particulate matter

Particulate matter is the mix of tiny solid and liquid particles floating in the air — the soot, dust and smoke too small to see individually. What makes it dangerous is size: the smaller a particle is, the deeper it can travel into the lungs, slipping past the nose and throat that normally filter out larger debris.

Particles are classified by diameter in micrometres. PM10 (10 micrometres or less) can reach the lower airways; PM2.5 (2.5 micrometres or less, the so-called fine particles) penetrate to the alveoli; and ultrafine particles smaller still can even cross into the bloodstream. They come from combustion — vehicle exhaust, power plants, wildfires, cooking and tobacco smoke — as well as dust and industrial processes.

Once deposited, these particles trigger inflammation and oxidative stress in the airways and alveoli, and the finest can drive effects beyond the lungs, including on the heart and blood vessels. Short-term peaks worsen asthma and COPD and raise infection risk, while long-term exposure contributes to reduced lung function, COPD and lung cancer. PM2.5 in particular is the air-pollution measure most consistently tied to harm, which is why air-quality standards focus on it.