Toxins may be classified as being either acute or chronic. Because of this, the EA is moving towards whole effluent testing. This also gets over the problem of setting limits for substances which only may be present in tiny amounts. Temperature also affects toxicity, and a rise of 10 degC usually halves the survival time. Toxins may also undergo transformation once they enter the aquatic system and become more toxic.
Toxins can also biomagnify up the food chain or bioaccumulate.
Tolerance can also build up. Potentiation occurs when previous exposure to a toxin increases one s sensitivity the next time. These sensitises may not be dose dependent. Effects of a toxin do not always result in immediate death. The immune, respiratory, reproductive or nervous system may all be affected, or there may be a change in the behaviour of the organism.