The index is divided into six categories, each corresponding to a different level of health concern. An AQI of over 100 triggers an alert for vulnerable groups.

Ultraviolet radiation, and thus UV index numbers, are higher when the sun is at a higher angle in the sky. Outside of the tropics, this means the UV index is highest during midday in the summer.

More: Air Quality Index (AQI) Basics What do the numbers mean? An AQI level of 50 or below represents good air quality, while an AQI value of more than 300 represents hazardous air quality.