Air Pollutants: Most Common Toxins By Quantity

Air pollution is a pressing issue that poses significant risks to both human health and the environment. Among the various air pollutants, some of the most prevalent and harmful substances include particulate matter (PM), carbon monoxide (CO), ground-level ozone (O3), nitrogen dioxide (NO2), and sulphur dioxide (SO2). These pollutants are released into the atmosphere through a range of human activities and natural processes, and their impact on air quality and public health is well documented. This introduction will discuss the primary sources, health effects, and regulatory considerations of these top air pollutants, providing a comprehensive overview of their quantity and impact.

Ground-level ozone

Ozone, a colourless gas composed of three oxygen atoms, exists both in the upper atmosphere and at ground level. While stratospheric ozone is beneficial as it shields us from the sun's harmful ultraviolet rays, ground-level ozone is detrimental. It can trigger various health issues, especially for vulnerable individuals such as children, the elderly, and those with lung diseases like asthma. Exposure to ground-level ozone has been linked to premature mortality and an increased number of hospital admissions for respiratory problems.

The Environmental Protection Agency (EPA) in the United States has designated ground-level ozone as one of the six "criteria" air pollutants that it regulates. These criteria pollutants are managed using human health-based and environmentally based guidelines. To improve air quality, the EPA works with states and tribes to monitor air quality and designate areas as attainment or nonattainment based on national ambient air quality standards. Nonattainment areas must develop state implementation plans (SIPs) to outline measures for improving air quality and reducing ground-level ozone.

Particle pollution

There are two types of particle pollution: primary and secondary particles. Primary particles are emitted directly from sources such as construction sites, unpaved roads, smokestacks, or fires. Secondary particles form through chemical reactions involving chemicals like sulfur and nitrogen oxides emitted from power plants, industries, and automobiles.

Nitrogen oxides

Nitrogen dioxide (NO2) is one of the most common nitrogen oxides and is considered a primary pollutant. It is a reddish-brown gas with a foul smell, soluble in water, and a strong oxidant. NO2 is emitted from vehicles, trucks, and other machinery that burn fuel, as well as power plants and other fuel-combusting equipment. It can also be formed when NOx reacts with other pollutants in the presence of sunlight.

The health effects of nitrogen dioxide are well documented. Short-term exposure can irritate the airways and aggravate respiratory diseases, especially asthma. Longer exposure to elevated NO2 concentrations may contribute to the development of asthma and increase susceptibility to respiratory infections. High levels of NO2 can cause inflammation of the airways, leading to respiratory issues such as wheezing, coughing, and bronchitis. Prolonged exposure can result in irreversible damage to the respiratory system.

In addition to NO2, other nitrogen oxides include nitric oxide (NO), nitrous oxide (N2O), nitrous acid (HONO), and nitric acid. While NO is not considered hazardous at typical ambient conditions, excess amounts can lead to respiratory ailments, hematologic side effects, metabolic disorders, low blood pressure, nausea, vomiting, and diarrhoea. Nitrous oxide, on the other hand, is a greenhouse gas with significant anthropogenic sources, while nitrous acid is a common pollutant in ambient and indoor environments, produced by the reaction of nitrogen dioxide with water.

Carbon monoxide

Indoor sources of CO include unvented kerosene and gas space heaters, leaking chimneys and furnaces, gas stoves, wood stoves, fireplaces, malfunctioning or improperly vented gas appliances (such as water heaters, furnaces, and clothes dryers), tobacco smoke, and car idling in attached garages. The highest levels of indoor CO typically occur during the colder months when inversion conditions trap air pollution near the ground.

In terms of its contribution to climate change, carbon monoxide participates in chemical reactions in the atmosphere that produce ozone, a potent greenhouse gas and air pollutant. CO is classified as a short-lived climate forcing agent, and reducing CO emissions is considered a potential strategy to mitigate the effects of global warming. While indoor CO levels can be higher than outdoors, elevated outdoor CO levels are still a concern, especially for individuals with heart disease.

Volatile organic compounds

VOCs are defined by their volatility, which is indicated by their vapor pressure. Volatility is the tendency of a substance to vaporize or the speed at which it vaporizes. VOCs with higher vapor pressure will vaporize more readily at a given temperature than substances with lower vapor pressure. The higher the volatility (or the lower the boiling point), the more likely the compound will be emitted from a product or surface into the air.

VOCs are released into the air from a wide range of solvent-using activities, such as printing, surface cleaning, vehicle coating, dry cleaning, and the manufacture of footwear and pharmaceutical products. VOCs are also found in many products used daily, including home cleaning products, building materials, personal care products, and outdoor sources that can enter homes.

Breathing in VOCs can cause eye, nose, and throat irritation, headaches, nausea, dizziness, and difficulty breathing. Long-term exposure can lead to more severe health issues, including damage to the liver, kidneys, and central nervous system. Some VOCs are linked to cancer and can worsen symptoms for people with asthma and COPD.

Regulations and directives have been implemented to reduce VOC emissions and protect human health. For example, the VOC Solvents Emissions Directive in the European Union aims to reduce industrial emissions of VOCs, while the Occupational Safety and Health Administration (OSHA) in the United States regulates VOC exposure in the workplace.

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