A tale of two measures
The controversy over PCBs in farmed salmon is driven by two very different measures for evaluating the health risks from exposure to chemicals in food and in the environment; one is cautious, the other is extremely cautious.
These approaches reflect the enormous scientific challenge in trying to observe and quantify the effects of trace amounts of substances as they accumulate and dissipate in the body over a lifetime. How do you separate their impact — if any — from the influence of a vast array of other far more deleterious environmental and lifestyle factors such as lack of exercise and poor diet? How do you account for mechanisms that can repair or eliminate damage?
In terms of PCBs, the Food and Drug Administration and the World Health Organization (WHO) believe that most of them have to reach a certain “threshold” concentration in the body before they can inflict or promote damage. Their limits of 2000 parts per billion per fish takes a cautious view of the risk from PCBs.
The Environmental Protection Agency rejects the idea of a threshold, and takes the view that a substance must be considered a “probable” carcinogen in humans if it can be linked in any way to cancer in animals. Physiological differences across species are unimportant, as are the massive doses administered to the animals in the experiments.
For the EPA, there is a linear relationship between dosage and damage, and exposure to tiny amounts of the substance simply means a lower risk. Given that the risk only disappears when the substance disappears, the EPA’s guidelines reflect an extremely cautious view of the risk from PCBs in fish. Both methods have their advocates; however, the assumptions underlying the linear model are more controversial.
According to a 1993 STATS survey of 401 randomly chosen members of the AmericanAssociation for Cancer Research, only one in four thought that cancer-causing agents were unsafe regardless of the dose (28 percent). Similarly, only one in four (27 percent) endorsed the practice of assessing human cancer risks by giving animals a maximum tolerable dose of a suspected carcinogen. And only twelve percent thought that chemicals should be banned from food if they ever caused cancer in any species.* (*S. Robert Lichter and Stanley Rothman, Environmental Cancer — A Political Disease, Yale 1999.)
The EPA’s no-threshold approach to assessing the risk from other chemicals, such as dioxins, has also drawn criticism from its own scientific advisory board.* (* Michael Fumento “EPA’s Own Panel Says It Masquerades Dioxin Policy as Science.”)