Air Quality: Seven Criteria Pollutants You Need To Know

The Clean Air Act requires the US Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants known as criteria air pollutants. These pollutants are found all over the United States and can harm human health, the environment, and cause property damage. The six criteria air pollutants are particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.

Particle pollution: a mixture of solid and liquid droplets, including fine and coarse particles

Particle pollution, also called "particulate matter" or "soot", is a mix of solid and liquid particles that vary in size and are suspended in the air we breathe. These particles are often so small that they are invisible to the naked eye, but when their levels are high, they can blur the spread of sunlight, making the air appear hazy and thick.

Particulate matter is one of six widespread air pollutants for which there are national air quality standards in place to limit their levels in the outdoor air. The Clean Air Act requires the US Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for these six common air pollutants, also known as "criteria air pollutants". These pollutants are found all over the US and can harm human health, the environment, and cause property damage.

Particle pollution is made up of both fine and coarse particles, which differ in their sources, composition, dosimetry (deposition and retention in the respiratory system), and health effects. Fine particles, or PM2.5, are generally 2.5 micrometers in diameter or smaller, and can be made up of hundreds of different chemicals. They are primarily emitted from combustion sources, such as residential fireplaces, wildfires, industrial boilers, automobiles, and trucks. These particles can easily penetrate indoors and are the main cause of reduced visibility (haze) in many parts of the US. Coarse particles, or PM10-2.5, have diameters larger than 2.5 micrometers and smaller than or equal to 10 micrometers. They are typically of crustal (earth) or biological origin, such as dust from dust storms or particles from agricultural activities.

The inhalation of fine particles can pose serious health risks. Some particles less than 10 micrometers in diameter can penetrate deep into the lungs, and particles smaller than 2.5 micrometers may even enter the bloodstream. Short-term exposure to high levels of fine particles can trigger cardiovascular events, hospitalisations, and even mortality. Long-term exposure can increase the risk of strokes, coronary heart disease, and premature death. Research has also linked particle pollution to increased mortality from all causes, respiratory disease, and lung cancer.

Ground-level ozone: a gas composed of three oxygen atoms, which can be beneficial or harmful depending on where it forms

Ground-level ozone, or "bad" ozone, is a gas composed of three oxygen atoms (O3). It is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight. This process involves the reaction of oxides of nitrogen (NOx) and volatile organic compounds (VOCs). Ground-level ozone is a harmful air pollutant due to its detrimental effects on human health, the environment, and property. However, ozone at higher altitudes, known as stratospheric or "good" ozone, plays a protective role by forming a layer that shields us from the sun's harmful ultraviolet rays.

The Clean Air Act, enacted in 1970, addresses ground-level ozone as one of the six common air pollutants, or "criteria" air pollutants, found throughout the United States. These criteria air pollutants, including particle pollution, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead, are regulated by the Environmental Protection Agency (EPA) through the establishment of National Ambient Air Quality Standards (NAAQS). These standards are set to protect public health and welfare, with primary standards focused on safeguarding sensitive populations such as asthmatics, children, and the elderly, and secondary standards aimed at preventing environmental and property damage.

The health effects of ground-level ozone have been extensively studied, and it is now understood to pose serious risks to human health, even at relatively low levels. People with asthma are particularly vulnerable, as ozone can aggravate their condition and lead to increased medication use, hospital visits, and admissions. Long-term exposure to ozone is also linked to the development of asthma, especially in children whose lungs are still developing. Additionally, ozone can cause constriction of the muscles in the airways, trapping air in the alveoli, and it has been associated with increased school absences and emergency room visits.

The impacts of ground-level ozone exposure are not limited to humans but extend to vegetation and ecosystems as well. Elevated ozone levels can harm sensitive vegetation during the growing season and have negative consequences for forests, parks, wildlife refuges, and wilderness areas. Furthermore, ground-level ozone contributes to smog formation, reducing visibility and causing regional haze. To mitigate these harmful effects, the EPA has implemented regulations and standards to reduce emissions from sources such as power plants, industrial sites, and vehicles, resulting in improvements in air quality across the nation.

Carbon monoxide: a poisonous, odourless, and colourless gas formed from the incomplete burning of carbon in fuels

Carbon monoxide is a harmful and poisonous gas that is formed from the incomplete burning of carbon-rich compounds, or fuels. It is colourless and odourless, making it impossible to detect with our senses. This means that carbon monoxide can be extremely dangerous, as it can build up indoors without people realising until they start to feel unwell. Symptoms of carbon monoxide poisoning are often similar to the flu, causing victims to ignore the early warning signs. According to the CDC, around 400 people die from unintentional exposure to carbon monoxide in the US every year.

Carbon monoxide is produced whenever a material burns, but it is a by-product of incomplete combustion. This occurs when there is insufficient oxygen present to react with the fuel and form carbon dioxide (CO2). Instead, the combustion reaction produces carbon monoxide (CO) and water, with some impurities such as ash. Incomplete combustion is common because the process requires heat, fuel, and oxygen, and it is difficult to achieve ideal conditions. The colour of a flame is a visual indicator of incomplete combustion, with a yellow flame and soot indicating that carbon has not been fully utilised in the reaction.

Carbon monoxide is a common air pollutant, along with particle pollution, ground-level ozone, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are found all over the US and can harm human health, the environment, and cause property damage. The US EPA has set National Ambient Air Quality Standards (NAAQS) for these six common air pollutants, which are known as criteria air pollutants. Primary standards aim to protect public health, while secondary standards aim to prevent environmental and property damage.

Carbon monoxide is often produced in homes with fuel-burning appliances, such as clothes dryers, water heaters, furnaces, fireplaces, and stoves. Motor vehicles, grills, generators, and power tools are also common sources of carbon monoxide. To prevent carbon monoxide poisoning, it is important to install a CO alarm and maintain fuel-burning appliances properly. Regular inspections of heating equipment and proper ventilation are crucial to minimise the risk of carbon monoxide exposure.

Sulfur dioxide: one of the six criteria air pollutants, along with nitrogen dioxide, particulate matter, carbon monoxide, and lead

Sulfur dioxide (SO2) is one of six criteria air pollutants, alongside nitrogen dioxide, particulate matter, carbon monoxide, and lead. These pollutants are harmful to human health, the environment, and property, and are found all over the US.

SO2 is a highly reactive gas and part of a group of gases known as oxides of sulfur. It is emitted into the air as a result of fossil fuel combustion at power plants and other industrial facilities, as well as other industrial processes.

SO2 is dangerous as it can contribute to the formation of other sulfur oxides (SOx) in the atmosphere. These sulfur oxides can react with other compounds to form small particles, contributing to particulate matter (PM) pollution. These particles can penetrate deeply into the lungs and cause serious health problems. At high concentrations, SOx can also harm trees and plants, and contribute to acid rain, which damages sensitive ecosystems.

To combat this, the EPA has set national and regional rules to reduce SO2 emissions and other pollutants that form sulfur oxides. These rules are designed to help state and local governments meet the Agency's national air quality standards. The EPA identifies areas where air quality does not meet SO2 standards, and governments then develop plans to reduce SO2 levels.

Nitrogen dioxide (NO2), another highly reactive gas, is formed from the burning of fuel, primarily from vehicle and power plant emissions. It can irritate the human respiratory system and exposure can aggravate respiratory diseases, leading to coughing, wheezing, and difficulty breathing. Longer-term exposure may contribute to the development of asthma and increased susceptibility to respiratory infections.

Particulate matter refers to a mixture of solid particles and liquid droplets found in the air, including dust, dirt, soot, and smoke. Some of these particles are large enough to be seen, while others are so small they can only be detected using an electron microscope. These particles can be inhaled and cause serious health issues, especially those less than 2.5 micrometers in diameter, known as PM2.5, which can get into the bloodstream.

Carbon monoxide (CO) is a colorless, odorless gas formed by the incomplete reaction of air with fuel. It is produced by fossil fuel-powered engines, including motor vehicles, construction equipment, and boats, as well as industrial processes and residential wood burning. CO reduces oxygen delivery to the body's organs and tissues, causing serious health issues, especially for those with heart disease.

Lead is persistent in the environment and can be added to soils and sediments through deposition from sources of lead air pollution, such as ore and metals processing, waste incinerators, and lead-acid battery manufacturers. Lead can accumulate in the human body, particularly in the bones, and adversely affect multiple systems, including the nervous and cardiovascular systems.

Primary and secondary standards: the EPA sets standards to protect human life and the environment from harmful pollutants

The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, also known as "criteria air pollutants". These pollutants are found all over the United States and can harm human health, the environment, and cause property damage. The six criteria air pollutants include particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.

To protect public health and welfare, the EPA sets both primary and secondary standards for these criteria pollutants. The primary standards aim to protect public health, including sensitive populations such as asthmatics, children, and the elderly. These standards provide enforceable concentration limits, with the EPA listing maximum contaminant levels (MCLs) for various pollutants. If a water supplier exceeds the MCL for a toxin, they may face fines and penalties imposed by the EPA. The EPA reviews each primary contaminant with its MCL standard every six years.

The secondary standards, on the other hand, focus on public welfare protection. They aim to prevent environmental and property damage, including decreased visibility and harm to animals, crops, vegetation, and buildings. These standards are non-mandatory and set guidelines for water quality, addressing aesthetic considerations such as taste, colour, and odour. While not enforced by the EPA, these standards provide valuable references for homeowners to assess the safety of their water sources.

In addition to setting standards, the EPA also designates areas as meeting (attainment) or not meeting (nonattainment) the NAAQS. When an area is designated as nonattainment, states must develop a State Implementation Plan (SIP) to outline the steps necessary to achieve and maintain the required air quality standards. These plans are submitted to the EPA for approval, ensuring that actions are taken to improve air quality and protect human life and the environment from harmful pollutants.

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