What is PM2.5?
Quick Answer: PM2.5 is fine particulate matter measuring 2.5 microns or smaller, roughly 30 times thinner than a human hair, and it is usually the pollutant that drives a city's air quality index.
These tiny particles come from traffic exhaust, industry, the burning of wood and coal, and wildfires. Because they are so small, PM2.5 can travel deep into the lungs and even pass into the bloodstream, which is why it is linked to heart and respiratory problems. This makes it one of the most closely watched air pollutants worldwide.
PM2.5 is measured in micrograms per cubic metre (µg/m³). The World Health Organization recommends keeping the 24-hour average below 15 µg/m³. When you see an air quality reading climb, fine particles are very often the main reason, so checking PM2.5 levels is a good way to judge whether the air is safe to breathe.