Air Pollutants: Conventional Contaminants, A Comprehensive Overview

Air pollution is the emission of pollutants into the air, which are detrimental to human health and the planet. The World Health Organization (WHO) reports that air pollution causes nearly seven million deaths globally each year. The Clean Air Act, established in 1970, authorizes the U.S. Environmental Protection Agency (EPA) to regulate the emission of harmful air pollutants. The EPA identifies six criteria air pollutants that are found all over the United States and are regulated based on human health and environmental criteria. These six criteria air pollutants are particulate matter (often referred to as particle pollution), ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are known to have adverse effects on human health, the environment, and property.

Particulate matter (PM)

PM is capable of penetrating deep into the lungs and entering the bloodstream, causing cardiovascular and cerebrovascular issues, as well as respiratory problems. Both long-term and short-term exposure to PM are associated with morbidity and mortality from cardiovascular and respiratory diseases. Long-term exposure has been further linked to adverse perinatal outcomes and lung cancer. In 2013, the World Health Organization's (WHO) International Agency for Research on Cancer (IARC) classified PM as a cause of lung cancer. It is also the most widely used indicator for assessing the health effects of exposure to air pollution.

PM10 and PM2.5 often derive from different emission sources and have different chemical compositions. Sources of the largest particles, called coarse particles, will mainly consist of pollen, sea spray, and wind-blown dust from erosion, agricultural spaces, roadways, and mining operations. The finer particles, PM2.5, can be derived from primary sources, such as the combustion of fuels in power generation facilities, industries, or vehicles, and secondary sources, such as chemical reactions between gases.

Short-term exposures to PM10 have been associated primarily with the worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), leading to hospitalisation and emergency department visits. Long-term exposure to PM2.5 has been linked to premature death, particularly in people with chronic heart or lung diseases, and reduced lung function growth in children. In California, PM2.5 exposure contributes to about 2,800 hospitalisations for cardiovascular and respiratory diseases and about 6,700 emergency room visits for asthma each year.

PM has also been shown to adversely affect climate, ecosystems, and materials. For example, black carbon promotes climate warming, while nitrate and sulfate have a cooling influence. PM is also the main cause of reduced visibility (haze) in parts of the United States, including many national parks and wilderness areas.

Carbon monoxide (CO)

CO is dangerous because it reduces the amount of oxygen that can be transported in the bloodstream to critical organs like the heart and brain. At very high levels, which are more likely to occur indoors or in enclosed spaces, CO can cause dizziness, confusion, unconsciousness, and even death. Even at lower levels, exposure to CO can be serious for individuals with heart disease, triggering chest pain and reducing their ability to exercise. Repeated exposure to lower levels of CO may also contribute to other cardiovascular issues over time.

Health risks associated with CO exposure have been well-documented, leading the U.S. Environmental Protection Agency (EPA) to set and review standards for safe levels of CO in outdoor air under the Clean Air Act. The EPA works with state, tribal, and local agencies to ensure that CO levels are maintained within safe limits. These standards are crucial, as CO not only affects human health but also contributes to climate change. CO participates in chemical reactions in the atmosphere that produce ozone, a known climate change gas.

While outdoor air quality has improved since the 1990s, CO remains one of the six "criteria" air pollutants regulated by the EPA, alongside ground-level ozone, particle pollution, lead, nitrogen oxides, and sulfur oxides. These pollutants are regulated based on human health and environmental criteria, reflecting the importance of managing CO emissions to protect public health and the environment.

Ground-level ozone

Tropospheric ozone is not emitted directly into the air but is instead created through the interaction of various pollutants. These pollutants, primarily NOx and VOCs, are emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources. The presence of these pollutants, known as ozone precursors, leads to the formation of ground-level ozone through complex chemical reactions. While ground-level ozone is less concentrated than stratospheric ozone, it is a significant concern due to its detrimental health effects.

To address the health and environmental concerns associated with ground-level ozone, regulatory bodies like the US Environmental Protection Agency (EPA) have implemented measures to reduce emissions of pollutants that contribute to its formation. These efforts include setting National Ambient Air Quality Standards (NAAQS) and working with state, tribal, and local agencies to improve air quality in areas that do not meet the standards. The EPA designates areas as attainment or nonattainment based on their air quality, and states are required to develop implementation plans to improve air quality in nonattainment areas.

Additionally, the Tropospheric Ozone Lidar Network (TOLNet) in the United States employs lidar technology to measure ground-level ozone. This network of ozone-observing lidars provides valuable data for monitoring and understanding ground-level ozone concentrations across the country. By combining regulatory measures, monitoring efforts, and public awareness, the goal is to mitigate the harmful impacts of ground-level ozone on human health and the environment.

Nitrogen dioxide (NO2)

Road traffic is the principal outdoor source of nitrogen dioxide. The burning of fuels, such as diesel, in power generation facilities, industries, and vehicles, also contributes to NO2 emissions. The use of coal in power stations and the increasing popularity of diesel cars have been linked to higher levels of nitrogen dioxide.

Indoor sources of nitrogen dioxide include tobacco smoke and gas-, wood-, oil-, kerosene- and coal-burning appliances such as stoves, ovens, heaters, and fireplaces, especially if they are unflued or poorly maintained. A Canadian study found that wood-burning appliances were not associated with elevated nitrogen dioxide concentrations. However, a study in Ethiopia, where wood, crop residues, and animal dung were the main household fuels, found a mean 24-hour concentration of nitrogen dioxide of 97 μg/m3.

Short-term exposure to high concentrations of NO2 can cause inflammation of the airways and may increase susceptibility to respiratory infections. It can also exacerbate symptoms in individuals with pre-existing lung or heart conditions. Long-term exposure to NO2 can cause significant harm to sensitive habitats by increasing nitrogen concentrations in soil or water, leading to changes in plant community structure and affecting associated animal species.

Sulfur dioxide (SO2)

SO2 emissions that lead to high concentrations of SO2 in the air can also lead to the formation of other sulfur oxides (SOx). These SOx gases can react with other compounds in the atmosphere to form small particles that contribute to particulate matter (PM) pollution. These particles may penetrate deeply into the lungs and, in sufficient quantities, can cause a range of harmful respiratory effects. People who live and work near large sources of SO2 emissions, such as ports and smelters, get the highest exposure to SO2.

At high concentrations, SO2 and other sulfur oxides can harm trees and plants by damaging foliage and decreasing growth. They can also contribute to acid rain, which can further damage trees and plants, inhibit plant growth, and harm sensitive ecosystems and waterways. SO2 and other sulfur oxides can also react with other compounds in the atmosphere to form fine particles that reduce visibility (haze) in parts of the United States and other countries.

To address the issue of SO2 emissions, the EPA in the United States has implemented national and regional rules to reduce emissions of SO2 and pollutants that form sulfur oxides (SOx). These rules are designed to help state and local governments meet the Agency's national air quality standards. Similar efforts to reduce pollution and advocate for healthy air are also being undertaken by organizations like the American Lung Association.

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