Air Pollution
According to the World Health Organization, several million people are estimated to experience premature deaths each year due to air pollution. In 2021, the US alone emitted 67,000,000 tons of pollutants into the air.
Air pollutants can be split into two distinct categories: primary pollutants and secondary pollutants. Primary pollutants are those that are formed and emitted directly from the pollution source. Secondary pollutants are formed in the atmosphere via chemical reactions, rather than being emitted right from a particular source.
Air Pollutants and their Sources
Particulate Matter (PM) consists of very small particles suspended in the air. It has natural sources, such as volcanic eruptions, dust, wildfires, pollen, and sea salt. In addition, there are also anthropogenic sources such as fossil fuel and biomass combustion (with coal being the worst). Anything that pushes sand, soil, or dust in the air, such as construction or agriculture, will also increase PM. It can contribute to haze and industrial smog. When inhaled, PM can lead to a decrease in lung function and respiratory irritation, worsening preexisting respiratory issues. It can be categorized by its size, with PM2.5 (being 2.5 micrometers and smaller) reaching deeper into the lungs and causing more issues than PM10 (being between 2.5 and 10 micrometers).
Sulfur Dioxide (SO2) is primarily released from the combustion of coal, although any fuels that contain sulfur (including oil and gasoline) can emit it. If inhaled, it can cause respiratory irritation, with high enough concentrations being potentially fatal. In addition, it is a precursor to sulfuric acid, causing acid rain, and is a component of industrial smog.
Nitrogen Oxides (NOx) include both Nitric Oxide (NO) and Nitrogen Dioxide (NO2), which readily undergo chemical reactions and turn into each other in the atmosphere. They are released primarily by fossil fuel combustion but can be released by the combustion of anything. It is a precursor to secondary pollutants causing acid rain and smog. It is also a respiratory irritant when inhaled.
Carbon Monoxide (CO) is released through the incomplete combustion of anything, with humans mainly being exposed to it due to poor ventilation. It is an asphyxiant, binding to the hemoglobin of our red blood cells and preventing the transport of oxygen through the blood.
Lead (Pb) is released into the atmosphere nowadays primarily from waste incineration, coal combustion, and metal plants. It used to be added to paints and gasoline (which is why gas stations say "unleaded" on them, since the Clean Air Act banned leaded gasoline), which released large amounts of it into the atmosphere. Aviation gasoline still currently has lead in it, although the Federal Aviation Administration recently announced a plan to phase out lead-based fuel by 2030. Lead is a neurotoxin, resulting in impairment of the central nervous system and decreasing abilities to concentrate and learn. It is particularly dangerous for children, as their brains are still undergoing significant development.
Tropospheric Ozone (O3) is a secondary pollutant formed from the chemical reactions of nitrogen oxides, atmospheric oxygen, and sunlight. While ozone is helpful when found in the stratosphere, in the troposphere it reduces lung function, damages the stomata of plants, and is a component of photochemical smog.
Volatile Organic Compounds (VOCs) have a high vapor pressure and low boiling point, meaning they easily aerosolize. They have strong scents due to this property. There are natural VOCs, such as those released by plants to attract predators or signal tissue damage, as well as a variety that are anthropogenic in origin (such as gasoline, solvents, and paints). Man-made VOCs are associated with respiratory and allergic effects in humans. In addition, some VOCs react with nitrogen oxides in the creation of photochemical smog.
Mercury (Hg) and other toxic metals are released primarily from the combustion of coal and mining. They can bioaccumulate and biomagnify in food chains, result in the death of a variety of organisms, and, depending on the metal, damage the central nervous system.
The Clean Air Act
One of the most influential environmental laws in the United States, the Clean Air Act sought to improve air quality within the nation. It required the EPA to create "National Ambient Air Quality Standards" that regulate the amount of six criteria air pollutants (tropospheric ozone, carbon monoxide, particulate matter, lead, sulfur dioxide, and nitrogen dioxide).
The Clean Air Act has undergone several amendments, adding to the EPA's powers and what it was meant to regulate. One of the most notable was the cap-and-trade provisions of the 1990 amendments. In order to reduce acid rain, power plants were required to reduce sulfur emissions. The factories could decide how they wanted to cut their emissions, and if they lowered them more than required, they could sell their extra "credits" to factories that were over the allowed emissions.
This legislation has made a significant difference. According to the EPA, between 1970 and 2022, criteria air pollutant emissions decreased by 78%.
What about CO2?
Carbon Dioxide is a major greenhouse gas, with its emissions being the leading cause of anthropogenic climate change. It is normal for there to be some amount of CO2 in the atmosphere, as it is a part of the carbon cycle. It is naturally released into the atmosphere from things such as cellular respiration, aerobic decomposition, wildfires, and volcanic eruptions.
That said, the reason it is a major concern is that human activities have been throwing the carbon cycle off balance and leading to continually increasing concentrations of it in the atmosphere. It is primarily released through the combustion of fossil fuels, with other human impacts such as deforestation also increasing its concentration in the atmosphere.
You might be wondering why it isn't included in that list of criteria air pollutants regulated by the Clean Air Act. To put it simply, the United States has a bit of a muddled history with the regulation of CO2.Â
You could look at the Kyoto Protocol, an international agreement to reduce greenhouse gas emissions (including CO2) in order to fight climate change, which the US did not ratify.
The EPA has had legal battles over whether or not it had the ability to regulate GHGs, with a Supreme Court decision (Massachusetts v. EPA) granting the agency the ability to regulate them, and a later decision (West Virginia v. EPA) taking away said power. Most recently, the Inflation Reduction Act of 2022 listed CO2 as a pollutant and gave the EPA the authority to regulate its emissions.