The result is that animal experiments have consistently been allowed to confuse or undermine human evidence.
In cancer research the effect has been devastating. In 1956 British epidemiologists drew attention to a link between X-rays during pregnancy and subsequent childhood cancers. Within a few years similar findings were reported in American children. But for a quarter of a century, scientists questioned whether X-rays were actually the cause and cited animal experiments to show that the foetus is not especially sensitive to radiation. However, it seems that compared with other species, the human foetus is more susceptible to the carcinogenic effects of X-rays, and during the 1980s further observational studies confirmed the hazards, particularly in early pregnancy.
Animal tests have produced conflicting evidence of the cancer-causing effects of metals. According to an analysis by the US Environmental Protection Agency, ‘Arsenic, chromium and nickel present an interesting paradox - convincing evidence of respiratory cancer in humans but weak evidence in animals despite substantial testing.38 ‘ In the case of arsenic, animal testing began in 1911, more than a century after the chemical was first suspected of causing human cancer. A 1947 review described how dozens of experiments had been performed but had given ‘only doubtful results’.16 The tests continued but still proved negative, and in 1969 America’s National Cancer Institute stated that ‘arsenic has been suspected by many investigators as a carcinogen in man, though there is no supporting evidence from animal experiments.’16 And in 1977, a further summary of the data concluded ‘there is little evidence that arsenic compounds are carcinogenic in experimental animals.’16 In the late 1980s animal researchers finally managed to produce cancer in animals, at last replicating the mass of human evidence from patients and workers exposed to arsenic. The picture is similar for nickel and chromium: both are known to cause cancer in people yet most animal studies have given negative results.38
|Animal Cancer Tests|
|Metal||No. of Animal Trials||Developed Cancer?|
Recognition of two major cancer hazards - smoking and asbestos - was delayed due to experiments on animals. By the mid-1950s more than a dozen epidemiological studies had identified the link between smoking and >>
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