Organisms exposed to ammonia may recover from toxic effects if the exposure time is shorter than that required to cause permanent effects (Milne et al., 2000; Hariprasad and Neeraja, 2006). Diamond et al. (2006) observed that latent toxicity did not occur in the fish, Pimephales promelas, and water-flea, Daphnia magna, when exposed to ammonia. However, the influence of exposure time on organism response may be different for different species. Increasing the interval between pulses of dissolved ammonia resulted in lower toxicity in D. magna, but greater toxicity in P. promelas (Diamond et al.,
2006). It is not possible to provide procedures for the modification of guidelines for effects based on exposure time beyond specifying guidelines for acute or chronic effects, respectively.Given that an organism is likely to be able to recover from
pulsed doses, unless the concentration is excessively high, the guidelines can be considered very conservative. The majority of acute and chronic effects data are derived from tests with durations greater than 24 h and often 96 h. Consequently, the concentrations of ammonia required to cause permanent toxic effects for short exposure times (e.g. 1–2 h) could be significantly higher than those required to cause effects over 96 h.