Eight-Hour Average Ozone Concentrations

Ground level ozone, or "smog," is a problem throughout New England during the warm months of the year. As a result, the New England states collect ozone data from a series of approximately 60 monitors placed throughout New England. This monitoring network records and provides hourly data for ozone from April through September.

In July 1997, EPA revised the former 1-hour ozone standard and replaced it with a more protective 8-hour standard at a level of 0.08 parts per million (ppm). The 1997 0.08 ppm, 8-hour primary standard is met at an air quality monitor when the 3-year average of the annual fourth-highest daily maximum 8-hour average ozone concentration is less than or equal to 0.08 ppm (that is, 0.084 due to rounding).

In March 2008, EPA again revised the ozone standard. The 2008 ozone standard is set at a level of 0.075 ppm averaged over an 8-hour period. This standard is met at an air quality monitor when the 3-year average of the annual fourth-highest daily maximum 8-hour average ozone concentration is less than or equal to 0.075 ppm.

Based on extensive scientific evidence about the effects of ozone on public health and welfare, on October 1, 2015, EPA strengthened the ground-level ozone standard to 0.070 ppm, averaged over an 8-hour period. This standard is met at an air quality monitor when the 3-year average of the annual fourth-highest daily maximum 8-hour average ozone concentration is less than or equal to 0.070 ppm.

The four highest daily maximum eight-hour average concentrations for each of the ozone monitors in New England for the ozone seasons of 1994 through 2022, as well as the number of days exceeding the 2008 and 2015 ozone standards, can be accessed by selecting the appropriate state below.