Over which averaging period is the ammonia critical level most precautionary?

Abstract

The ammonia critical level set at the UNECE Egham Workshop in 1992 (van der Eerden et al. 1994) was based on mean NH3 concentrations for different averaging periods. These values were derived from experimental studies and a toxicological model, so that a high value for the critical level was given for short exposure periods (acute exposure), and a low for long exposure periods (chronic exposure). The critical level values set applied to all vegetation types and are summarized in Table 8.1. It may be noted that in a subsequent review, WHO (2000) recommended the discontinuation of the monthly and hourly critical levels in regulatory assessment, although the arguments for doing so were not clearly explained. For the purpose of the present scientific assessment, all the time periods are used (i.e., hourly, daily, monthly, yearly). © 2009 Springer Netherlands.