weather term
Air Quality Index (AQI)
A standardised scale, usually 0 to 500, that reports how polluted the air is and what health precautions, if any, are advisable.
AQI combines readings of several pollutants, commonly fine particulate matter, ozone, nitrogen dioxide and carbon monoxide, into a single number, with higher values meaning greater health risk. Different countries use slightly different scales and category names, so a reading considered 'moderate' in one country's system might fall into a different category elsewhere, but in general values above roughly 100 warrant extra caution for sensitive groups such as children, older adults and people with respiratory conditions.