The concept of dose-load is used widely in risk assessment literature. This concept is based on animal experiments in which the animals were exposed to constant concentration during a set period of time. However, in most accident scenarios, people are exposed to time varying concentration of the toxic materials. The extension of the dose-load concept to such conditions is not straightforward. The assumption that the dose-load is additive leads to a paradox. We suggest a different approach for extending the experimental results for time-varying concentration. We introduce the concept of the effective dose, which considers physiological recovery processes. It is found that in many cases, especially those which include intermittent time series of the concentration, the number of casualties is reduced when considering the recovery process. It is also shown that by using the effective dose concept we can resolve the apparent paradox in the dose-load concept for intermittent concentration time series. We demonstrate the importance of buildings as shelter against toxic gases especially for an instantaneous release, a fact that should be considered in hazard evaluation. (C) 2008 Elsevier B.V. All rights reserved.