Damstra, T, S Barlow, A Bergman, R Kavlock and G Van Der Kraak (editors). 2002. Global Assessment of the State-of-the-Science of Endocrine Disruptors. International Programme On Chemical Safety.

In a scientifically comprehensive and cautious global assessment of the state of endocrine disruption science, the World Health Organization and the US National Institute of Environmental Health Sciences conclude that the breadth of evidence from laboratory, wildlife and human studies justifies concerns about the possible human health impacts of endocrine disruptors.

The report acknowledges considerable scientific uncertainty, particularly in establishing definitive causal links between known exposures to endocrine-disrupting chemicals in people and endocrine-mediated adverse health effects.

For example,

• even though specific contaminants are known, via animal experiments, to act as endocrine disruptors;
• and even though people are known to be exposed to these compounds;
• and even though the types of health effects expected on the basis of animal experiments are occurring in people;

there is no scientific evidence proving in people that effect is caused by the contaminant acting through an endocrine pathway.

This is an appropriate high level of scientific skepticism for research. There may be other pathways, not endocrine-mediated, that are the means by which the contaminant is having an effect.

And thus on this basis, the assessment concludes that while it is plausible that endocrine disruption is the cause of the health effects, it is not scientifically proven.

While such caution is warranted with respect to the scientific understanding of the core mechanisms involved, UK toxicologist Gwynne Lyons argues that this sort of residual uncertainty should not prevent public health measures being taken: "It is worth remembering that epidemiological research in 1952 demonstrated that smoking caused lung cancer, but the probable causal mechanism was not found until 1996, and even this is still not universally accepted."

Lyon's also cites DDT as another good example of the delay between evidence of effect and proof of the mechanism. According to Lyons, "The IPCS report acknowledges that DDT, via its degradation product, caused eggshell thinning, leading to broken eggs and other adverse reproductive effects in several bird species. However, as the report states, “[the] mechanism of eggshell-thinning has never been completely deduced” and there are several hypotheses. Therefore, the report highlights that “it cannot be stated with certainty that it is indeed a result of endocrine disruption”. However, what really matters from a regulatory point of view is the end-result of the exposure, not whether the mechanism of action is known with certainty.

Lyon's concludes that it would be "totally unacceptable for regulation of chemicals of concern to have to wait until the precise mechanism of action was known."

The report also observes (Chapter 1.2):