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High Ozone in Your Metro Area

This table summarizes high one-hour and eight-hour ozone averages in the metropolitan areas where the Virginia DEQ monitors ozone.

Please Note: Data in this table is based on the number of monitors operating in an area, as well as on the severity of ozone.
This data should not be used to perform comparisons between cities or across years without factoring in the number of monitors.

The peak one-hour and eight-hour averages are color-highlighted based on the EPA-defined Air Quality Index colors. (See Interpreting the AQI.)

High Ozone Averages for 2017

   
Highlight with AQI colors
Metropolitan AreaOne-Hour Averages
>= 125 ppb
Eight-Hour Averages
>= 71 ppb
Peak ValueHigh DaysPeak ValueHigh Days
DateppbDateppb
Richmond
Includes:
     Charles City County, VA
     Chesterfield County, VA
     Hanover County, VA
     Henrico County, VA
No averages at or above 125 07/19/2017871
Hampton Roads
Includes:
     Hampton City County, VA
     Suffolk City County, VA
No averages at or above 125 No averages at or above 71
Northern Virginia
Includes:
     Arlington County, VA
     Fairfax County, VA
     Fauquier County, VA
     Loudoun County, VA
     Prince William County, VA
No averages at or above 125 07/19/2017753
RoanokeNo averages at or above 125 No averages at or above 71
CharlottesvilleNo averages at or above 125 No averages at or above 71
HarrisonburgNo averages at or above 125 No averages at or above 71
WinchesterNo averages at or above 125 No averages at or above 71
Fredericksburg
Includes:
     Caroline County, VA
     Stafford County, VA
No averages at or above 125 No averages at or above 71
Shenandoah National ParkNo averages at or above 125 No averages at or above 71
Rockbridge County, VirginiaNo averages at or above 125 No averages at or above 71
Wythe County, VirginiaNo averages at or above 125 No averages at or above 71
Ozone averages are reported in parts per billion (ppb).
The High Days columns represent the number of days where one-hour or eight-hour ozone averages were at or above the thresholds of 125 ppb and 71 ppb respectively.

What do the Numbers Mean?

The information in the table is composed of ozone measurements from monitoring sites across Virginia. There are two averaging periods that are of interest for ozone: a one-hour average and an eight-hour average. There are national air quality standards that have been established by the EPA for both of these averaging periods.

For the one-hour ozone average, a threshold of 125 ppb has been used to determine high value days. For the eight-hour ozone average, a threshold of 71 ppb has been used. Only averages that are equal to or higher than one of these thresholds appear in this table. The High Days column is a count of the number of days each metropolitan area experienced high ozone measurements (either one-hour or eight-hour) for the selected year. Out of all the averages at or above the thresholds, the peak or highest one-hour and eight-hour averages and the date they occurred are shown.

Ozone formation tends to be highest from March through October. Before May, there will be few if any metropolitan areas with high ozone readings. Some metropolitan areas never experience high ozone. In either of these cases, the table will show "No averages at or above 125", or "No averages at or above 71".

Interpreting the AQI

Air Quality Index

Each NAAQS pollutant has a separate AQI scale, with an AQI rating of 100 corresponding to the concentration of the Federal Standard for that pollutant. Additional information about the AQI and how it can be used is available from the EPA's AirNow web site.

Place your mouse pointer over the scale displayed above to view information about the Air Quality Index, and each of the rating levels.

The actual index calculation is different for each parameter measured and is specified by the EPA. The following table shows the various breakpoints used in calculating the AQI.

AQI Breakpoint Definitions
AQI Range 1hr Ozone
in ppm
8hr Ozone
in ppm
0 - 50 Not Defined 0 - 0.054
51 - 100 Not Defined 0.055 - 0.070
101 - 150 0.125 - 0.164 0.071 - 0.085
151 - 200 0.165 - 0.204 0.086 - 0.105
201 - 300 0.205 - 0.404 0.106 - 0.200
301 - 400 0.405 - 0.504 Not Defined
401 - 500 0.505 - 0.604 Not Defined
500+ Not Defined Not Defined

PLEASE NOTE: This data has not been verified by the Virginia DEQ and may change. This is the most current data, but it is not official until it has been certified by our technical staff. Data is collected from Virginia DEQ ambient monitoring sites and may include data collected by other outside agencies. This data is updated hourly. All times shown are in local standard time unless otherwise indicated.