Air pollution refers to the contamination of the atmosphere by substances that are harmful to human health and the surrounding environment. These substances may be gases, solid particles, or liquid droplets. The air we breathe every day is composed of various chemical substances, with nitrogen being the most abundant element (about 78%), followed by oxygen (about 21%), along with trace gases like argon and carbon dioxide. The composition of each gas is not entirely fixed and can vary slightly over time. If the concentration of pollutants in the air remains low, the impact on human health and the environment will be relatively minor; however, when these pollutants reach dangerous levels, we must find ways to reduce or remove them from the air.

Air pollution can be broadly divided into three main categories: chemical pollution (gases and vapors like carbon monoxide or nitrogen oxides), biological pollution (pollen, mold, and airborne pathogens), and particulate matter pollution (PM10 and fine PM2.5 particles). While noise, heat, radiation, and light pollution are sometimes mentioned alongside atmospheric issues, modern environmental science classifies these specifically as physical hazards rather than air contaminants. Generally, the causes of air pollution can be categorized into natural and human-made sources.

• Natural sources of air pollution include:
• Volcanic activity releasing sulfur dioxide and ash
• Dust and sand blown from desert regions or areas lacking vegetation
• Gases emitted by biological processes, such as methane released by livestock during digestion
• Smoke and fine particulate matter produced by wildfires

Human-made (anthropogenic) air pollution is typically caused by industrial processes and the combustion of fossil fuels, which result from activities such as:

• Dust, ammonia, and chemical residues from agricultural operations
• General manufacturing and industrial activities
• Vehicles powered by internal combustion engines
• Emissions from coal, oil, and gas power generation
• Paints and volatile organic solvents (VOCs)
• Commercial and industrial aerosols

Air pollution is not limited to outdoor environments; indoor air can also become polluted—with concentrations often reaching levels 2 to 5 times higher than outdoors according to the U.S. Environmental Protection Agency—although the specific mechanisms and sources of indoor pollution differ.

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