Acute to chronic ratio ACR

The definition of the acute to chronic ratio (ACR) has been one of the methods used to predict the threshold concentration at which a toxicant does not produce noticeable effects during a chronic exposure. This ratio is based on the same concept as the application factors, but its numerical value is the inverse (Stephan, 1982). The ACR is the ratio between chemical concentrations exerting a lethal versus sublethal toxic effect and describes the ratio of a lethal to sublethal end-point  

This very simple value has proved to be an effective tool to predict chronic effects. Its main advantage stems from the fact that it can be used to predict, from known ACR values, possible chronic responses in similar species or with similar toxicants. It was possible to use this approach because at least 90% of the substances being evaluated showed that the concentration at which no effect was observed was found to be at least two orders of magnitude below the LC50 values (Kenaga, 1982a,b). 

Studies aimed at verifying this ratio have shown a 0.88 correlation coefficient for 11 freshwater species and 126 chemicals (Slooff et al., 1983). It was concluded on the basis of these results that compounds that produce acute toxicity also show chronic toxicity. Therefore, with the exception of substances with a very particular toxic action, it is possible to predict the chronic toxicity of a substance on a species based on the acute values obtained for the same compound with similar species (Giesy and Graney, 1989). 

Some environmental protection agencies have indicated that an ACR value of 10 can be used for industrial effluents and common chemicals when no 

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The extrapolation from acute responses to no-observed-effect concentrations or chronic responses is particularly important as chronic tests are more costly and time-consuming than acute tests. Traditionally, relationships between acute and chronic effects were estimated using a simple ratio, the acute-to-chronic ratio (ACR). Where acute and chronic effect measures are available for the same species, this ratio is used to estimate chronic responses in related organisms for which only acute data are available (Stephan and Rogers 1985). This approach is based on the assumption that there is a relationship between the responses in acute and chronic tests, an... 

As discussed in Section 4.8, and providing that some chronic data are available, use of acute-to-chronic ratios (ACRs) can be helpful if chronic data are limited to allow use of the large acute toxicity database for PNEC or EQS derivation. 

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