Air Pollutants: Primary Pollutants And Their Criteria

The six most common air pollutants, also known as criteria air pollutants, include carbon monoxide, lead, ground-level ozone, particulate matter, nitrogen dioxide, and sulfur dioxide. These pollutants are primarily the result of combustion products released by the burning of fossil fuels and have been linked to a variety of adverse health effects, including coughing, wheezing, and aggravation of respiratory illnesses such as asthma. Children, in particular, may be more susceptible to the adverse effects of these pollutants due to their developing organ systems and increased exposure through outdoor play. To protect public health, the EPA has established National Ambient Air Quality Standards (NAAQS) for these criteria pollutants, setting limits based on scientific information regarding their health and environmental impacts.

Carbon monoxide is a colourless, odourless and tasteless gas

Carbon monoxide (CO) is a colourless, odourless, and tasteless toxic gas. It is produced by the incomplete burning of fossil fuels such as natural gas, propane, oil, wood, and coal. Sources of carbon monoxide include fireplaces, furnaces, cars, trucks, buses, wood stoves, space heaters, charcoal grills, and gas appliances like water heaters, ovens, and clothes dryers. Carbon monoxide is dangerous because when inhaled, it displaces oxygen in the blood by binding with haemoglobin, leading to symptoms such as headache, dizziness, nausea, and fatigue. At high concentrations, carbon monoxide exposure can result in unconsciousness and even death, making it a leading cause of fatal poisoning in the United States.

Due to its lack of colour, odour, and taste, carbon monoxide can be challenging to detect without specialised equipment like carbon monoxide detectors. It may accumulate to harmful levels, especially during calm weather in winter and early spring when fuel combustion is at its peak and carbon monoxide stability is higher due to low temperatures. To prevent carbon monoxide poisoning, it is crucial to have functioning detectors in homes, especially near fuel-burning devices, and to ensure regular maintenance of fuel-burning appliances.

Federal tailpipe standards for new automobiles, traffic flow improvements, and annual emissions inspection programs have contributed to improved CO levels in some areas, such as Nashua, New Hampshire, which has been in attainment of the NAAQS for CO since the mid-1990s. However, carbon monoxide remains a significant concern, especially in areas with higher traffic volumes and congestion, such as poorly ventilated parking garages and tunnels.

Carbon monoxide is one of the six criteria air pollutants, along with particulate matter, ground-level ozone, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are given this designation by the EPA because it sets NAAQS (National Ambient Air Quality Standards) for them based on the latest scientific information about their effects on health and welfare. The Clean Air Act requires the EPA to establish both primary and secondary NAAQS for criteria pollutants. Primary standards focus on protecting public health, especially for vulnerable populations like asthmatics, children, and the elderly, while secondary standards aim to safeguard public welfare, including visibility and the well-being of animals, crops, vegetation, and buildings.

Nitrogen dioxide is a reddish-brown toxic gas

Nitrogen dioxide (NO2) is a reddish-brown toxic gas and a member of the nitrogen oxides (NOx) family of atmospheric pollutants. It is formed through the oxidation of nitric oxide by oxygen in the air. NO2 is introduced into the environment by natural causes, such as entry from the stratosphere, bacterial respiration, volcanic activity, and lightning. It is also a product of combustion processes, particularly the burning of fossil fuels in utility and industrial boilers, automobiles, trucks, and internal combustion engines.

Nitrogen dioxide is a significant air pollutant due to its harmful effects on human health and the environment. It is a strong oxidizer and can react with other chemicals in the atmosphere to form particulate matter, ozone, and acid rain. Ozone (O3) formation occurs when NO2 and certain volatile organic compounds react in the presence of sunlight. Acid rain, on the other hand, is formed when NOx combines with water, oxygen, and other atmospheric chemicals. These reactions contribute to reduced visibility, damage to ecosystems, and adverse health impacts.

The reddish-brown colour of nitrogen dioxide is a result of its preferential absorption of light in the blue region of the spectrum (400–500 nm). This absorption extends throughout the visible spectrum at shorter wavelengths and into the infrared at longer wavelengths. The colour can vary from reddish-brown to yellowish-brown, depending on temperature. At temperatures above 21.2 °C (70.2 °F), NO2 is a reddish-brown gas with a pungent odour, while at lower temperatures, it becomes a yellowish-brown liquid.

Nitrogen dioxide is a harmful pollutant that poses risks to human health, particularly the respiratory system. Acute exposure to high concentrations of NO2 can be fatal, with a median lethal dose (LC50) of 174 ppm for a 1-hour exposure. Lower concentrations over extended periods can also lead to respiratory issues, such as aggravating asthma and increasing susceptibility to respiratory infections. Studies have linked indoor exposure to gas stoves and heaters, which emit NO2, to poorer air quality and respiratory diseases. Additionally, NO2 has been associated with alterations in lung metabolism, structure, and function, as well as increased inflammation and susceptibility to pulmonary infections.

Regulators have recognised the importance of controlling NO2 levels to mitigate its harmful effects. Efforts to reduce NO2 emissions will also contribute to decreasing the presence of other NOx species and associated toxic substances. Standards and guidelines, such as the National Ambient Air Quality Standards (NAAQS) and state-specific regulations, have been established to protect public health and welfare. These standards aim to limit the concentration of pollutants like NO2 in outdoor air, ensuring that they remain within safe levels for human health and the environment.

Sulfur dioxide is a gaseous pollutant, primarily emitted by industrial furnaces

The Environmental Protection Agency (EPA) identifies six criteria air pollutants: particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are given this designation because the EPA sets National Ambient Air Quality Standards (NAAQS) for them based on the latest scientific information regarding their effects on health and welfare.

Sulfur dioxide (SO2) is a colorless, odorless gas or liquid with a strong, choking odor. It is a gaseous pollutant emitted primarily by industrial furnaces, among other sources. SO2 is produced by the burning of fossil fuels, such as coal and oil, and the smelting of mineral ores containing sulfur, including copper, zinc, lead, iron, and aluminum. The largest sources of SO2 emissions are power plants and other industrial facilities that burn fossil fuels.

In addition to industrial sources, SO2 emissions can also come from natural sources such as volcanoes. Smaller sources include locomotives, ships, and other vehicles and heavy equipment that burn fuel with a high sulfur content. SO2 can also be found in homes due to tobacco smoke, improperly vented gas appliances, gas or kerosene heaters, wood or coal stoves, automobile exhaust from attached garages, and malfunctioning chimneys.

Elevated concentrations of 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. SO2 and other sulfur oxides are a major component of acid rain, which can have detrimental effects on ecosystems, including damage to forests and crops, changes in soil acidity, and the acidification of lakes and streams, making them unsuitable for fish.

Short-term exposure to high levels of SO2 can be life-threatening, causing a burning sensation in the nose and throat, difficulty breathing, and potentially exacerbating respiratory illnesses, especially in at-risk populations such as children, the elderly, and individuals with asthma. Long-term exposure to SO2 can also lead to changes in lung function and aggravate existing heart disease.

Lead is the most abundant toxic heavy metal

Lead is a naturally occurring heavy metal with a high atomic weight and a density of at least five times that of water. It is the most common toxic metal in the US, with exposure often coming from lead-based paint in older homes. Lead is also found in contaminated water, batteries, gasoline, and construction materials. In 2021, 2,079 cases of lead exposure were reported in the US, with 957 of those cases involving children under 6. Even low blood lead levels can negatively affect brain development in children.

The toxicity of lead depends on several factors, including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals. Lead poisoning can cause serious health issues, including brain damage, and liver and kidney failure. It can also affect cellular organelles and components such as cell membranes, mitochondria, lysosomes, endoplasmic reticulum, nuclei, and some enzymes involved in metabolism, detoxification, and damage repair.

Lead is a primary pollutant and is one of the six criteria air pollutants, along with particulate matter, ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA sets National Ambient Air Quality Standards (NAAQS) for these pollutants based on the latest scientific information regarding their effects on health and welfare. The primary, health-based standard for lead is 0.15 micrograms per cubic liter (μg/m3). This standard is measured as total suspended particles (TSP) for a rolling three-month average.

Industrial sources of lead emissions include mining, construction, and manufacturing. Lead can also be released into the air through the burning of fossil fuels. In 2008, the EPA revised the ambient monitoring requirements for measuring lead in the air, expanding the nation's lead monitoring network to better assess compliance with the revised standard. All counties in Wisconsin meet the primary and secondary standards for lead.

Ground-level ozone is a secondary pollutant

The US EPA identifies six criteria air pollutants: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Ground-level ozone is a secondary pollutant, formed when two primary pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOCs), react with sunlight and stagnant air. This reaction occurs when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources are exposed to sunlight.

Nitrogen oxides (NOx) are a group of highly reactive gases, primarily produced by burning fuels in utility and industrial boilers, automobiles, and trucks. About 95% of NOx from human activity comes from burning coal, gasoline, and oil. NOx can also react with other chemicals in the air to form particulate matter, acid rain, and regional haze, contributing to reduced visibility.

Volatile organic compounds (VOCs) are released through gasoline combustion, upstream oil and gas production, residential wood combustion, and the evaporation of liquid fuels and solvents. Significant quantities of VOCs originate from natural sources such as coniferous forests.

Ground-level ozone is a harmful air pollutant due to its adverse effects on human health and the environment. It is the main ingredient in "smog" and can cause various health issues, particularly for children, the elderly, and people with lung diseases like asthma. Ozone is most likely to reach unhealthy levels on hot sunny days in urban areas, but it can also impact rural regions through wind transportation.

Ozone is a gas composed of three atoms of oxygen. While stratospheric ozone is beneficial as it protects against ultraviolet radiation, ground-level ozone is detrimental. Exposure to ground-level ozone has been linked to premature mortality, increased hospital admissions, and asthma exacerbations. In addition to health impacts, ground-level ozone can damage vegetation, decrease crop productivity, and harm synthetic materials.

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