PM2.5 stands for Particulate Matter of 2.5 micrometer (µm) in diameter. These airborne particles include dust, vehicle exhaust, smoke, industrial and construction sources, etc.
Studies have found a close link between exposure to fine particles and early death from heart and lung disease. Fine particles are also known to trigger or worsen chronic disease such as asthma, heart attack, bronchitis and other respiratory problems. Fine particles also impact climate change.
The WHO has set a daily average limit of 25µg/m³ and yearly average limit of 10 µg/m³. These limits are based on broad scientific research. Above these limits, the negative impact on our health is proven.
PM2.5 is usually measured in µg/m³, but US sources might recalculate it to AQI values. Although AQI tries to make one readable scale for all forms of air pollution, it is not a linear scale!