What are the types of air emissions from electrical power plants?

All power plants that burn fossil fuel create gases and fine particulate matter as a byproduct. The main categories of pollutants are:

• Nitrous oxide (N2O - dinitrogen oxide, or more commonly referred to as NOx) is a colorless, sweet smelling gas. When inhaled it has sedative and analgesic (pain relieving) effects, as well as causing amusing effects, which gives it the name "laughing gas". On a global scale NOx contributes to global warming and is the third most important greenhouse gas after carbon dioxide and methane. It has a high "global warming potential" (310 times that of carbon dioxide). Although not directly harmful to humans, NOx damages the ozone layer, thus reducing the protection offered from harmful UV sun rays. When released into the atmosphere, NOx reacts with water, oxygen, and other substances to form a mild solution of nitric acid. This acidic solution falls back to earth in a phenomenon called acid rain. When the acid rain reaches Earth, it flows across the surface in runoff water, enters water systems, and sinks into the soil. Acid rain is harmful to many types of aquatic creatures and plants. Acid rain was first identified as a problem in North America in the early 1970s. To combat the problem, the US Congress imposed strict emission regulations on industry in 1970 through the Clean Air Act, which was strengthened in 1990.
  
• Sulfur dioxide (SO2, and together with other sulfur oxides commonly referred to as SOx) is an invisible gas with a nasty, sharp smell. It reacts easily with other substances to form harmful compounds, such as sulfuric acid, sulfurous acid and sulfate particles. In the atmosphere, SOx react with other compounds to form fine particles that reduce visibility, often referred to as yellow haze. Short-term exposures to SO2 can harm the human respiratory system and make breathing difficult, with children, the elderly, and those who suffer from asthma being particularly sensitive. SOx also reachts with water, which creates a mild solution of sulfuric acid that is another component of acid rain.
  
• Heavy metal refers to any metallic chemical element that has a high density and is toxic or poisonous at low concentrations. Burning of coal is the primary source of heavy metal emissions, including lead, mercury, nickel, tin, cadmium, antimony, arsenic, and radioisotopes of thorium and strontium. Heavy metal toxicity can result in damage to the central nervous system, blood cells, lungs, kidneys, liver, and other vital organs. Long-term exposure can result in allergies; slowly progressing physical, muscular, and neurological degenerative processes that mimic Alzheimer's disease, Parkinson's disease, muscular dystrophy, and multiple sclerosis; and cancer. Coal plants are responsible for approximately half of the US human-caused emissions of mercury, one of the more dangerous of the heavy metals.
  
• particulate matter (PM, also referred to as particle pollution or simply smoke and soot) is mixture of solids and liquid droplets floating in the air. Some particles are released directly from the burning of fossil fuels, while others form through chemical reactions in the atmosphere. Particles come in a wide range of sizes. Coarse particles, considered greater than 2.5 up to 10 microns in diameter, are designated PM10. Ten microns is less than the width of a single human hair (see Figure 1), and these particles are so small that they can get into the lungs, potentially causing serious health problems. Fine particles, PM2.5, are 2.5 microns in diameter or smaller and can only be seen with an electron microscope. Numerous scientific studies connect particle pollution exposure to a variety of health issues, including: irritation of the eyes, nose and throat; coughing, chest tightness and shortness of breath; reduced lung function; irregular heartbeat; asthma attacks; heart attacks; and premature death in people with heart or lung disease.
  
• carbon dioxide (CO2) is generated by the burning of fossil fuel, which by nature is carbon based, and is the primary greenhouse gas emitted through human activities. In 2015 burning fossil fuels to generate electricity was the largest single source of CO2 emissions in the US, accounting for about 35 percent of total US CO2 emissions and 29 percent of total US greenhouse gas emissions. Although carbon dioxide is not a direct health hazard to humans, the increase in greenhouse gasses in the atmosphere is the primary cause for the global warming being experienced by the planet and the resulting changes to the climate. To produce a given amount of electricity, burning coal will produce approximately twice as much CO2 as burning natural gas.

See Figure 2 for a summary of air emissions from coal-fired power plants in 2007.

Burning natural gas generates the least amount of these pollutants, and burning low quality coal generates the most. Early power plants emitted these directly into the atmosphere, but over the years the EPA has continually tightened allowable pollution levels. With the exception of carbon dioxide, all these emissions are controllable using modern emissions control technology that must be built into any new coal-fired power plants and that can be retrofitted to older plants to meet EPA limits. For more information on modern pollution control techniques, see the OWOE topic "How do electric power plants manage air emissions?".

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