Air Pollution: Understanding The Different Types Of Contaminants

Air pollution is the contamination of the atmosphere by chemical, physical, or biological agents, which can have detrimental effects on human health and the planet. The World Health Organization (WHO) reports that 99% of the global population breathes air that exceeds the recommended guideline limits for pollutants, with low- and middle-income countries suffering the most. The main sources of air pollution include household combustion devices, motor vehicles, industrial facilities, and forest fires. The pollutants of major public health concern include particulate matter, carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. These pollutants can lead to respiratory and other diseases, causing morbidity and mortality.

Particulate matter (PM)

PM10 and PM2.5 often derive from different emissions sources and have different chemical compositions. Emissions from the combustion of gasoline, oil, diesel fuel, or wood produce much of the PM2.5 pollution found in outdoor air, as well as a significant proportion of PM10. PM10 also includes dust from construction sites, landfills, agriculture, wildfires, brush/waste burning, industrial sources, wind-blown dust from open lands, pollen, and fragments of bacteria.

PM may be directly emitted from sources (primary particles) or formed in the atmosphere through chemical reactions of gases (secondary particles) such as sulfur dioxide (SO2), nitrogen oxides (NOX), and certain organic compounds. These organic compounds can be emitted by both natural sources, such as trees and vegetation, and from man-made (anthropogenic) sources, such as industrial processes and motor vehicle exhaust.

Both PM2.5 and PM10 can be inhaled, with some depositing throughout the airways. The locations of particle deposition in the lung depend on particle size. PM2.5 is more likely to travel into and deposit on the surface of the deeper parts of the lung, while PM10 is more likely to deposit on the surfaces of the larger airways of the upper region of the lung. Particles deposited on the lung surface can induce tissue damage and lung inflammation.

A number of adverse health impacts have been associated with exposure to both PM2.5 and PM10. For PM2.5, short-term exposures (up to 24 hours) have been linked to premature mortality, increased hospital admissions for heart or lung causes, acute and chronic bronchitis, asthma attacks, emergency room visits, respiratory symptoms, and restricted activity days. These adverse health effects have been reported primarily in infants, children, and older adults with pre-existing heart or lung diseases. In addition, of all the common air pollutants, PM2.5 is associated with the greatest proportion of adverse health effects related to air pollution, both in the United States and worldwide, based on the World Health Organization’s Global Burden of Disease Project.

Short-term exposures to PM10 have been associated primarily with the worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), leading to hospitalization and emergency department visits. Long-term (months to years) exposure to PM2.5 has been linked to premature death, particularly in people with chronic heart or lung diseases, and reduced lung function growth in children. The effects of long-term exposure to PM10 are less clear, although several studies suggest a link between long-term PM10 exposure and respiratory mortality. The International Agency for Research on Cancer (IARC) published a review in 2015 that concluded that particulate matter in outdoor air pollution causes lung cancer.

Particulate matter has been shown in many scientific studies to reduce visibility and adversely affect climate, ecosystems, and materials. PM, primarily PM2.5, affects visibility by altering the way light is absorbed and scattered in the atmosphere. With reference to climate change, some constituents of the ambient PM mixture promote climate warming (e.g., black carbon), while others have a cooling influence (e.g., nitrate and sulfate). PM can adversely affect ecosystems, including plants, soil, and water through deposition and its subsequent uptake by plants or its deposition into water, affecting water quality and clarity. The metal and organic compounds in PM have the greatest potential to alter plant growth and yield. PM deposition on surfaces leads to soiling of materials.

Carbon monoxide (CO)

CO is harmful because it binds to haemoglobin in the blood, reducing the blood's ability to carry oxygen. This interferes with oxygen delivery to the body's organs, including the heart and brain. The most common effects of CO exposure are fatigue, headaches, confusion, and dizziness due to inadequate oxygen delivery to the brain. For people with cardiovascular disease, short-term CO exposure can further reduce their body's already compromised ability to respond to the increased oxygen demands of exercise, exertion, or stress. Inadequate oxygen delivery to the heart muscle leads to chest pain and decreased exercise tolerance. Unborn babies whose mothers experience high levels of CO exposure during pregnancy are at risk of adverse developmental effects.

People with heart disease are especially vulnerable to the effects of CO. They already have a reduced ability to get oxygenated blood to their hearts in situations where the heart needs more oxygen than usual. In these situations, short-term exposure to elevated CO may result in reduced oxygen to the heart, accompanied by chest pain, also known as angina. Even healthy people can be affected by high levels of CO, potentially developing vision problems, reduced ability to work or learn, reduced manual dexterity, and difficulty performing complex tasks. At very high levels, CO is poisonous and can cause death.

Indoor sources of CO include gas stoves, leaking chimneys and furnaces, unvented gas and kerosene space heaters, malfunctioning or improperly vented gas appliances (such as water heaters, furnaces, and clothes dryers), fireplaces, tobacco smoke, and car or truck exhaust that enters from attached garages. Indoor CO levels can be considerably higher than outdoors, especially during the colder months of the year when inversion conditions are more frequent. During the cold season, CO poisoning cases tend to increase due to the increased use of improperly vented space heaters and gas ranges to heat homes.

Ozone (O3)

Ground-level ozone is a major component of smog and is considered one of the most dangerous and widespread pollutants in the United States. It can cause serious health issues, especially for individuals with pre-existing respiratory conditions like asthma. When inhaled, ozone chemically reacts with lung tissue, causing respiratory problems and accumulating damage over time. This damage can lead to increased susceptibility to respiratory infections, pulmonary inflammation, and even premature death when combined with other risk factors.

The presence of ground-level ozone is particularly concerning during hot and sunny weather, as it is more likely to reach unhealthy levels under these conditions. Climate change, by driving warmer temperatures, is contributing to increased levels of ground-level ozone in many regions. Additionally, ozone can be transported by wind over long distances, affecting even rural areas far from the sources of pollutant gases.

To address the issue of ground-level ozone, policies and regulations, such as the Clean Air Act in the United States, have been implemented to reduce emissions and mitigate their harmful effects. These efforts have led to improvements in air quality, but there is still a significant portion of the population breathing unhealthy levels of ozone. Individual actions, such as reducing vehicle usage and supporting clean energy initiatives, can also contribute to lowering ground-level ozone pollution.

In summary, ground-level ozone (O3) is a harmful air pollutant that poses significant risks to human health and the environment. Its formation through the interaction of human-induced emissions and natural factors underscores the importance of ongoing efforts to reduce ozone pollution and protect public health.

Nitrogen dioxide (NO2)

Nitrogen dioxide primarily gets into the air from the burning of fuel. NO2 forms from emissions from cars, trucks, and buses, power plants, and off-road equipment. Road traffic is the principal outdoor source of nitrogen dioxide. The combustion of fossil fuels such as coal, oil, methane gas (natural gas), and diesel are also sources of NO2.

Breathing air with a high concentration of NO2 can irritate the airways in the human respiratory system. Exposures over short periods can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms such as coughing, wheezing, or difficulty breathing. Longer exposures to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. People with asthma, as well as children and the elderly, are generally at greater risk for the health effects of NO2.

NO2 and other NOx react with other chemicals in the air to form both particulate matter and ozone, which are harmful when inhaled due to their effects on the respiratory system. NOx in the atmosphere also contributes to nutrient pollution in coastal waters and the formation of acid rain, which harms sensitive ecosystems such as lakes and forests.

Sulphur dioxide (SO2)

SO2 can affect both human health and the environment. Short-term exposure to SO2 can harm the human respiratory system and make breathing difficult. People with asthma, particularly children, are sensitive to these effects of SO2. High concentrations of SO2 in the air can also lead to the formation of other sulphur oxides (SOx), which can react with other compounds in the atmosphere to form small particles. These particles contribute to particulate matter (PM) pollution, which may penetrate deeply into the lungs and, in sufficient quantities, can cause health problems.

At high concentrations, gaseous SOx can harm trees and plants by damaging foliage and decreasing growth. SO2 and other sulphur oxides can also contribute to acid rain, which can harm sensitive ecosystems. They can react with other compounds in the atmosphere to form fine particles that reduce visibility (haze) in parts of the world, including many national parks and wilderness areas. The deposition of particles can also stain and damage stone and other materials, including culturally important objects such as statues and monuments.

Total SO2 emissions have decreased substantially due to changes in fuel use and international agreements such as the UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP) and the EU "Sulphur Protocols". The UK, for example, has seen substantial reductions in SO2 emissions due to the decline of heavy industry from the late 1970s to the 1990s, as well as the switch from coal to gas in the domestic, industrial and electricity-generating sectors.

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