Air Pollutants: Common Toxins In Our Air

Air pollution is responsible for millions of premature deaths worldwide each year. Common air pollutants, often referred to as criteria pollutants, include particulate matter, ground-level ozone, carbon monoxide, lead, nitrogen oxides, and sulfur oxides. These pollutants can be emitted from a variety of sources, including household activities, industrial processes, and vehicle emissions, and have been shown to have significant impacts on human health, the environment, and property. Particle pollution, for example, can irritate the eyes, nose, and throat, while smaller particles can enter the deep parts of the lungs or even the bloodstream. Other pollutants, such as benzene, perchloroethylene, and asbestos, are classified as hazardous air pollutants (HAPs) due to their potential to cause cancer and other serious health issues.

Particulate matter (particle pollution)

Particulate matter, or particle pollution, is a mixture of solid particles and liquid droplets found in the air. These particles can be large, such as dust, dirt, soot, or smoke, and visible to the naked eye, or they can be so small that they can only be detected using an electron microscope.

The particles that make up particulate matter come in various sizes and shapes and can be composed of hundreds of different chemicals. Some of these particles are emitted directly from sources such as construction sites, unpaved roads, fields, smokestacks, or fires. However, most particles form in the atmosphere as a result of complex reactions between chemicals like sulfur dioxide and nitrogen oxides, which are themselves pollutants emitted from power plants, industries, and motor vehicles.

Particle pollution is categorized into two types: inhalable coarse particles and fine particles. Inhalable coarse particles have diameters ranging from 2.5 to 10 micrometers, while fine particles, also known as PM2.5, are 2.5 micrometers or smaller. These fine particles pose the greatest risk to human health as they can penetrate deep into the lungs and even enter the bloodstream. They are also the primary cause of reduced visibility (haze) in many regions of the United States, including national parks and wilderness areas.

The U.S. Environmental Protection Agency (EPA) has established standards and regulations to address particulate matter pollution. The Air Quality Index (AQI) is used to inform the public about the air quality in their region and provide guidance on necessary actions to protect their health. The EPA also sets National Ambient Air Quality Standards (NAAQS) for six common pollutants, including particulate matter, and develops rules to reduce emissions of pollutants that contribute to particle pollution, helping state and local governments meet the Agency's air quality standards.

Ground-level ozone

Tropospheric, or ground-level ozone, is not emitted directly into the air. Instead, it is a secondary pollutant formed by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight and stagnant air. These primary pollutants, NOx and VOCs, originate from both natural sources and human activities. Human-induced NOx emissions primarily come from burning coal, gasoline, and oil in motor vehicles, homes, industries, and power plants. VOCs, on the other hand, are largely associated with gasoline combustion, upstream oil and gas production, residential wood combustion, and the evaporation of liquid fuels and solvents.

Ozone at ground level can have detrimental effects on human health, particularly for vulnerable groups such as children, the elderly, and individuals with lung diseases like asthma. Exposure to ground-level ozone has been linked to an increased risk of hospital admissions, asthma attacks, and even pre-mature mortality. Additionally, ground-level ozone can impact vegetation, reducing the productivity of certain crops and causing damage to flowers and shrubs.

To address the issue of ground-level ozone pollution, the United States Environmental Protection Agency (EPA) has implemented regulations to help states reduce ozone levels in outdoor air. These regulations include setting National Ambient Air Quality Standards (NAAQS) and designating areas as attainment or nonattainment based on their air quality. States with nonattainment areas are required to develop state implementation plans (SIPs) to improve air quality and meet the EPA's standards.

Carbon monoxide

Indoors, carbon monoxide can be released by a variety of household items, including unvented kerosene and gas space heaters, leaking chimneys and furnaces, and gas stoves. In homes in developing countries, the burning of biomass fuels is also a significant source of carbon monoxide. When carbon monoxide is released indoors, it can affect air quality and be harmful to human health. This is because breathing air with a high concentration of CO reduces the amount of oxygen transported in the bloodstream to critical organs like the heart and brain. At very high levels, which are possible indoors, CO can cause dizziness, confusion, unconsciousness, and even death.

The US Environmental Protection Agency (EPA) has set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, including carbon monoxide. These standards are based on human health and/or environmental criteria. The EPA also sets and reviews standards for CO in outdoor air under the Clean Air Act, helping state, tribal, and local agencies ensure that CO is kept at a safe level.

The World Health Organization (WHO) has also published guidelines for indoor air quality, which include information on carbon monoxide. Various studies have investigated the health effects of carbon monoxide, including its impact on mortality, birth weight, asthma, congestive heart failure, and psychiatric admissions.

Nitrogen dioxide oxides

Nitrogen dioxide (NO2) is a highly reactive gas and a major air pollutant. It is one of a group of gases known as oxides of nitrogen (NOx). Other nitrogen oxides include nitric oxide (NO), nitrous oxide (N2O), nitrous acid (HONO), and nitric acid. NO2 is a significant component of vehicle exhaust and is often used as a proxy for tracking traffic-related air pollution.

Nitrogen dioxide is generated primarily through the burning of fuels in vehicles, power plants, and industrial facilities. High combustion temperatures increase the production of nitric oxide, which is rapidly oxidised in the air to form nitrogen dioxide. As a result, road traffic is the principal outdoor source of NO2. Indoor sources of nitrogen dioxide include tobacco smoke and gas-, wood-, oil-, kerosene- and coal-burning appliances such as stoves, ovens, heaters, and fireplaces, particularly if they are unflued or poorly maintained.

NO2 and other nitrogen oxides react with other chemicals in the air, including volatile organic compounds (VOCs), to form secondary pollutants such as ozone, particulate matter, acid rain, and other toxic chemicals. These secondary pollutants can have harmful effects on human health, particularly the respiratory system. NO2 irritates the airways and is linked to more frequent and severe asthma symptoms. Exposure to NO2 may also impair lung development in children, intensify allergies, and increase susceptibility to respiratory infections.

The World Health Organization (WHO) has established guidelines for indoor air quality, including limits for nitrogen dioxide concentrations. Despite these guidelines, many people worldwide breathe unhealthy levels of air pollutants. For example, 42% of the global population is exposed to NO2 levels above the annual guideline value recommended by WHO. High exposures to NO2 are seen in North Africa, the Middle East, high-income countries, and Central and Eastern Europe and Central Asia. However, sustained policy action and technological advancements have led to a decline in NO2 exposures over time in some regions.

Sulfur oxides

SO2 is a harmful pollutant that can cause a range of negative health effects, especially for those who live or work near large sources of emissions. It can cause wheezing, shortness of breath, chest tightness, and other respiratory problems, especially during physical activity. Long-term exposure to high levels of SO2 can increase respiratory symptoms and reduce lung function. Even short exposures to peak levels of SO2 in the air can cause breathing difficulties for people with asthma.

In addition to its impacts on human health, SO2 and other sulfur oxides can contribute to acid rain, which can harm sensitive ecosystems. They can also 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 health problems. At high concentrations, gaseous SOx can also harm trees and plants by damaging foliage and decreasing growth.

The US Environmental Protection Agency (EPA) has established national ambient air quality standards for SO2 to protect against exposure to the entire group of sulfur oxides (SOx). Control measures that reduce SO2 emissions can generally be expected to reduce people's exposure to all gaseous SOx. EPA rules to reduce SO2 emissions are designed to help state and local governments meet the Agency's national air quality standards.

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