Helena Joyce
There are two types of pollutants: visible and invisible. Invisible air pollutants are among the most dangerous to our health. They are invisible gaseous substances that cannot be seen or smelled. They include nitrogen oxides, sulfur dioxide, volatile organic compounds (VOCs), and carbon monoxide. These pollutants can irritate the sensitive tissues of the airway and are particularly harmful to people with pre-existing lung conditions such as asthma or emphysema. They can also damage plants and other parts of the environment.
| Characteristics | Values |
|---|---|
| Visibility | Invisible |
| Odor | Odorless |
| Types | Particulate matter, nitrogen oxides, ozone, volatile organic compounds, carbon monoxide, lead, nitrogen dioxide, sulfur dioxide |
| Sources | Industrial processes, wildfires, dust storms, vehicle exhaust, combustion engines, forest fires, power plants |
| Effects | Harmful to human health, harmful to the environment, contributes to climate change, forms acid rain |
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What You'll Learn
- Carbon monoxide: Colourless, odourless, poisonous gas that displaces oxygen in red blood cells
- Nitrogen oxides: Irritate sensitive tissues of the airway, especially for those with asthma or emphysema
- Sulfur dioxide: Irritating to humans and plants
- Volatile organic compounds: Cause smog, with serious health effects
- Ground-level ozone: Created from chemical reactions of pollutants, VOCs, and NOx
Carbon monoxide: Colourless, odourless, poisonous gas that displaces oxygen in red blood cells
Carbon monoxide (CO) is a colourless, odourless, and poisonous gas that is a product of the incomplete combustion of carbon-based fuels, including coal, wood, and oil. It is one of the six principal air pollutants and is particularly dangerous due to its invisible and undetectable nature.
Carbon monoxide is a significant health hazard as it displaces oxygen in red blood cells, leading to serious illness or even death, especially in high concentrations. It is crucial to be aware of potential sources of carbon monoxide exposure, such as combustion appliances and vehicles, and to ensure proper ventilation and maintenance to mitigate the risks associated with this invisible pollutant.
The combustion of low-grade solid fuels and biofuels, such as those used in stoves or fireplaces, can generate high levels of carbon monoxide emissions. Unless the flue gases are vented outdoors through a chimney, the occupants may be at risk of carbon monoxide poisoning. Similarly, devices using high-grade fuels like natural gas, butane, or propane can also lead to lethal carbon monoxide intoxication if not properly maintained, vented, or if the air-to-fuel ratios are not correctly adjusted.
Carbon monoxide is a trace gas in the Earth's atmosphere and does not directly impact global temperatures. However, it plays a crucial role in atmospheric chemistry and affects the atmosphere's ability to cleanse itself of other polluting gases. Carbon monoxide also contributes to the formation of lower-atmospheric ozone and urban smog when combined with other pollutants and sunlight.
The concentration of carbon monoxide varies across different regions and seasons. For example, in Africa, seasonal shifts in carbon monoxide levels are associated with agricultural burning that moves north and south of the equator. In contrast, fires are a significant source of carbon monoxide pollution in the Amazon, Southeast Asia, and other regions of the Southern Hemisphere. In urban areas of the United States, Europe, and eastern China, vehicle and industrial emissions are the primary sources of carbon monoxide pollution.
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Nitrogen oxides: Irritate sensitive tissues of the airway, especially for those with asthma or emphysema
Invisible air pollutants are dangerous to human health. They cannot be seen or smelled, but when concentrated in large amounts in certain areas, they become visible as smog. One such invisible pollutant is nitrogen oxides, which irritate the sensitive tissues of the airway. Nitrogen oxides are a group of highly reactive gases, including nitrogen dioxide (NO2), nitrous acid, and nitric acid. NO2 is the most commonly referred to nitrogen oxide and is used as an indicator for the larger group.
Nitrogen oxides are irritant gases with low water solubility. Their low water solubility means they do not cause early mucus membrane irritation, which could otherwise prompt individuals to remove themselves from exposure. Without this warning, individuals may receive a larger cumulative inhaled dose of nitrogen dioxide, allowing the gas to travel deeper into the respiratory tract. This delayed reaction is why nitrogen oxides are particularly dangerous to those with pre-existing lung conditions such as asthma or emphysema.
The primary source of NO2 in the air is the burning of fuel, including emissions from cars, trucks, buses, power plants, and off-road equipment. When inhaled, NO2 oxidizes protective antioxidants in the epithelial lining fluid (ELF) of the airways, triggering extracellular damage. This reaction also causes oxidative stress, which can lead to edema, bronchoconstriction, and reduced forced expiratory volume. These effects can be particularly harmful to high-risk patients, including those with respiratory issues, heart failure, or the elderly, and can result in symptoms such as coughing, wheezing, or difficulty breathing.
Occupations that expose individuals to nitrogen oxides, such as working with agricultural grain silos, put employees at risk for toxicity. They may be unaware of ongoing exposures, and longer exposures allow higher concentrations of nitrogen oxides to reach the lower airways, causing delayed lower respiratory tract injuries. It is crucial for individuals exposed to high concentrations of nitrogen oxides to remove themselves from the source of exposure and seek medical attention if necessary.
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Sulfur dioxide: Irritating to humans and plants
Invisible air pollutants are among the most dangerous to human health. One such pollutant is sulfur dioxide (SO2), which is released into the atmosphere through the burning of fossil fuels by power plants and other industrial facilities. Natural sources such as volcanoes also emit sulfur dioxide.
SO2 is irritating and somewhat toxic to humans, particularly when inhaled in large quantities over a sustained period. It can affect the human respiratory system, making breathing difficult, and people with asthma are especially vulnerable to this effect. A study of 74,671 pregnant women in Beijing found that exposure to SO2 was the largest attributable risk factor for low birth weight. In addition, SO2 can react with other compounds in the atmosphere to form fine particles that reduce visibility (haze) and contribute to particulate matter (PM) pollution.
SO2 is also irritating to plants. At high concentrations, it can harm trees and other plants by damaging foliage and decreasing growth. It also contributes to acid rain, which can harm sensitive ecosystems. Deposition of particles can stain and damage stone and other materials, including culturally important objects such as statues and monuments.
To control sulfur emissions, various methods have been developed to remove sulfur from coal and fuels before burning, preventing the formation of SO2. The US EPA's Acid Rain Program has helped reduce emissions and improve air quality.
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Volatile organic compounds: Cause smog, with serious health effects
Volatile organic compounds (VOCs) are emitted as gases from certain solids or liquids. They include a variety of chemicals, some of which may have adverse health effects in the short and long term. VOCs are emitted by thousands of products, including paints, varnishes, waxes, cleaning products, disinfectants, cosmetics, degreasers, hobby products, and fuels. These products can release organic compounds during use and, to some degree, when stored.
The concentration of VOCs is consistently higher indoors than outdoors. The US EPA's Total Exposure Assessment Methodology (TEAM) Study found levels of common organic pollutants to be 2 to 5 times higher inside homes than outside. During activities such as paint stripping, levels can be 1,000 times higher than background outdoor levels.
The health effects of VOC exposure include eye, nose, and throat irritation, headaches, nausea, dizziness, and difficulty breathing. Long-term exposure can damage the liver, kidneys, and central nervous system, and some VOCs are linked to cancer. They may also worsen symptoms for people with asthma and COPD.
VOCs contribute to smog formation, which has serious health implications. Ground-level ozone, the principal component of smog, is created from chemical reactions of pollutants, VOCs, and NOx. To reduce VOC exposure, it is important to increase ventilation when using products that emit them and dispose of unwanted containers safely.
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Ground-level ozone: Created from chemical reactions of pollutants, VOCs, and NOx
Ground-level ozone is a harmful air pollutant and the main ingredient in smog. It is a colourless gas and a major component of air pollution. Unlike stratospheric ozone, which protects the Earth from excess UV radiation, ground-level ozone is harmful to human health.
Ground-level ozone is created by chemical reactions between NOx gases (oxides of nitrogen) and volatile organic compounds (VOCs). These chemicals, in the presence of sunlight, form ozone. The NOx gases are produced by combustion, and the VOCs are often emitted by cars, power plants, industrial boilers, refineries, and chemical plants. These sources of pollutants chemically react in the presence of sunlight to create ground-level ozone.
Ozone at ground level is not emitted directly into the air but is a secondary pollutant, formed by the reaction of other pollutants. The chemical reactions involved in ground-level ozone formation are complex cycles in which carbon monoxide and VOCs are oxidised to water vapour and carbon dioxide. The oxidation begins with the reaction of carbon monoxide with the hydroxyl radical. The radical intermediate formed reacts rapidly with oxygen to create a peroxy radical. These reactions occur in ambient air and can be estimated using a modified Leighton relationship.
Ground-level ozone is of particular concern as it can react with common indoor pollutants, skin oils, and other chemicals. 'Green' cleaning products based on citrus or terpene extracts can react with ozone to form toxic and irritating chemicals and fine particles. Ozone is most likely to reach unhealthy levels on hot, sunny days in urban environments, but it can also be transported by wind, affecting rural areas.
Ozone is one of the six common air pollutants identified in the Clean Air Act. These criteria air pollutants are designated by the EPA, and their levels in outdoor air are limited based on health criteria.
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Frequently asked questions
Invisible pollutants are air pollutants that cannot be seen or smelled. Some examples include nitrogen oxides, sulfur dioxide, volatile organic compounds (VOCs), and carbon monoxide. These pollutants can have serious health effects, such as irritating the sensitive tissues of the airway and, in the case of carbon monoxide, displacing oxygen in red blood cells, which can lead to death.
Invisible pollutants can have negative effects on the environment as well. For example, nitrogen oxides and sulfur dioxide can irritate plants. Additionally, when large amounts of invisible pollutants are concentrated in areas such as cities, they can be seen as smog, which is harmful to the environment.
One way to reduce the emissions of invisible gaseous pollutants is by increasing fuel efficiency. This means that less fuel needs to be burned to go the same distance in a vehicle or produce electricity in a power plant, resulting in fewer emissions.
