Air Quality Standards: Pollutants And Their Impact

Air pollution is a serious issue that poses a significant threat to human health and the environment. The World Health Organization (WHO) estimates that there are 7 million premature deaths each year due to the combined effects of outdoor and household air pollution. To address this problem, various organizations such as the WHO and the US Environmental Protection Agency (EPA) have developed air quality standards that focus on specific pollutants. These pollutants include particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), carbon monoxide (CO), and sulfur dioxide (SO2). These pollutants are commonly found in urban and rural areas and have harmful effects on human health, including respiratory and cardiovascular diseases, as well as damage to the environment. By setting standards and guidelines, organizations aim to improve air quality and protect the health and well-being of people worldwide.

Particulate matter (PM2.5 and PM10)

Particulate matter (PM) is a mixture of many chemical species. It is a complex combination of solids and aerosols composed of small droplets of liquid, dry solid fragments, and solid cores with liquid coatings. PM10 and PM2.5 are particulate matter 10 micrometres or less and 2.5 micrometres or less in diameter, respectively. PM2.5 is generally described as fine particles. By way of comparison, a human hair is about 100 micrometres, so roughly 40 fine particles could be placed on its width.

PM10 and PM2.5 are produced by a wide range of industrial processes, including bulk material handling, combustion, and minerals processing. The industries that use these processes include brickworks, refineries, cement works, iron and steelmaking, quarrying, and fossil fuel power plants. Particulates are released from a wide range of diffuse sources, including lawn mowing, wood stoves, fires, and wind-generated dust. Natural sources of PM10 and PM2.5 include bushfires, dust storms, pollen, and sea spray. Vehicles also generate particulates, either from direct emissions from burning fuel (especially diesel-powered vehicles) or from tyre wear or vehicle-generated air turbulence on roadways.

PM10 and PM2.5 can affect animals in the same way they affect humans. They can also affect the aesthetics and utility of areas through visibility reduction and may affect buildings and vegetation. Once in the air, particulate matter generally takes a long time to settle. In the water, particulates may settle, dissolve, or both. PM10 and PM2.5 are very fine and light and are therefore easily entrained into the air by wind or disturbances.

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. The effects of long-term exposure to PM10 are less clear, although several studies suggest a link between long-term PM10 exposure and respiratory mortality. The International Agency for Research on Cancer (IARC) published a review in 2015 that concluded that particulate matter in outdoor air pollution causes lung cancer. Research points to older adults with chronic heart or lung disease, children, and asthmatics as the groups most likely to experience adverse health effects with exposure to PM10 and PM2.5.

The World Health Organization's Air Quality Guidelines (AQG) are a set of evidence-based recommendations of limit values for specific air pollutants developed to help countries achieve air quality that protects public health. The guidelines are neither standards nor legally binding criteria, but governments across the world use them in different ways depending on their technical capabilities, economic capacities, air quality management policies, and other political and social factors. The latest global version of the guidelines was published in 2005, and the guidelines are regularly updated to support a broad range of policy options for air quality management.

Ozone (O3)

Ozone is a major air pollutant and a key component of smog, which is commonly found in urban areas. It is formed when volatile organic compounds (VOCs) and nitrogen oxides (NOx) react with sunlight. Ground-level ozone is harmful to human health, particularly for sensitive groups such as children, the elderly, and people with asthma. It can cause respiratory issues, aggravate lung diseases, and reduce lung function. The World Health Organization (WHO) has developed Air Quality Guidelines (AQG) to help countries protect public health by reducing air pollution. These guidelines are not legally binding but serve as a global target for governments to improve air quality and safeguard citizens' health.

The EPA has been working to maintain and improve air quality by setting NAAQS for ozone and other pollutants. The agency has developed a Menu of Control Measures (MCM) to provide state, local, and tribal air agencies with information on emission reduction measures. This document assists these agencies in developing strategies, plans, and programs to reduce emissions and ensure compliance with NAAQS. The EPA also conducts periodic reviews of the NAAQS for ozone, with the most recent review completed in 2020. The agency has published various documents related to these reviews, including the Integrated Science Assessment, Policy Assessment, and Additional Ozone Air Quality Analyses.

Ozone pollution is a serious issue, and the EPA's efforts to regulate and reduce it are crucial for protecting public health and the environment. By setting standards, conducting reviews, and providing resources, the EPA works collaboratively with other agencies to address the harmful effects of ozone pollution. These measures aim to reduce the impact of ozone on human health, the environment, and overall air quality.

Nitrogen dioxide (NO2)

NO2 is introduced into the environment through natural processes, such as bacterial respiration, volcanic activity, and lightning. However, human activity is a significant contributor to NO2 pollution. The burning of fossil fuels, including from cars, trucks, buses, power plants, and off-road equipment, releases NO2 into the atmosphere. Indoor exposure to NO2 can arise from cigarette smoke and butane and kerosene heaters and stoves. On average, homes with gas stoves have at least three times higher NO2 levels than those with electric stoves.

Breathing air with high concentrations of NO2 can irritate the airways and aggravate respiratory diseases, especially asthma. Short-term exposure to NO2 can lead to coughing, wheezing, and difficulty breathing, and may require hospital admission. Prolonged exposure to elevated NO2 levels may contribute to the development of asthma and increase susceptibility to respiratory infections.

To address NO2 pollution, the United States Environmental Protection Agency (EPA) has established National Ambient Air Quality Standards (NAAQS). The EPA works with state, tribal, and local air agencies to implement emission reduction strategies and attain and maintain NAAQS. The Menu of Control Measures (MCM) provides these agencies with information on existing emission reduction measures and their efficiency and cost-effectiveness. Additionally, the EPA identifies areas that do not meet the national NO2 standards, and local governments develop plans to reduce NO2 levels in those areas.

Carbon monoxide (CO)

The World Health Organization (WHO) has established Air Quality Guidelines (AQG) that serve as a global target for governments to strive towards improving the health of their citizens by reducing air pollution. The guidelines are neither standards nor legally binding criteria, but they offer recommendations for limit values of specific air pollutants, including carbon monoxide.

In the United States, the Environmental Protection Agency (EPA) is required by the Clean Air Act to set National Ambient Air Quality Standards (NAAQS) for carbon monoxide and five other pollutants considered harmful to public health and the environment. These other pollutants are ozone (O3), particulate matter (PM2.5 and PM10), nitrogen oxides (NO2), sulfur dioxide (SO2), and lead (Pb). The EPA periodically reviews and updates these standards to ensure they provide adequate protection.

The current NAAQS for carbon monoxide is 10 mg/m3 for an 8-hour average. However, activities such as incense burning, especially common in certain cultural and religious rituals, can cause indoor carbon monoxide concentrations to exceed this standard. In the absence of indoor sources, carbon monoxide levels in indoor air in European and North American cities are generally below the levels of existing air quality guidelines. Nevertheless, sources like unvented gas appliances, tobacco smoking, and proximity to busy traffic can contribute to elevated carbon monoxide concentrations indoors.

Sulfur dioxide (SO2)

The NAAQS are reviewed and updated periodically to ensure they provide adequate protection for public health and the environment. The EPA's decisions are based on scientific evidence and exposure/risk information. The primary NAAQS for SO2 was last reviewed in 2019, and the EPA decided to retain the existing standard of 75 parts per billion (ppb), which was established in 2010. This standard is based on the three-year average of the 99th percentile of the yearly distribution of one-hour daily maximum SO2 concentrations.

The purpose of the primary NAAQS is to protect public health, including vulnerable populations such as people with asthma, children, and the elderly. The secondary NAAQS, on the other hand, focuses on public welfare protection, including the protection of animals, crops, vegetation, and buildings from negative effects such as reduced visibility and damage.

SO2 is a gaseous air pollutant that can come from various sources, including industrial chimneys, traffic exhausts, power generation, and the burning of household fuel. It is a significant contributor to air pollution, which the World Health Organization (WHO) identifies as the greatest environmental threat to human health. WHO's Air Quality Guidelines (AQG) provide global targets for governments to work towards improving air quality and protecting the health of their citizens. While not legally binding, these guidelines offer recommended levels and targets for reducing common air pollutants, including SO2.

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