Maximum legal limits for pollutants: how safe are they?

Posted on March 4, 2011

Freshwater contamination is usually assessed by comparing the observed level of a given pollutant in water samples with the corresponding Maximum Permissible Concentration (MPC).  If the MPC is exceeded, measures must be taken to reduce the concentration in surface water bodies.  A procedure commonly used in identifying MPCs involves exposing water fleas (Daphnia magna) to water samples and recording the number of survivors after a set time, usually 24-96 hours.  In these tests Daphnia are treated as representative primary consumers for the purposes of assessing the ecosystem impacts of pollution.  A drawback with current assessment procedures is that MPCs are developed for particular chemical compounds but not for mixtures – in other words, they don’t address the possibility that exposure to several contaminants at the same time may reduce an animal’s tolerance to one or more of them.  In a recent Dutch study, the authors concluded that some pesticide mixtures and metal mixtures have this effect.  They found that measured levels of cadmium in surface waters were close to the lethal concentration and that exposed Daphnia populations can go extinct because of the co-occurrence of other metals in the water, even when no MPC is exceeded.  A similar effect was identified for mixtures of pesticides such as pirimiphos-methyl and diazinone.  These findings indicate that it’s wrong to assume that aquatic ecosystems are safe as long as no MPC is surpassed, and that stricter legal limits are required for some of the more potent compounds. 

Reference:  Baas, J. & Kooijman, B.  2010.  Chemical contamination and the ecological quality of surface water.  Environmental Pollution 158, 1603–1607.