Misconception 3: synthetic chemicals are causing many cancers and other diseases

The chemical reality is that many claims about chemicals being 'linked' to diseases simply tell us that that a chemical was present when an effect occurred, rather than showing that the chemical causes the effect. Broadly speaking, it can be helpful to think of three kinds of 'links':

• The chemical was present when the effect occurred

There are many examples of these kinds of ‘links’, not least because pathologists record things like the presence of prescribed medication during an autopsy. Such ‘links’ are usually of no consequence.

• The chemical and the effect appear to be related

There is a correlation between the dose of the chemical and the extent of the effect. This is usually tested scientifically by increasing and decreasing exposure and/or by comparing effects in different groups of people. A correlation is not the same as a causal relationship. For example, heavy drinkers might be more likely to develop lung cancer, but this could be because they are likely to be heavy smokers too, rather than because alcohol causes the cancer. Caution is needed in reporting apparent correlations: it is in the nature of scientific experiments that many disappear when a further test is done or they turn out to be explained in other ways.

• The chemical causes the effect

This means that there is a plausible mechanism to explain how the chemical might cause the effect. You would generally expect this to be consistent with what is known about how the chemical or the organism works and to be supported by evidence of predictable effects.

HOW CAN YOU EVALUATE CLAIMS ABOUT CHEMICALS AND THEIR RISKS?

In order to evaluate the risks of chemicals scientists would normally try and answer some of the following questions.

1. What is the status of the claim about the particular chemical? For example:
   • Is it anecdotal?
   • Is it based on objective scientific experiments or observations? (Look for mention of ‘Randomised Controlled Trials’ or whether it is published in a peer-reviewed science journal, for example.)
   • Has the effect been widely observed in relation to the chemical?
2. Who is the individual or organisation making the claim about this chemical; do they have experience in the area of concern? (This is much more important if claims are from sources other than a scientific journal.)
   • What else have they published?
   • Is the main aim of the information to promote something? (This doesn’t mean it’s wrong but it can be useful to ask.)
3. Is the exposure to the chemical always followed by the claimed effect?
4. Does the effect occur in the absence of the chemical? If so, there might be another explanation for the effects.
5. Does there appear to be a relationship between the level of exposure to the chemical and the severity of the effects?
6. Is there a plausible mechanism to explain how the chemical could produce this particular result?
7. Does the evidence that is presented fit the known facts or data?

Go to www.senseaboutscience.org/chemicals for examples of how these questions can be used to test chemical stories.