The use of the cumulative exposure index with a threshold concentration had some limitations. Depending upon the threshold concentration used, the parameter ignored the lower hourly mean concentrations. R ecognizing the disadvantage of using a threshold concentration with the cumulative index, a modification was suggested that applied differential weighting to the hourly mean concentrations of ozone and summing over time. Lefohn et al. W hile future research should focus on identifying the family of cumulative indices that best describe the relationship between ozone exposure and vegetation effects, one needs to be aware that exposure indices will continue to produce inconsistent results when trying to predict growth losses.
Most exposure indices are insensitive to diurnal periods of maximum sensitivity of the plant.
The sensitivity of vegetation as a function of the time of day has not been well defined. Most cumulative-type and other exposure indices cannot adequately describe some of the subtle differences in the two different types of exposure regimes. It is unclear how important sensitivity and the amount and chemical form of the pollutant that enters the target organism are in an overall weighting scheme when predicting vegetation effects. If both the sensitivity of the target organism and the actual dose that enters the organism are as important as ambient air pollutant exposure, then a given pollutant exposure will elicit varying biological responses at different times for the same crop. Recognizing the limitations of applying exposure indices as dose surrogates, at this time, the cumulative exposure index may still be the best family of indices available for relating exposure and biological response.