Air Pollution: Classifying The Invisible Threat

Air pollution is defined as the presence of harmful substances in the Earth's atmosphere, which can include gases, dust, and smoke. These pollutants can be released into the atmosphere through various anthropogenic processes, such as vehicles, industries, and construction, or natural sources like pollen and volcanic eruptions. The classification of air pollutants is essential for understanding and managing air quality. The US EPA, for instance, categorizes pollutants into Common Air Pollutants and Hazardous Air Pollutants, with the former being further divided into six criteria air pollutants that are regulated by the National Ambient Air Quality Standards. These criteria pollutants include particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Hazardous Air Pollutants, on the other hand, are known to cause serious health and environmental issues, including cancer and reproductive problems.

Physical composition: Solid, liquid or gaseous

Air pollution is classified into two main categories based on physical composition: solid matter and gaseous particles.

Solid Particles

Solid particles are formed by the suspension of extremely small solid or liquid particles called "particulates" in the air. These particles are less than 10 micrometres (μm) in size and are significant air pollutants due to their harmful effects on human health. Examples of solid particles include soot, dust, smoke, fumes, and mists. Particulate matter (PM) is a common term used to describe these solid particles, and it varies in size and composition. Ultrafine to coarse particles make up PM, with black carbon being one of the most toxic variants. PM 2.5 particles, which are 2.5 μm or less in diameter, are considered highly harmful as they can be inhaled deep into the lungs, causing respiratory issues, irritation, inflammation, and pneumoconiosis.

Gaseous Particles

Gaseous particles, on the other hand, refer to various gases released into the atmosphere. These include ground-level ozone, nitrogen dioxide, and volatile organic compounds (VOCs). Ground-level ozone, a major secondary air pollutant, forms when volatile organic compounds (VOCs) and nitrous oxides (NOx) interact with sunlight and heat. These primary pollutants can be transported over long distances by wind, affecting even rural areas. Gaseous particles also contribute to global warming, as an increase or decrease in the composition of certain gases in the atmosphere can lead to an imbalance, resulting in a rise in Earth's temperature.

Health and Environmental Impact

Both solid and gaseous particles have detrimental effects on human health and the environment. Solid particles, such as lead fumes, can be toxic, especially when inhaled. Gaseous particles, including nitrogen dioxide and ground-level ozone, are known to cause respiratory issues and environmental damage. Additionally, the interaction between different pollutants, wind, and climatic factors can lead to complex and dynamic changes in air quality, making it crucial to monitor and regulate these pollutants to minimize their negative impacts.

Regulatory Measures

Regulatory bodies, such as the US EPA, play a crucial role in managing air quality by setting standards and regulations for common and hazardous air pollutants. The Clean Air Act, for example, mandates the EPA to establish National Ambient Air Quality Standards (NAAQS) for six common air pollutants, including particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These standards aim to protect human health and the environment by limiting the concentrations of these pollutants in the atmosphere.

Sources: Anthropogenic or natural

Air pollution sources can be classified as natural or anthropogenic. Natural sources include wildfires, windblown sand or dust, and ash from volcanoes. While these sources can be significant, they usually do not create ongoing air pollution problems. For example, smoke from wildfires can reduce visibility and create haze, but it is typically a temporary issue.

Anthropogenic sources, on the other hand, are of greater concern as they are increasing in number and concentration due to the growing global population and rising energy demands. These sources include the burning of fossil fuels such as coal, natural gas, and oil for energy production, transportation, and industrial processes. The major sources of anthropogenic air pollution are stationary sources, like power plants, and mobile sources, like automobiles. Stationary sources emit large amounts of pollution from a single location, while mobile sources, including cars, trucks, planes, and trains, collectively account for more than half of all air pollution in the United States.

In addition to the energy sector, other significant anthropogenic sources include industrial processes, such as metallurgy, construction, and oil and gas development, which release pollutants like ozone and nitrogen dioxide. The manufacturing and use of consumer products also contribute to air pollution. For example, chlorofluorocarbons (CFCs) used in refrigerators and aerosol sprays deplete the ozone layer, and the burning of plastics emits polychlorinated biphenyls (PCBs).

Indoor air pollution is another concern, with sources such as secondhand cigarette smoke, radon gas, and toxic mould. Overall, anthropogenic sources of air pollution have led to a modification of the air quality beyond the limits stipulated in legislation, causing adverse effects on humans and other living organisms.

Health impact: Toxicity and health effects

Air pollution is a mix of hazardous substances from both human-made and natural sources. It is a major threat to global health and prosperity and is the single largest environmental health risk in Europe. In 2019, it was the largest environmental risk to health globally. It is responsible for more than 6.5 million deaths each year, a number that has increased over the past two decades.

The main pathway of exposure from air pollution is through the respiratory tract. Pollutants are breathed in, leading to inflammation, oxidative stress, immunosuppression, and mutagenicity in cells throughout the body, impacting the lungs, heart, and brain, among other organs. Almost every organ in the body can be impacted by air pollution. Some pollutants are small enough to penetrate into the bloodstream via the lungs and circulate throughout the entire body, leading to systemic inflammation and carcinogenicity.

Pollutants have differences in physical and chemical properties, which leads to discrepancies in their capacity for producing toxic effects. For example, aerosol compounds have greater toxicity than gaseous compounds due to their tiny size in the atmosphere; they have a greater penetration capacity. Gaseous compounds are eliminated more easily by our respiratory system. These particles are able to damage the lungs and can even enter the bloodstream, leading to the premature deaths of millions of people yearly.

Fine particulate matter is an especially important source of health risks, as these very small particles can penetrate deep into the lungs, enter the bloodstream, and travel to organs, causing systemic damage to tissues and cells. Particulate matter (PM) is composed of chemicals such as sulfates, nitrates, carbon, or mineral dusts. Vehicle and industrial emissions from fossil fuel combustion, cigarette smoke, and burning organic matter, such as wildfires, all contain PM. A subset of PM, fine particulate matter (PM 2.5), is 30 times thinner than a human hair. It can be inhaled deeply into lung tissue and contribute to serious health problems. PM2.5 accounts for most health effects due to air pollution in the US.

Ozone, an atmospheric gas, is often called smog when at ground level. It is created when pollutants emitted by cars, power plants, industrial boilers, refineries, and other sources chemically react in the presence of sunlight. Hundreds of studies have confirmed that ozone harms people at levels currently found in many parts of the United States.

Air pollution exposure is associated with oxidative stress and inflammation in human cells, which may lay the foundation for chronic diseases and cancer. In 2013, the International Agency for Research on Cancer of the World Health Organization (WHO) classified air pollution as a human carcinogen. Many studies have established that short-term exposure to higher levels of outdoor air pollution is associated with reduced lung function, asthma, cardiac problems, emergency department visits, and hospital admissions. Mortality rates related to air pollution are also a concern. Exposure to the air pollutant PM2.5 is associated with an increased risk of death.

Both short- and long-term exposure to air pollution can lead to a wide range of diseases, including stroke, chronic obstructive pulmonary disease, trachea, bronchus and lung cancers, aggravated asthma, and lower respiratory infections. The World Health Organization (WHO) provides evidence of links between exposure to air pollution and type 2 diabetes, obesity, systemic inflammation, Alzheimer’s disease, and dementia. Maternal exposure to air pollution is associated with adverse birth outcomes, such as low birth weight, pre-term birth, and small for gestational age births. A growing body of evidence also suggests that air pollution may affect diabetes and neurological development in children.

Regulation: Common or hazardous

The US EPA broadly classifies air pollutants into two categories: Common Air Pollutants and Hazardous Air Pollutants.

Common Air Pollutants

Common Air Pollutants are a group of six pollutants that are regulated by the National Ambient Air Quality Standards (NAAQS). These pollutants, also known as "criteria pollutants", are found all over the United States and can harm human health, damage the environment, and cause property damage. The six common air pollutants are:

• Particulate matter (particle pollution)
• Ground-level ozone
• Carbon monoxide
• Sulfur dioxide
• Nitrogen dioxide
• Lead

Particulate matter is made up of solid particles that vary in size and composition. These particles can come from anthropogenic processes such as vehicles, industries, and construction, or natural sources like pollen and volcanic eruptions. Particle pollution is one of the most widespread health threats, as the tiny particles can be inhaled deep into the lungs, causing respiratory problems, irritation, and inflammation.

Hazardous Air Pollutants (HAPs)

Also known as toxic air pollutants, HAPs are known or suspected to cause serious health effects, including cancer, reproductive issues, birth defects, and adverse environmental effects. These pollutants are regulated in the United States through amendments to the Clean Air Act. Some examples of HAPs include:

• Benzene
• Dioxins
• Asbestos
• Toluene
• Heavy metals such as cadmium, mercury, chromium, and lead compounds

In addition to the composition of the pollutants, air pollution can also be classified by its physical state. Pollutants can be composed of solid matter or gaseous particles. Gaseous pollutants include ground-level ozone, nitrogen dioxide, and volatile organic compounds (VOCs).

Pollutant type: Particulate or gaseous

Air pollutants can be composed of solid matter or gaseous particles. Particulate matter (PM) refers to a mixture of solid particles and liquid droplets found in the air. These particles vary in size, ranging from ultrafine to coarse, and in composition, including toxic substances like black carbon. PM2.5, fine inhalable particles with diameters of 2.5 micrometres or less, pose the greatest risk to human health as they can penetrate deep into the lungs and even enter the bloodstream. Sources of PM2.5 include combustion of fuels, industrial activities, and vehicle emissions.

The health risks associated with PM are well-documented. Short-term exposure to PM2.5 has been linked to increased hospital admissions for cardiovascular and respiratory issues, including asthma attacks and bronchitis. Long-term exposure has been associated with premature mortality and adverse perinatal outcomes. PM10, particles with diameters of 10 micrometres or less, can also induce adverse health effects, particularly for those with pre-existing respiratory conditions.

The USEPA classifies pollutants into Common Air Pollutants and Hazardous Air Pollutants (HAPs). Particulate matter, along with ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead, are considered Common Air Pollutants. HAPs, on the other hand, are known or suspected to cause serious health and environmental issues, including cancer and reproductive problems. Examples of HAPs include benzene, asbestos, mercury, and lead compounds.

While particulate matter is a significant concern, it is important to note that gaseous pollutants also play a role in air quality. Ground-level ozone, nitrogen dioxide, and volatile organic compounds (VOCs) are examples of gaseous air pollutants. These gaseous forms can interact with particulate matter and climatic factors like wind to influence the development of air pollution in specific areas. Therefore, understanding the interplay between particulate and gaseous pollutants is crucial for effective air quality management.

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