Tiny solid or liquid particles that are suspended in air. Particulate matter is produced from a wide variety of sources like natural and human-caused, large and small. They are comprised of directly emitted particles, and secondary particles formed in the atmosphere through interactions of directly emitted pollutants such as sulphur oxides, nitrogen oxides, ammonia, and volatile organic compounds.
Wood and fossil-fuel burning are the two main human-caused sources. Particulate matter that is 10 Micron in diameter or less called PM10. Of major concern are particles that are 2.5 micrometres or smaller in diameter (PM2.5) because they can lodge deep in the lungs, and cause respiratory and cardiac problems.