Inside Air Pollution : Understanding the World’s Most DangerousPollutants

Air pollution kills 7 to 8 million people each year. It is a significant risk factor for a number of diseases, including stroke, heart disease, chronic obstructive pulmonary disease (COPD), coronavirus, and lung cancer. Ozone affects crops, and forests are damaged by the pollution that causes acid rain. Overall, the World Bank has estimated that welfare losses (premature deaths) and productivity losses (lost labor) caused by air pollution cost the global economyover $8 trillion per year.

Major pollutants

Air pollutants can be tiny solid or liquid particles dispersed in the air (called aerosols) or gases. Pollutants are classified as primary or secondary. Primary pollutants are produced directly by a source and remain in the same chemical form after being emitted into the atmosphere. Examples include carbon monoxide gas from car exhausts and sulfur dioxide from factories.

Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants react with each other or with other parts of the atmosphere. Ground-level ozone is one example of a secondary pollutant. Some pollutants may be both primaryand secondary — both are emitted directly and formed from other primary pollutants. 

Ammonia

Ammonia (NH3) is emitted mainly by overuse of synthetic nitrogen fertilizers on farmland, and from manure and urine from livestock. At typical concentrations in the air, it is not harmful to health directly. However, ammonia can react with other pollutants in the air to form ammonium sulfate or nitrate salts, contributing to particulate matter pollution. Furthermore, when ammonia is deposited onto the soil, it can harm ecosystems via eutrophication. 

Carbon dioxide

Carbon dioxide (CO2) is mainly emitted by the burning of fossil fuels. CO2 is sometimescalled an air pollutant, because it is the main greenhouse gas responsible for climatechange. Although the World Health Organization recognizes CO2 as a climate pollutant, it does not include the gas in its Air Quality Guidelines or set recommended targets for it. This question of terminology has practical consequences, for example, in determining whether the U.S. Clean Air Act (which is designed to improve air quality) is deemed to regulate CO2 emissions. The Inflation Reduction Act of 2022 amended the Clean Air Act to define CO2 from fossil fuel burning explicitly as an air pollutant. 

Carbon monoxide

Carbon monoxide (CO) is a colorless, odorless, and toxic gas. It is a product of the combustionof fuel such as natural gas, coal, or wood. In the past, emissions from vehicles were the main source of CO, but modern vehicles do not emit much of it. Now, wildfires and bonfires are the main source of outdoors CO. Indoors, CO is a larger problem and mainly comes fromcooking and heating. In poorly ventilated spaces, CO can accumulate to dangerous levels, and exposure may cause people to lose consciousness and die. When CO is destroyed in the atmosphere, it can raise levels of CO2 and CH4. 

Ground-level ozone

Ground-level ozone (O3) is mostly created when NO

x and volatile organic compounds mix in the presence of sunlight. It can also form from carbon monoxide or methane. Due to the influence of temperature and sunlight on this reaction, high ozone levels are most common on hot summer afternoons. It is the main gas in photochemical smog. 

3 can be harmful to human health, but also to some materials, forests, plants, and crops. Smog is a particular problem in big cities where it cannot easily be transported away by wind (e.g., cities built in valleys surrounded by mountains). When ground-level ozone is produced, it canlinger in the air for days or weeks, and therefore be transported far from where it was first formed. 

Nitrogen oxides

Nitrogen oxides (NOx), particularly nitric oxide (NO), are mostly created by the burning of fossil fuels, and in lesser amounts by lightning. Nitrogen dioxide (NO

2) is formed from NO in a reaction with other atmospheric gases. NO and NO

2 can form acid rain, can form into a haze, and can cause nutrient pollution in water. NO 2 is a reddish-brown toxic gas with a strong odor, whereas NO is odorless and colorless.