality standards.
Ambient air quality standards are regulations passed by the U.S. Environmental Protection Agency (EPA) that limit how much of certain pollutants can be present in the air anywhere in the United States. These standards are based on scientific evidence about the effects of air pollution on human health and welfare.
Ambient air quality standards are set at levels that minimize potential harm to human health and welfare, taking into account the latest available scientific information, our current understanding of the relationship between pollutant concentrations and effects on humans or ecosystems, and the technical feasibility of measurement.
Major types of Ambient Air Pollutants
The major types of Ambient Air Pollutants are:
Particulate matter (PM): A complex mixture of solid and liquid particles suspended in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small that they can only be detected using an electron microscope. Particles in the PM2.5 size range (fine particulates) can be inhaled deeply into the lungs.
Nitrogen dioxide (NO2): A gas that can be emitted from motor vehicles and power plants. It is also produced when fuels are burned at high temperatures.
Carbon monoxide (CO): An odorless, colorless gas that is emitted from vehicle exhaust and other sources that burn fuel. It restricts the ability of blood to carry oxygen.
Sulfur dioxide (SO2): A colorless, reactive gas that is produced when fossil fuels containing sulfur are burned at power plants and other industrial facilities. SO2 reacts with other compounds in the atmosphere to produce fine particulates and ground-level ozone.
Ground-level ozone (O3): A highly reactive gas that is not emitted directly into the air but forms when other pollutants react in sunlight and stagnant air conditions.