Very small airborne particles that are less than 2.5 micrometres across (these are a component of PM10) so can be easily breathed deep into the lungs where the smallest particles can enter the bloodstream. Health effects include respiratory diseases, heart attacks, and lung cancer.  Most particulate matter from natural sources is larger than 2.5 micrometres across.  These concentrations are reported in micrograms per cubic metre (µg/m3) of air.