Urban air pollution overview
Cities concentrate both people and the activities that sustain them: transport, industry, energy production, construction. Each of these releases substances into the atmosphere that degrade air quality. The result is urban air pollution: a set of inputs to atmospheric systems that alter their composition in ways harmful to living organisms and built structures.
The main pollutants driving urban air quality problems are:
- Nitrogen oxides (): a family of gases dominated by nitric oxide () and nitrogen dioxide ()
- Sulfur dioxide ()
- Carbon monoxide ()
- Particulate matter (PM): airborne solid and liquid particles
These substances enter the atmosphere as primary pollutants, meaning they are harmful at the point of emission. Some then undergo chemical reactions to form secondary pollutants, which can be more damaging than the originals. Understanding both categories, and where each comes from, is the foundation for managing urban air quality.
Urban air pollution is best understood as a systems problem: human activities are the inputs, atmospheric chemistry is the processing stage, and the outputs (degraded air, acid deposition, smog) feed back to affect human and ecosystem health.
Particulate matter categories
Particulate matter is not a single substance but a broad category defined by particle size, because size determines how far particles travel into the respiratory system and what damage they cause.
Fine particulate matter with a diameter of 2.5 micrometres or less. Particles this small can penetrate deep into the lungs and pass into the bloodstream, contributing to heart disease, lung cancer, and stroke.
Particulate matter with a diameter of 10 micrometres or less (including PM2.5). Coarser particles in this range are typically trapped in the upper airways but can still cause bronchitis and aggravated asthma.
For reference, a human hair is roughly 70 micrometres across, so PM2.5 particles are about 1/30th that width. Their tiny mass allows them to remain suspended in air for days and travel long distances from their source.
Sources of particulate matter span both categories:
- Combustion of fossil fuels and biomass generates mostly PM2.5
- Construction sites, unpaved roads, and dust storms tend to produce coarser PM10
PM2.5 and PM10 are not two distinct particle types. PM2.5 is a subset of PM10. The distinction matters for health risk assessment, because finer particles carry greater risk of systemic harm.