We know that an AQI (Air Quality Index) value under 50 is healthy and over 300 is hazardous, but what do these numbers mean?

— Lauren G., Medford

Lauren, air quality websites — airnow.gov, deq.state.or.us/aqi/ — are among the most popular for Southern Oregonians these days (hence the sites don't always load properly), but we have to admit the actual data is not the easiest to comprehend.

The Air Quality Index, which made its debut in 1968, is calculated using four major air pollutants: ground-level ozone, particle pollution, carbon monoxide and sulfur dioxide. These major pollutants are recorded by air monitors every day, then converted to the AQI value using standard formulas developed by the Environmental Protection Agency.

In the U.S., AQI value is divided into six categories indicating increasing levels of health concern — the higher the AQI value, the greater the health concern. An AQI value below 50 is "good" and over 300 is "hazardous." Air quality in the Medford and Ashland area, for example, was categorized as "hazardous" on Wednesday but had moved to "unhealthy for sensitive groups" by Friday. Other categories include "moderate," "unhealthy" and "very unhealthy."

People who are most at risk when the AQI value exceeds 100 are people with lung diseases, children and the elderly. The EPA also recommends residents in "unhealthy" air quality area to avoid outdoor activities, so keep an eye on those air-quality websites so you can see what the latest risks are from the air we breathe.

— Send questions to “Since You Asked,” Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. To see a collection of columns, go to mailtribune.com/youasked. We’re sorry, but the volume of questions received prevents us from answering all of them.