Ozone data are not official, are not quality assured, and are subject to change. These data are gathered quickly and from a variety of sources. Some data are unavailable from some states or regions. The data are incomplete and should only be viewed as a general indication of region-wide exceedances during 1999. 8-hour ozone health standard up to the given date.
Bold type indicates the last known exceedance day at an individual monitor.
The number of exceedance days is given as a general indication of the frequency the 8-hour standard has been exceeded at a site during the 1999 ozone season. Based on 6 or 7 hour average due to missing data.