Understanding Criteria Pollutant Identification Methods

The Clean Air Act of 1970 empowers the US Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) for six criteria air pollutants: sulfur dioxide, nitrogen dioxide, particulate matter, carbon monoxide, ozone, and lead. These pollutants are chosen because they are harmful to human health and the environment, and they are common across the United States. The EPA sets standards and regulations to control these pollutants, aiming to protect public health and welfare. The criteria for these pollutants have remained largely unchanged since 1978, but regulatory activity and legislation continue to evolve to address their wide-ranging impacts on communities, the environment, and industries.

Particulate matter (PM)

PM is classified into two main categories based on particle size: PM2.5 and PM10. PM2.5 refers to fine particles with diameters of 2.5 micrometers or smaller, while PM10 refers to inhalable coarse particles with diameters between 2.5 and 10 micrometers. For context, a human hair is approximately 70 micrometers in diameter. PM2.5 is generally considered more harmful than PM10 due to its smaller size, which allows it to penetrate deeper into the respiratory system.

The health effects of PM exposure have been well-documented in scientific studies. Short-term exposure to PM10 has been linked to the worsening of respiratory diseases, such as asthma and chronic obstructive pulmonary disease (COPD). Long-term exposure to PM2.5 has been associated with premature death, particularly in individuals with chronic heart or lung diseases, and reduced lung function growth in children. Overall, PM2.5 is associated with the greatest proportion of adverse health effects related to air pollution, both in the United States and worldwide.

The Air Quality Index (AQI) is a useful tool that provides information about the level of PM and other pollutants in the air. It translates air quality data into numbers and colours, helping individuals understand when they may need to take action to protect their health. Additionally, the International Agency for Research on Cancer (IARC) has concluded that particulate matter in outdoor air pollution causes lung cancer.

Ozone (O3)

NOx is primarily produced when fossil fuels like gasoline, oil, or coal are burned, such as in power plants, motor vehicles, furnaces, and other sources of high-heat combustion. VOCs are common ingredients in solvents and products that easily evaporate, including gasoline vapors, chemical solvents, paints, and lawn and garden equipment. Major contributors to ground-level ozone pollution include motor vehicle exhaust, industrial emissions, and agricultural and construction vehicles.

Ground-level ozone is harmful to both human health and the environment. It can irritate the eyes, nose, and throat, and cause or aggravate lung diseases, including asthma, bronchitis, and chronic obstructive pulmonary disease (COPD). Long-term exposure to ground-level ozone has been associated with increased respiratory illnesses, metabolic disorders, nervous system issues, reproductive issues, and increased respiratory and cardiovascular-related mortality. Studies have also shown that even short-term exposure to high levels of ozone can lead to immediate health problems, such as increased airway obstruction and breathing difficulties.

Due to its harmful effects, ground-level ozone is identified as a criteria pollutant by the U.S. Environmental Protection Agency (EPA). The EPA is required by the Clean Air Act to set National Ambient Air Quality Standards (NAAQS) for criteria pollutants, including ground-level ozone, to protect human health and the environment. These standards aim to regulate the permissible levels of ground-level ozone and other criteria pollutants to mitigate their adverse impacts.

Carbon monoxide (CO)

CO is harmful to human health and the environment because it can reduce the amount of oxygen delivered to the body's organs, such as the heart and brain. High levels of exposure to CO, which are possible indoors or in other enclosed spaces, can lead to dizziness, confusion, unconsciousness, and even death. People with certain types of heart disease may be more severely affected by short-term exposure to elevated levels of CO.

The Clean Air Act requires the United States Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six commonly found air pollutants, known as criteria air pollutants, which include carbon monoxide. These standards are set to protect human health and the environment from the harmful effects of these pollutants.

To monitor and regulate carbon monoxide levels, specialised instruments are employed to detect and measure its concentration in the air. These instruments utilise various technologies, such as gas chromatography, infrared spectroscopy, and electrochemical sensing, to provide accurate measurements of CO levels. By continuously monitoring carbon monoxide concentrations, particularly in areas susceptible to high emissions, such as urban centres with dense vehicle traffic or industrial zones, authorities can ensure compliance with the established standards.

In summary, carbon monoxide is identified as a criteria pollutant due to its harmful effects on human health and the environment, and its prevalence as a byproduct of combustion processes, particularly in urban areas with high traffic congestion. The EPA, in conjunction with local and state agencies, plays a pivotal role in setting standards, monitoring levels, and implementing measures to mitigate the adverse impacts of carbon monoxide pollution.

Nitrogen dioxide

Indoors, exposure to nitrogen dioxide arises from cigarette smoke, butane and kerosene heaters and stoves, and gas cookers. Indoor exposure levels of NO2 are, on average, at least three times higher in homes with gas stoves compared to electric stoves. Outdoor air concentrations of nitrogen dioxide can be slightly higher or lower depending on the year.

Breathing air with a high concentration of NO2 can irritate the airways in the human respiratory system. Exposure to NO2 over short periods can aggravate respiratory diseases, especially asthma, leading to respiratory symptoms such as coughing, wheezing, or difficulty breathing. Longer exposure to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. Studies in mice and ferrets have shown that exposure to nitrogen dioxide can lead to progressive airway inflammation, increased mucus hypersecretion in the central airways, progressive airflow obstruction, and focal parenchymal inflammation associated with airspace enlargement.

In addition to its harmful effects on human health, nitrogen dioxide also has negative environmental impacts. NO2 is one of the pollutants that contribute to the formation of ground-level ozone, which can inflame and damage lungs, increase the frequency of asthma attacks, and make lungs more susceptible to infection.

Primary and secondary standards

The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for six commonly found air pollutants known as criteria air pollutants. These pollutants are particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are harmful to human health and the environment and can cause property damage.

The EPA sets both primary and secondary NAAQS for criteria pollutants. The primary standards are based on human health and provide public health protection, including protecting the health of sensitive populations such as asthmatics, children, and the elderly. These standards are set to ensure that the levels of pollutants in the air do not pose a significant risk to human health. The primary standards are based on the latest scientific information and health guidelines and are designed to protect the most vulnerable members of the population.

The secondary standards, on the other hand, focus on public welfare and environmental protection. These standards aim to protect against decreased visibility and damage to animals, crops, vegetation, and buildings. They are designed to maintain the overall quality of life and preserve the natural environment. For example, elevated concentrations of sulfur dioxide (SO2) in the atmosphere can react with other compounds to form small particles that contribute to particulate matter pollution, reduce visibility, and cause regional haze. Additionally, high concentrations of gaseous sulfur oxides can harm trees and plants by damaging foliage and hindering growth due to acid rain.

The primary and secondary standards work together to safeguard both human health and the environment. These standards are periodically reviewed and revised to ensure that they remain effective and up-to-date. State, local, and tribal agencies play a crucial role in developing emission reduction strategies, plans, and programs to attain and maintain these standards. By implementing measures to reduce the emission of criteria pollutants, these agencies strive to protect public health and welfare.

It is important to note that the primary and secondary standards are not static but are subject to change over time as new scientific information becomes available and a better understanding of the health and environmental impacts of these pollutants is gained. This flexibility allows for the continuous improvement of air quality and the well-being of communities across the United States.

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