Air Pollution: Understanding Its Complex Components

Air pollution is a pressing issue that poses a significant threat to both human health and the environment. It refers to the contamination of indoor or outdoor air by various physical, biological, or chemical agents, which can have detrimental effects on living beings and the ecosystem. The major sources of outdoor air pollution include residential energy use, vehicles, power generation, agriculture, waste incineration, and industrial activities. These sources release a range of pollutants, including particulate matter (such as dust and smoke), carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. Additionally, the combustion of fossil fuels and the use of toxic products contribute to both indoor and outdoor air pollution, leading to respiratory and cardiovascular health issues. Recognizing the presence of invisible air pollutants, such as nitrogen oxides and sulfur dioxide, is crucial as they can be more harmful than visible pollutants. Addressing air pollution through policies and interventions is essential for mitigating climate change and improving public health.

Particulate matter (PM)

PM is classified based on the diameter of the particles. "Inhalable coarse particles" have diameters between 2.5 micrometers and 10 micrometers, while "fine particles" are 2.5 micrometers or smaller. Fine particles, also known as PM2.5, pose the greatest risk to health as they can be inhaled deeply into the lung tissue and contribute to severe health problems. They are 30 times thinner than a human hair, making them difficult to detect without an electron microscope. PM2.5 accounts for most health effects due to air pollution in the US and worldwide. Short-term exposures to PM2.5 have been linked to respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), leading to hospitalisations. Long-term exposure has been associated with premature death, particularly in individuals with pre-existing heart or lung conditions, and impaired lung development in children.

The sources of PM vary and are context-specific. Outdoor PM sources include residential energy for cooking and heating, vehicles, power generation, agriculture/waste incineration, and industry. Indoor sources include biological allergens like pollen, mould spores, dust mites, and cockroaches. Indoor activities such as smoking tobacco, cooking, and burning candles or incense also contribute to PM levels. Additionally, particles can form indoors from the complex reactions of gaseous pollutants emitted from household cleaning products and air fresheners.

The US Environmental Protection Agency (EPA) has implemented rules to reduce emissions of pollutants that form PM, aiming to improve air quality and protect public health. The Air Quality Index (AQI) is a useful tool that provides daily information on outdoor air quality and associated health risks, helping individuals take necessary precautions to safeguard their health.

Ground-level ozone

The concentration of ground-level ozone increases with height above sea level, with a maximum concentration at the tropopause. It accounts for about 10% of total ozone in the atmosphere, while the remaining 90% is in the stratosphere. Although ground-level ozone is less concentrated than stratospheric ozone, it is a concern due to its proximity to the Earth's surface and its health effects. As a greenhouse gas, ground-level ozone contributes to global warming and is the third most important greenhouse gas after CO2 and CH4.

To address the issue of ground-level ozone pollution, the United States Environmental Protection Agency (EPA) has implemented regulations to help states reduce ozone levels in outdoor air. The EPA designates areas as attainment or nonattainment based on air quality standards, and states must develop implementation plans to improve air quality in nonattainment areas. Additionally, the EPA provides air quality forecasts and notifications to the public through websites and mobile applications.

Carbon dioxide and other greenhouse gases

Carbon dioxide (CO2) is the most important greenhouse gas on Earth. It is a gas that absorbs and radiates heat. Unlike oxygen or nitrogen, which constitute most of Earth's atmosphere, greenhouse gases absorb heat from the Earth's surface and re-release it in all directions, including back toward the surface. This natural greenhouse effect is essential for maintaining the average global surface temperature above freezing. However, human activities have been adding more carbon dioxide to the atmosphere, amplifying the greenhouse effect and causing global temperatures to rise.

Carbon dioxide enters the atmosphere through the burning of fossil fuels (coal, natural gas, and oil), solid waste, trees, and other biological materials, as well as certain chemical reactions like cement production. Each year, human activities release more carbon dioxide into the atmosphere than natural processes can remove, leading to a net increase in atmospheric carbon dioxide. The annual rate of increase in atmospheric carbon dioxide over the past 60 years is about 100 times faster than previous natural increases. The global average carbon dioxide concentration reached a new record high of 419.3 parts per million in 2023, 50% higher than pre-Industrial Revolution levels.

The rise in carbon dioxide and other greenhouse gas concentrations is closely linked to the global temperature increase. According to observations, carbon dioxide alone was responsible for about two-thirds of the total heating influence of all human-produced greenhouse gases in 2021. This warming has been distributed unevenly across the globe, with the Northern Hemisphere experiencing more warming than the Southern Hemisphere, and the poles showing particularly strong warming. In some regions, temperatures have risen by more than 5°C.

In addition to carbon dioxide, other significant greenhouse gases include methane (CH4) and nitrous oxide. These gases are emitted during the production and transport of fossil fuels, agricultural practices, land use, and the decay of organic waste in landfills. Methane is also released during the combustion of fossil fuels and biomass, such as wood, for energy generation.

The impact of greenhouse gases on climate change is measured by their Global Warming Potential (GWP), which quantifies the amount of energy absorbed by emissions over a given period. Gases with higher GWP values contribute more to global warming. To stabilize or reduce atmospheric CO2 concentrations, a transition to net-zero emissions is necessary, involving transformations in energy systems, industries, and food production.

Nitrogen oxides

Nitrogen dioxide (NO2) is often used as an indicator for the presence of other nitrogen oxides. High levels of NO2 in the air can irritate the airways in the human respiratory system and aggravate respiratory diseases, especially asthma. Prolonged exposure to elevated levels of NO2 may even contribute to the development of asthma and increase susceptibility to respiratory infections. Fine particulate matter (PM2.5), the smallest and most harmful type of particle pollution, is formed from nitrogen oxides in combination with volatile organic compounds (VOCs), sulfur oxides, ammonia, and other particulates. This type of particle pollution can easily penetrate deep into the lungs, causing respiratory diseases like emphysema and bronchitis.

In addition to the direct health impacts of nitrogen oxides on the respiratory system, NOx in the atmosphere also has indirect effects on human health. For example, NOx contributes to nutrient pollution in coastal waters and the formation of ground-level ozone, or "bad" ozone. Ground-level ozone acts as a barrier that prevents pollution from escaping the lower atmosphere, leading to increased pollution in the air we breathe. This type of ozone is not typically emitted directly but is instead formed from chemical reactions between NOx and VOCs in the presence of sunlight and heat. Areas near urban centers tend to have higher levels of ground-level ozone due to increased emissions from cars, power plants, industrial boilers, refineries, and chemical plants.

Volatile organic compounds (VOCs)

Volatile organic compounds, or VOCs, are emitted from thousands of everyday products and can have serious health and environmental impacts. They are called organic because they contain carbon and they are considered volatile because they vaporize at or near room temperature. VOCs are emitted as gases from certain solids or liquids and are found in both indoor and outdoor air. Concentrations of VOCs indoors are often much higher than outdoors—up to 10 times higher according to the US EPA.

Sources of VOCs include paints and lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials including glues and adhesives, permanent markers, and photographic solutions. VOCs are also found in tobacco smoke, as well as in some fuels, such as gasoline and natural gas, which are major sources of VOCs released during combustion.

The health effects of VOCs can be severe. Breathing VOCs can irritate the eyes, nose, and throat, cause headaches and nausea, and worsen symptoms for people with asthma and COPD. Long-term exposure to VOCs can damage the liver and kidneys, and some VOCs are linked to cancer. VOCs may also react with other gases to form other air pollutants.

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. Keeping buildings smoke-free and ensuring effective ventilation systems are also important measures to reduce VOC concentrations indoors.

Policies and regulations have been implemented in various regions, such as the VOC Solvents Emissions Directive in the European Union, to address the reduction of industrial emissions of VOCs and limit their use in certain products.

Frequently asked questions