Brian Barton
Carbon dioxide is a greenhouse gas and is the most abundant pollutant in the transportation sector. However, it is not one of the six criteria air pollutants as defined by the US Environmental Protection Agency (EPA). These six pollutants, which are commonly found all over the US, include carbon monoxide, nitrogen dioxide, lead, sulfur dioxide, ozone, and particulate matter. These criteria pollutants are regulated by the EPA, which sets National Ambient Air Quality Standards (NAAQS) to limit their allowable concentrations in outdoor air. While carbon dioxide is not a criteria pollutant, it is a significant contributor to global warming and climate change.
| Characteristics | Values |
|---|---|
| Number of pollutants | 6 |
| Pollutants | Carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2) |
| Harm | Health, environment, and property damage |
| Regulator | United States Environmental Protection Agency (U.S. EPA) |
| Standards | National Ambient Air Quality Standards (NAAQS) |
| Standards review | Periodic, based on health, environmental, and technological advances |
| Air quality designations | Attainment or nonattainment |
| Non-criteria pollutants | TOGs, VOCs, hydrocarbons |
| Carbon dioxide (CO2) | Greenhouse gas, most abundant in the transportation sector |
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What You'll Learn
Carbon dioxide as a greenhouse gas
Carbon dioxide is not one of the six criteria air pollutants. These pollutants, found all over the United States, are:
- Particulate matter
- Ground-level ozone
- Carbon monoxide
- Sulfur dioxide
- Nitrogen dioxide
- Lead
These pollutants can cause harm to human health, the environment, and property. The Clean Air Act requires the Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for these six criteria pollutants.
However, carbon dioxide (CO2) is the most abundant greenhouse gas in the transportation sector. Greenhouse gases are global pollutants, and fluorinated refrigerants used in mobile air conditioners are also major contributors due to their high global warming potential.
Carbon dioxide is released into the atmosphere through the burning of fossil fuels in boilers, engines, automobiles, trucks, and buses. It is found in higher concentrations in areas with poor ventilation, such as parking garages, tunnels, or areas with high traffic congestion.
As a greenhouse gas, carbon dioxide contributes to the greenhouse effect, which is essential for making the Earth habitable. However, human activities, particularly the burning of fossil fuels, have significantly increased the concentration of carbon dioxide in the atmosphere, leading to global warming and climate change.
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Criteria air pollutants and their health effects
Carbon dioxide is not considered a criteria pollutant. Criteria air pollutants are six common and widespread outdoor ambient air pollutants that are found all over the United States. These pollutants can harm human health and the environment and cause property damage. The Clean Air Act requires the United States Environmental Protection Agency (US EPA) to set National Ambient Air Quality Standards (NAAQS) for these six criteria air pollutants:
- Carbon monoxide (CO): CO is an odorless gas that can come from the incomplete combustion of fossil fuels in boilers and engines, but it is primarily produced by automobiles, trucks, and buses.
- Nitrogen dioxide (NO2): NO2 is part of a group of highly reactive gases known as oxides of nitrogen (NOx). It is harmful to human health, particularly the respiratory system. It contributes to the formation of other air pollutants, including particulate matter (PM), ozone, acid rain, and regional haze.
- Ozone (O3) or "ground-level ozone": While ozone naturally occurs high in the atmosphere and protects the Earth from ultraviolet rays, ground-level ozone is a harmful air pollutant. It can inflame and damage lungs, increase asthma attacks, and make lungs more susceptible to infection. Ground-level ozone is formed by a chemical reaction between other directly emitted pollutants, including NOx and volatile organic compounds (VOCs) emitted by motor vehicles.
- Lead (Pb): Lead is a toxic heavy metal that can have significant health impacts, especially on children's brain development and neurological function.
- Sulfur dioxide (SO2): 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 can also harm trees and plants by damaging foliage and decreasing growth due to acid rain.
- Particulate Matter (PM): Fine particulate matter, such as dust, dirt, soot, and smoke, can penetrate deep into the lungs and even enter the bloodstream. It can aggravate asthma, make breathing difficult, and contribute to particle formation with associated health effects.
These criteria air pollutants are regulated by the US EPA, which develops human health-based and environmentally based criteria (science-based guidelines) for setting permissible levels. The limits based on human health are called primary standards, while those intended to prevent environmental and property damage are called secondary standards.
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Carbon monoxide and its toxicity
Carbon monoxide (CO) is one of the six criteria air pollutants, which are commonly found and harmful to health and the environment. It is released into the environment during the incomplete combustion of carbon compounds. This occurs when fuel is burned at high temperatures, including in industrial, commercial, and residential combustion, motor vehicles, and electric utilities.
CO is a colorless, odorless, and tasteless gas, making it essentially undetectable by human senses. This allows toxicity to develop gradually, as the gas enters the body through the lungs and binds with hemoglobin in the bloodstream, forming COHb and disseminating throughout the body. As COHb levels rise, cerebral blood vessels dilate, and coronary blood flow and capillary density increase, potentially leading to central respiratory depression due to cerebral hypoxia.
The toxicity of carbon monoxide can have a range of effects on the human body, from asymptomatic to cardiovascular collapse and death. The most commonly reported symptoms of unintentional carbon monoxide toxicity include headache, nausea, dizziness or vertigo, drowsiness or fatigue, and vomiting. Less common symptoms include cough, confusion, shortness of breath, syncope, chest pain, weakness, and throat irritation. The broad range of symptoms can make diagnosis challenging, as they may mimic those of more common diseases.
Treatment for carbon monoxide toxicity includes hyperbaric oxygen therapy (HBOT), which accelerates the elimination of carbon monoxide from the body. Extracorporeal membrane oxygenation (ECMO) has also been used successfully to facilitate carbon monoxide removal and provide circulatory and respiratory support. Ongoing research is exploring the use of additional therapies, such as reactive oxygen species scavengers and agents that directly bind to carbon monoxide.
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Nitrogen dioxide and its impact on human health
Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides (NOx). It is primarily released into the air through the burning of fuel, such as natural gas, wood, or gas, and emissions from cars, trucks, buses, power plants, and off-road equipment.
Nitrogen dioxide is a significant health concern, particularly for the respiratory system. Exposure to NO2 can irritate the airways and aggravate respiratory diseases, especially asthma. Short-term exposure to high concentrations of NO2 can lead to coughing, wheezing, and difficulty breathing, resulting in hospital admissions and visits to emergency rooms.
Longer-term exposure to elevated levels of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. Scientific evidence also suggests a link between NO2 exposure and adverse effects on the heart, lungs, pregnancy and birth outcomes, and a potentially increased risk of kidney and neurological harm, autoimmune disorders, and cancer.
Those living near emission sources, such as power plants and major roadways, are at a higher risk of experiencing the health impacts of nitrogen dioxide pollution. Indoor exposure can also occur through the use of appliances like stoves, dryers, and space heaters that burn natural gas or liquified petroleum gas (LPG). Inadequate ventilation can lead to unhealthy levels of NO2 accumulation indoors.
To mitigate the health risks associated with nitrogen dioxide, regulatory measures, such as the Clean Air Act, have been implemented to drive down emissions and improve air quality. While progress has been made in reducing NO2 levels, it remains a criteria air pollutant, and continued efforts are necessary to protect public health.
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Particulate matter and its adverse effects
Carbon dioxide is one of the six criteria air pollutants, along with ozone, carbon monoxide, lead, sulfur dioxide, and nitrogen dioxide. These pollutants are found all over the US and can harm health, the environment, and cause property damage.
Particulate matter (PM) is a mixture of solid particles and liquid droplets found in the air. Some particles, like dust, dirt, soot, or smoke, are large or dark enough to be visible to the naked eye. Others are so small that they can only be detected using an electron microscope. PM2.5, for example, refers to fine inhalable particles with diameters of 2.5 micrometers or smaller. These particles come in various sizes and shapes and can be composed of hundreds of different chemicals.
PM has adverse effects on both the environment and human health. In terms of the environment, PM deposition on vegetated surfaces depends on the size distribution of the particles and, to a lesser extent, their chemistry. Dust coating can cause abrasion and radiative heating, reducing the photosynthetically active photon flux reaching photosynthetic tissues. Acidic and alkaline materials may damage leaves, while other materials may be absorbed across the cuticle. PM deposited directly onto soil can influence nutrient cycling, particularly nitrogen cycling, through its effects on rhizosphere bacteria and fungi. Alkaline cation and aluminum availability are dependent on soil pH, which can be significantly altered by PM deposition.
PM also has negative consequences for human health. Individuals who spend time near major roadways are at higher risk of short-term exposure to nitrogen dioxide, which contributes to the formation of PM. High concentrations of PM in livestock houses can also threaten the health and welfare of humans and animals. Livestock housing is a significant source of PM emissions, and the particles can adsorb and contain gases, odorous compounds, and microorganisms, enhancing their biological effects.
To protect human health, the Air Quality Index (AQI) informs people about the cleanliness of their outdoor air and associated health risks. The AQI translates air quality data into numbers and colors, helping individuals understand when to take action to protect themselves.
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Frequently asked questions
A criteria pollutant is one of six common and widespread outdoor ambient air pollutants, including carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2). These pollutants are harmful to human health and the environment.
No, carbon dioxide (CO2) is not one of the six criteria pollutants. However, it is a greenhouse gas and is the most abundant GHG in the transportation sector.
Nitrogen dioxide is emitted from many sources when fuel is burned at high temperatures, including industrial, commercial, and residential combustion, motor vehicles, and electric utilities.
