Would additional protection be provided by setting standards for more than one averaging period for NO2?
With regard to protection against acute health effects, either the peak-hour average or 24hr (daily) average NO2 concentrations can be used as a measure of direct short-term exposure, since they are highly correlated in urban areas. Having a longer-term guideline value is also supported by the evidence on possible direct effects of NO2, and on its indirect consequences through the formation of secondary pollutants. Rationale: The limited data on short-term responses to controlled NO2 exposures in chamber studies does not provide any guidance on the relevant averaging time for the physiological responses on the scale of hours to days. However, this is a less important issue in terms of a guideline because of the high correlation of hourly maximum with daily average values. In terms of the long-term effects of NO2 and/or the secondary pollutants that result from the presence of NO2 in the ambient air, maintaining an annual NO2 WHO guideline value is most appropriate. Effective implementa
