Brian Barton
Air pollution is caused by the release of various gases, finely divided solids, or finely dispersed liquid aerosols into the atmosphere at rates that exceed the environment's capacity to dilute or absorb them. These pollutants can have undesirable health, economic, and aesthetic effects. The most common air pollutants include particulate matter (often referred to as particle pollution), ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are regulated by organizations like the U.S. EPA, which establishes National Ambient Air Quality Standards to protect public health.
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What You'll Learn
Particulate matter
Sources of particulate matter are both natural and anthropogenic. Natural sources include wildfires, dust storms, volcanic eruptions, and sea spray. Man-made sources, on the other hand, include combustion in mechanical and industrial processes, vehicle emissions, and tobacco smoke. In California, for example, the combustion of gasoline and diesel fuel in motor vehicles is the major contributor to particle pollutants, especially in urban areas.
The health effects of particulate matter exposure are well-documented. It is associated with an increased risk of morbidity and mortality, particularly due to cardiovascular diseases. Studies have shown that both acute and chronic exposure to particulate matter can lead to ischemic heart disease, heart failure, and ischemic/thrombotic stroke. In addition, particulate matter has been identified as an endocrine disrupter, contributing to the development of metabolic diseases such as obesity and diabetes mellitus.
To reduce exposure to particulate matter, individuals can take several measures. These include using air conditioning and particulate filters, reducing indoor combustion for heating and cooking, and smoking cessation. High-efficiency portable air cleaners and electric or hybrid vehicles are also recommended. For those living in areas with high levels of particle pollution, such as near busy roads or freeways, it is advised to limit outdoor activity during peak traffic periods or when air quality is poor.
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Ground-level ozone
Ozone is a gas composed of three oxygen atoms, and it can be beneficial or detrimental depending on its location in the atmosphere. Stratospheric ozone, which occurs naturally in the upper atmosphere, acts as a protective layer, shielding the Earth from the sun's harmful ultraviolet radiation. In contrast, ground-level ozone is considered "bad" because it can trigger a range of health issues, especially for vulnerable individuals such as children, the elderly, and people with lung diseases like asthma. Exposure to ground-level ozone has been linked to increased hospital admissions, asthma exacerbations, and even premature mortality.
The sources of primary pollutants that contribute to ground-level ozone formation include motor vehicles, homes, industries, and power plants. About 95% of NOx from human activity comes from burning coal, gasoline, and oil in these sectors. VOCs, on the other hand, originate from various human activities, including gasoline combustion, oil and gas production, residential wood combustion, and the evaporation of liquid fuels and solvents. Natural sources, such as coniferous forests, also contribute significant amounts of VOCs.
The U.S. Environmental Protection Agency (EPA) has implemented regulations to help states reduce ground-level ozone levels. These include developing state implementation plans (SIPs) to outline measures for improving air quality in areas that do not meet the national standards. Additionally, the Canada-United States Air Quality Agreement, with the addition of the Ozone Annex in December 2000, addresses the transboundary air pollution contributing to high levels of ground-level ozone.
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Carbon monoxide
CO is a harmful air pollutant that can have adverse health effects. It reduces the amount of oxygen that can be transported in the blood to vital organs like the heart and brain. Exposure to carbon monoxide can cause dizziness, chest pain, confusion, unconsciousness, and even death. People with heart disease are particularly vulnerable to the effects of CO, and even low levels can cause chest pain, reduced exercise capacity, and other cardiovascular issues. Healthy individuals can also be impacted by high levels of CO, experiencing vision problems, reduced manual dexterity, and difficulty with complex tasks.
Indoor sources of CO include gas stoves, malfunctioning or improperly vented gas appliances (such as water heaters, furnaces, and clothes dryers), space heaters, fireplaces, tobacco smoke, and wood stoves. CO levels tend to be higher indoors and during the colder months when inversion conditions trap air pollution near the ground. Outdoor CO levels are typically higher in areas with heavy traffic congestion. Industrial processes, residential wood burning, and natural sources such as forest fires also contribute to CO emissions.
To protect public health, the U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) for carbon monoxide under the Clean Air Act. These standards help state, tribal, and local agencies ensure that CO levels remain safe. While no ecological effects of CO were identified at ambient levels in a 2011 review, CO contributes to climate change through chemical reactions that produce ozone, a potent climate change gas. As a result, reducing CO emissions is considered a potential strategy to mitigate global warming.
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Nitrogen dioxide / oxides
Nitrogen dioxide (NO2) is a highly reactive gas and is one of the six "criteria" pollutants that the US EPA is required to regulate under the Clean Air Act. These pollutants can harm human health, damage the environment, and cause property damage. Nitrogen dioxide is a primary pollutant and is particularly harmful when inhaled due to its effects on the respiratory system. It is one of a group of gases known as oxides of nitrogen (NOx), which also includes nitric oxide (NO), nitrous oxide, nitrous acid, and nitric acid.
Nitrogen dioxide is formed through the combustion of fuels, such as vehicle emissions, power plants, and off-road equipment. The higher the combustion temperature, the more nitric oxide is generated. Typically, 90-95% of nitrogen oxides are emitted as nitric oxide, and only 5-10% as nitrogen dioxide. However, nitric oxide is rapidly oxidized in the air to form nitrogen dioxide, and this oxidation process is faster outdoors than indoors. As a result, road traffic is the primary outdoor source of nitrogen dioxide, while tobacco smoke and fuel-burning appliances, such as stoves and heaters, are significant indoor sources.
Nitrogen dioxide and other nitrogen oxides can react with other chemicals in the air, including volatile organic compounds, to form secondary pollutants such as particulate matter, ozone, acid rain, and other toxic chemicals. These secondary pollutants can also be harmful to human health and the environment. For example, particulate matter and ground-level ozone are considered the most widespread health threats among the criteria pollutants.
To protect public health, the US EPA has set National Ambient Air Quality Standards (NAAQS) for NO2, with a limit of 100 parts per billion (ppb) averaged over one hour and 53 ppb averaged annually. These standards are based on the health effects of NO2, as exposure to this pollutant can trigger inflammation and cause other respiratory issues.
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Sulphur oxides
Sulphur dioxide has harmful effects on both human health and the environment. In terms of human health, SO2 can cause respiratory problems, including wheezing, shortness of breath, and chest tightness. Long-term exposure to high levels of SO2 increases respiratory symptoms and impairs lung function. It is particularly harmful to individuals with asthma, making it difficult for them to breathe during outdoor activities when SO2 levels are high.
The environmental impact of sulphur dioxide is also significant. SO2 can damage trees and plants, inhibit their growth, and harm sensitive ecosystems and waterways. When SO2 dissolves in water vapour in the air, it forms acids that can contribute to acid rain. Acid rain, in turn, can have further detrimental effects on trees, plants, and ecosystems, as well as buildings and materials.
Once released into the air, sulphur dioxide can create secondary pollutants, such as sulphate aerosols and particulate matter, and acid rain. These secondary pollutants contribute to overall air pollution and can have additional impacts on human health and the environment. SO2 is one of the six "criteria" air pollutants identified by the U.S. Environmental Protection Agency (EPA) and is regulated through the Clean Air Act to protect human health and the environment.
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Frequently asked questions
The six most common air pollutants, as defined by the US EPA, are particulate matter (often referred to as particle pollution), ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are found all over the United States and can harm human health, the environment, and cause property damage.
These pollutants are emitted by various industrial processes, coal- or oil-burning power plants, residential heating systems, and automobiles.
Health problems can occur as a result of both short- and long-term exposure to these pollutants. For example, particulate matter (PM) can penetrate deep into the lungs and enter the bloodstream, causing cardiovascular and respiratory issues. Long-term exposure to PM has also been linked to adverse perinatal outcomes and lung cancer. Nitrogen dioxide (NO2) is a strong oxidant and can cause respiratory problems, while carbon monoxide can have harmful effects on the cardiovascular system.
