Particulate matter 2.5 (PM2.5) refers to tiny particles in the air that are 2.5 microns or smaller. PM2.5 can penetrate into the lungs and bloodstream and cause irritation of the airways and aggravation of heart and lung diseases.
According to the new World Health Organization (WHO) guidelines, annual average concentrations of PM2.5 should not exceed 5 µg/m3, while 24-hour average exposures should not exceed 15 µg/m3.
