Ozone Exposure

Latest Score:


in 2019

Weight: 8%
score trend is up over time

Ground-level ozone is formed through a chemical reaction between air-born organic compounds (VOCs) and nitrogen oxides (NOx) when exposed to sunlight1. High levels of ozone exposure can cause leaf damage, which can reduce the efficiency of processes such as photosynthesis. Foliar damage due to ozone exposure is the first visible sign of injury, and indicates impaired physiological processes in the leaves1. For some trees, ozone exposure can increase the likelihood of secondary stressors, like damage caused by diseases, insects, or weather events. Ozone is expressed as the average concentration (in parts per million) between the hours of 8:00 am and 8:00 pm2 from April 1 to September 30. Due to regulations, ground-level ozone pollution has improved gradually over the past decade. Here, we selected the W126 standard3 because it is thought to be better representation of ozone injury to plants. The target ozone concentration has been set by the EPA at 0.07 ppm-hour2. A high score means that ozone levels are below this threshold.

1US EPA. 2013. Integrated science assessment for ozone and related photochemical oxidants. EPA 600/R-10/076F.
2US EPA. 2015. 2015 National Ambient Air Quality Standards (NAAQS) for Ozone. Available at
3Environmental Protection Agency. 2015. W126 Standard. Available at:

-- Expert interpretation for Ozone Exposure is not available--

The score is calculated using a target value and the historical range of the the entire long-term dataset. The higher the score, the closer this year's value is to the target.

Once the score is computed for each year, the trend in scores over time is calculated. If the trend is significantly positive or negative, the long-term trend is marked as increasing or decreasing respectively.

Component Description
Scored as

Distance between target and maximum (scaled 1-5)

Target value


Directionality of scores

Lower values in the data are better.

Minimum value used in scoring


Maximum value used in scoring

Data maximum + 10% of range

Daily ozone data (ppm-hour) was accessed from the air quality monitoring sites at Underhill (Proctor Maple Research Forest) and Bennington (Morse Airport), Vermont1. To compute an ozone exposure index, we selected the W126 standard2 because it is thought to be better representation of ozone injury to plants. Under this, Vermont’s ozone monitoring season runs April 1 to September 30 and growing hours are 8:00 am to 8:00 pm. We determined the minimum detection limit per site (here 0.005 for both sites over the entire record). We backfilled missing values with the minimum value observed per year, restricted to the established monitoring season and growing hours. All months must have >75% completeness in daily records to be utilized. Data were transform following the equation: OZ*(1/(1+4403*EXP((OZ*(-126))))). A daily index value was calculated by summing the transformed values per day; these were then summed per month to compute a monthly index value. These monthly indices were multiplied by the ratio of collected and backfilled samples to total possible samples within the month. For each month, we computed a three-month maximum -- the current month plus the two preceding months (i.e., the3-month maximum for June is the maximum of June, May and April). In Vermont, this results in 3-monthmaximum values for June, July, August and September. To compute the annual W126, we took the mean of the largest 3-month maximum over the current year and the previous two years. We set the target for ozone exposure at 7 ppm-hour3. The annual score was computed as a difference between 7 and the upper scoring bounds (maximum value in the dataset plus 10% of the range), scaled between 1 and 5. Values below 7 were given a score of 5.

1 Environmental Protection Agency. 2019. Available at:
2 Environmental Protection Agency. 2015. W126 Standard. Available at:
3 Environmental Protection Agency. 2015. National Ambient Air Quality Standards (NAAQS) for Ozone. Available at:

Dataset: VT Ozone