Ozone Exposures Based on material published in the literature and summarized by A.S.L. & Associates, it was recommended that the analysis should be limited to vegetation studies in which the SUM06 or W126 indices had been used to describe ozone exposure. In addition, because of the importance of high hourly average concentrations, A.S.L. & Associates identified the number of hourly average concentrations greater than or equal to 0.10 ppm that were associated with the SUM06 and W126 exposures for each of the experiments. Thus, the three exposure indices used to describe ozone exposures were the   SUM06 - The sum06 index uses an artificial threshold of 0.06 ppm. To calculate the index, all hourly average concentrations over the exposure period equal to and greater than 0.06 ppm are summed;   W126 - Alternatively, the W126 exposure index does not utilize a threshold value, but weights differentially all hourly average concentrations. Significant weighting greater than 0 occurs at all hourly average concentrations above 0.04 ppm. The use of a 0 weighting for hourly average concentrations less than 0.04 ppm was intentionally designed into the W126 so as to reflect near background levels. The W126 has weighting of approximately 1 for all hourly average concentrations equal to and above 0.10 ppm. The weighting of 1 at these concentrations was based on informal discussions with vegetation researchers in California whose vegetation experienced repeated occurrences of hourly values equal to and above 0.10 ppm. This was a subjective decision and it is recognized that every species will have a different weighting function. N100 - The number of hourly average concentrations equal to and greater than 0.10 ppm. The N100 is required to make the W126 exposure index consistent in predicting vegetation injury and damage. Please click here for additional information about the use of the W126 and the N100 exposure indices.   Home Page | News | Corporation | Maps | Publications | Table of Contents Multimedia Center Copyright © 1995-2008 A.S.L. & Associates. All rights reserved.