Air Pollutants: The Most Common Toxins Affecting Our Health

Air pollution is a pressing issue that significantly impacts human health and the environment. The most common air pollutants include ozone, carbon monoxide, sulfur dioxide, nitrogen oxides, particulate matter, and lead. These pollutants are known as criteria pollutants and are regulated by organizations like the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), which establish health-based ambient air quality standards to protect public health. Ozone, a key component of smog, forms when sunlight reacts with volatile organic compounds and nitrogen oxides. Carbon monoxide, a colorless and odorless gas, is produced by the incomplete combustion of fossil fuels. Sulfur dioxide, emitted from volcanic eruptions and industrial processes, contributes to acid rain. Nitrogen oxides, released from burning fossil fuels, play a role in the formation of ground-level ozone and smog. Particulate matter, such as PM2.5, consists of tiny particles that can penetrate deep into the lungs and cause health issues. Additionally, hazardous air pollutants like benzene, perchloroethylene, and asbestos pose serious health risks, including potential links to cancer. Addressing these air pollutants is crucial to safeguard human well-being and the planet we inhabit.

Particulate matter (PM2.5 and PM10)

Particulate matter is a mixture of various chemical species, composed of solids and aerosols. It is made up of small droplets of liquid, dry solid fragments, and solid cores with liquid coatings. These particles vary in size, shape, and chemical composition and may contain inorganic ions, metallic compounds, elemental carbon, organic compounds, and compounds from the earth's crust. The two most common types of particulate matter are PM2.5 and PM10, which are defined by their diameter.

PM10 refers to particulate matter that is 10 micrometres or less in diameter. These particles are inhalable and can enter the lungs, potentially causing adverse health effects. Sources of PM10 include industrial processes such as bulk material handling, combustion, and mineral processing in industries like brickworks, refineries, and steelmaking. Natural sources include bushfires, dust storms, pollen, and sea spray.

PM2.5, on the other hand, is fine particulate matter with a diameter of 2.5 micrometres or less. These particles are so small that they can be drawn deep into the lungs, increasing the risk of respiratory issues and other health problems. PM2.5 is primarily produced by the burning of fuels, especially in diesel-powered vehicles, and from industrial processes. It can also come from indoor sources such as tobacco smoke, cooking, and burning wood or incense.

The health effects of exposure to PM2.5 and PM10 can be severe, particularly for sensitive groups such as children, the elderly, and individuals with pre-existing heart or lung conditions. Long-term exposure to PM2.5 has been linked to premature death, reduced lung function growth in children, and increased hospitalisations for cardiovascular and respiratory diseases. PM10 exposure has also been associated with respiratory mortality and adverse effects on buildings and vegetation.

Particulate matter, especially PM2.5, can also affect visibility by altering how light is absorbed and scattered in the atmosphere. This phenomenon can impact both local aesthetics and industries such as solar energy production. Overall, particulate matter is a significant contributor to air pollution and has detrimental effects on human health, the environment, and various industries.

Ozone (O3) and nitrogen oxides (NOx)

Ozone is a highly reactive gas molecule composed of three oxygen atoms. It is primarily found in the Earth's upper atmosphere, where it forms a protective layer that shields us from the sun's harmful ultraviolet (UV) radiation. However, at ground level, ozone becomes a pollutant and a health hazard. Ground-level ozone is not directly emitted into the air but is formed by the reaction of volatile organic compounds (VOCs) and nitrogen oxides in the presence of sunlight. While ozone itself is not emitted directly, its precursor pollutants, such as nitrogen oxides, are released into the atmosphere through human activities and natural processes.

Nitrogen oxides (NOx) refer to a group of gases that contain nitrogen and oxygen, including nitric oxide (NO) and nitrogen dioxide (NO2). These gases are major contributors to air pollution and have harmful effects on human health and the environment. NOx gases are primarily produced during the combustion of fuels, especially in car engines and areas with high motor vehicle traffic, such as large cities. They are also formed naturally by lightning strikes, with the occurrence of lightning-induced NOx varying with season and geographic location.

The presence of NOx in the atmosphere can lead to the formation of smog, acid rain, and the depletion of ozone. NOx gases react with other compounds in the air, such as ammonia and moisture, to form nitric acid vapour and related particles. This nitric acid contributes to acid rain, which has detrimental effects on forests, crops, and soil health. Additionally, NOx plays a dual role in ozone formation and reduction. During the daytime and in the presence of sunlight, NO2 is a major precursor for the formation of ozone. However, during the night and in winter, NO becomes an important scavenger of ozone through a process called NOx titration.

The reduction of NOx emissions has been a focus of environmental efforts, and significant decreases in NOx precursors have been achieved in some regions. However, the relationship between NOx and O3 is complex, and reducing NOx emissions has not always led to proportionate decreases in O3 levels. This is partly due to the dual role of NOx in ozone formation and quenching, and other factors such as VOC reductions and meteorological conditions also influence ozone trends.

Ozone pollution has adverse effects on human health, causing damage to lung tissue and reducing lung function, especially in vulnerable populations such as children, the elderly, and people with asthma. Similarly, NOx exposure can trigger and exacerbate asthma symptoms and has been associated with respiratory issues and other health risks, including heart disease and diabetes.

Carbon monoxide (CO)

CO is produced whenever a material burns, and it is a key ingredient in many industrial chemistry processes. It is an important biological compound across phylogenetic kingdoms and is produced by many organisms, including humans. In mammalian physiology, carbon monoxide acts as a gasotransmitter at low concentrations, while high concentrations can be toxic, resulting in carbon monoxide poisoning.

Carbon monoxide is a common indoor air pollutant, often caused by the use of biomass, such as wood, for cooking and heating. Homes with fuel-burning appliances, attached garages, or tobacco smoke are more likely to have elevated CO levels. Common sources of CO indoors include fuel-burning appliances such as clothes dryers, water heaters, furnaces, fireplaces, gas stoves, and ovens. CO can also enter the home from attached garages, especially if vehicles are left running inside. Additionally, unvented kerosene and gas space heaters, leaking chimneys, and back-drafting from furnaces can contribute to indoor CO levels.

The danger of carbon monoxide lies in its ability to displace oxygen in the human body, leading to poisoning. As CO is undetectable by human senses, it can quickly build up to dangerous levels without being noticed. The early signs of CO poisoning may be mild, such as a headache and breathlessness, and are often mistaken for the flu. However, as exposure continues, individuals may experience more severe symptoms, including angina, impaired vision, and reduced brain function. At very high concentrations, carbon monoxide exposure can be fatal.

To prevent carbon monoxide poisoning, it is recommended to install CO alarms and properly maintain fuel-burning appliances. It is also crucial to ensure proper ventilation when using items such as camp stoves, charcoal grills, and portable generators, as they can produce high levels of CO.

Volatile organic compounds (VOCs)

Volatile organic compounds, or VOCs, are gases emitted from solids or liquids that contain certain organic chemicals. VOCs are released into the air from a wide array of products numbering in the thousands, including paints, varnishes, waxes, cleaning products, disinfectants, cosmetics, degreasers, hobby products, and fuels. They are also found in tobacco smoke and can be emitted by plants, animals, or microorganisms. Concentrations of VOCs are often much higher indoors than outdoors, and they can have serious short-term and long-term health effects.

Immediate symptoms of VOC exposure include eye, nose, and throat irritation, headaches, nausea, dizziness, and difficulty breathing. Long-term exposure to VOCs has been linked to liver and kidney damage, central nervous system issues, and cancer. Some VOCs are known to cause cancer in animals, and some are suspected or known to cause cancer in humans.

To reduce exposure to VOCs, it is recommended to read product labels, avoid or limit the use of items with harmful ingredients, safely dispose of unwanted products, and increase ventilation when using products containing VOCs. It is also advised to keep buildings smoke-free, open windows, and use fans to pull indoor air outside when using products with high VOCs. Additionally, new carpets or building products should be aired outside before installation, and products with VOCs should not be stored indoors.

VOCs are regulated in various ways worldwide. For example, the VOC Solvents Emissions Directive in the European Union aims to reduce industrial emissions of VOCs, while the U.S. Environmental Protection Agency (EPA) focuses on controlling VOCs as precursors of photochemical smog. However, there are currently no federally enforceable standards for VOCs in non-industrial settings in the United States.

Polycyclic aromatic hydrocarbons (PAHs)

The detection of PAHs in materials is often done using gas chromatography-mass spectrometry or liquid chromatography with ultraviolet, visible, or fluorescence spectroscopy. The most common subclass of PAOM (polycyclic organic matter) is the polycyclic aromatic hydrocarbons. PAHs with low molecular weights occur in the atmosphere mainly in the vapour phase, while those with multiple rings are typically bound to particles. Particle-bound PAHs are considered very hazardous to human health.

The geometry of PAHs is a result of the merger of sp2 hybrid orbitals of adjacent carbons, which lie in the same plane as the carbon atom. This geometry is characteristic of the benzenoid subclass of PAHs, which includes compounds like phenanthrene, anthracene, and coronene. The aromaticity of PAHs can vary, and according to Clar's rule, the resonance structure with the largest number of disjoint aromatic pi sextets is the most important for characterizing the properties of a given PAH.

PAHs have been detected in both indoor and outdoor air, with higher concentrations found in Asia, Africa, and Latin America compared to other regions. Sources of indoor PAH pollution include diesel engine emissions, incense burning, candle emissions, and cooking and heating with biomass. PAHs are also present in outdoor air due to industrial processes, the burning of fossil fuels, waste management, and agriculture.

Frequently asked questions