If the combined effect of two or more chemicals is simply the sum of the effects of the individual chemicals, the effect is said to be additive (2 + 2 = 4). For example, if a dose of drug A that produces 25% of the maximum response is combined with a dose of drug B that produces 50% of the maximum response, then 75% of the maximum response is produced. This is possible when the stressors are sufficiently similar. For example, most stable organochlorines (PCBs and pesticides) are found together in the environment. These organochlorines have similar chemistry and similar biological effects; hence the response from a combination of these stressors may add up to something damaging though the response from each chemical stressor may be negligible. Diazinon and chlor-pyrifos pesticide have been found to exhibit additive toxicity when present together.
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