Air pollution is the presence of harmful substances in the air. It comes from vehicles, factories, power plants, farms, and cooking fires. The pollutants include particulate matter, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. Some are visible as smog. Others are invisible.

Health effects are severe. Fine particulate matter, known as PM2.5, penetrates deep into the lungs and bloodstream. It causes heart disease, stroke, lung cancer, and respiratory infections. The World Health Organization estimates that outdoor and household air pollution together cause about 7 million premature deaths each year. Most occur in low- and middle-income countries.

Solutions exist. Catalytic converters, scrubbers, and filters cut emissions at the source. Cleaner fuels and electric vehicles reduce tailpipe pollution. Zoning and public transit limit traffic. Indoor cooking with clean stoves saves lives. Cities like Beijing and Mexico City have improved air quality significantly, though the work is far from finished.

Common pollutants

• Particulate matter (PM2.5 and PM10)
• Ground-level ozone
• Nitrogen dioxide
• Sulfur dioxide
• Carbon monoxide