Each month, from March through October, EWG prepares the daily (9 am – 9 pm) maximum one-hour and eight-hour ozone concentration, in parts per billion (ppb), for each of the air quality monitors located in the St. Louis (MO-IL) region and generates the Air Quality Index (AQI) calendar. The Environmental Protection Agency (EPA)’s AQI conversion calculator is used to convert daily maximum eight-hour ozone average into an AQI assigned to color-coded general health categories. An AQI of 100 corresponds to the 2015 eight-hour ozone standard (70 ppb). Any value greater than that is considered to be unhealthy.
