Air pollutants can broadly be classified into gaseous pollutants and particulate matter. In reality, air pollution is a complex mixture of gases, particles, and other substances, and pollutants can affect both outdoor and indoor air. Among the pollutants with the strongest evidence of adverse effects on human health are particulate matter, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide.

Common gaseous pollutants include sulfur dioxide (SO₂) and nitrogen dioxide (NO₂), which are associated with the combustion of fossil fuels and industrial processes. Nitrogen oxides and sulfur dioxide can contribute to the formation of acid deposition. Other important gaseous pollutants include carbon monoxide (CO), a toxic gas produced by incomplete combustion; ground-level ozone (O₃), a major component of photochemical smog; and various volatile organic compounds (VOCs), some of which are toxic or contribute to the formation of ozone.

Carbon dioxide (CO₂) is a greenhouse gas and an important driver of climate change, although it is not generally classified as a conventional toxic air pollutant at the concentrations normally found outdoors. Chlorofluorocarbons (CFCs) are also powerful greenhouse gases and, historically, were widely used as refrigerants and in other applications. They are particularly important environmentally because they also destroy stratospheric ozone. International controls under the Montreal Protocol have greatly reduced their production and use.

Ozone requires particular attention because its effects depend strongly on where it is found. Ground-level, or tropospheric, ozone is an air pollutant that can irritate and damage the respiratory system and is an important component of smog. In contrast, most atmospheric ozone is found in the stratosphere, where the ozone layer absorbs harmful ultraviolet radiation from the Sun and protects human health and ecosystems.

Airborne particulate matter (PM) consists of tiny solid particles and liquid droplets suspended in the air. It can come from sources such as vehicle and industrial emissions, combustion of fuels and biomass, wildfires, construction, road dust, and natural sources. Particle size is an important factor in determining where particles can travel in the respiratory system and what health effects they may cause.

Two commonly monitored categories are PM10 and PM2.5. PM10 refers to particles with an aerodynamic diameter of 10 micrometers or less, while PM2.5 refers to particles with an aerodynamic diameter of 2.5 micrometers or less. PM10 can penetrate into the respiratory tract and reach deep into the lungs. PM2.5 is small enough to penetrate particularly deeply into the lungs, and exposure is associated with cardiovascular and respiratory disease as well as premature mortality.

The smaller size of PM2.5 does not mean that every particle simply passes through the lungs and enters the bloodstream. Rather, some components or particles can cross the lung barrier and enter the circulation, and fine particulate pollution can produce effects throughout the body. Current scientific evidence shows that exposure to PM2.5 is associated with cardiovascular and respiratory effects, and the World Health Organization considers fine particulate matter to be a major public-health concern.

It is also important not to confuse particle size with the chemical composition of the particle. For example, inhalation of crystalline silica dust can cause silicosis, a serious and potentially permanent lung disease. This is different from “selenium-containing dust”; selenium is not the substance responsible for silicosis. Occupational exposure to respirable crystalline silica is therefore an important concern in industries and activities that generate fine silica-containing dust.

Tobacco smoke is another important source of airborne particulate matter. Cigarette smoke contains a complex mixture of gases and extremely small particles, including fine and ultrafine particles. Second-hand tobacco smoke can also contribute substantially to indoor air pollution. Exposure to tobacco smoke is associated with numerous serious health effects, including cardiovascular and respiratory disease and lung cancer. The health risks from tobacco smoke result from its many chemical constituents as well as its particulate component.

Air-pollution science has also advanced considerably since this article was originally written. It is now clear that health effects can occur from both short- and long-term exposure and that there is not necessarily a single concentration below which every pollutant is harmless. Consequently, modern air-quality guidelines place particular emphasis on reducing exposure to PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide rather than simply classifying air as either “polluted” or “clean.”

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026