It stands to reason that a two-pack-a-day smoker faces a far greater risk of developing lung cancer than someone who puffs just one pack, while anyone who indulges in the occasional cigarette is at little or no risk at all.
Among scientists, this notion that harm escalates as the exposure to something harmful increases seems so commonsensical that it's gone unchallenged since first postulated in the 16th century by Paracelsus, the Swiss alchemist considered the father of toxicology.
Immortalized as "the dose makes the poison," the concept is used by regulators around the world to determine the safety of compounds. It's why laboratories give test animals huge quantities of chemicals to establish what will damage them, then lesser amounts to determine at what point exposure is harmless.
But even pillars of science can be turned on their heads, Critics such as Pete Myers, chief scientist at Environmental Health Sciences, a public-health advocacy group based in Charlottesville, Va., argue that in at least one chemical category, toxicology has it all wrong. When it comes to synthetic substances able to mimic the hormones in our endocrine system, low doses only seem insignificant.
The idea that less of a chemical may pose an even greater threat seems "almost incredible," Dr. Myers admits - "until you understand the incredible potency of hormones."
Not only are small doses of hormone disruptors dangerous, he says, they may well explain the mysterious rash of modern ailments - attention-deficit disorders, thyroid problems, obesity, precocious puberty in girls, hormonally influenced cancers - that have gone from rare to commonplace.
Man-made compounds able to imitate or skew natural hormones are found everywhere, leaking from many consumer products to become uninvited visitors in the cells in our bodies. For instance, many chemicals used to make plastics and pesticides have, upon closer testing, been found to be able to play havoc with hormones.
One culprit is phthalates, the chemicals used to make that rubber-duck bath toy so supple. But the poster child for this controversy is bisphenol A, the reason so many Canadians have been pitching out their plastic water bottles, baby bottles and sippy cups. U.S. surveys are finding that more than 90 per cent of the public carries traces of BPA, while researchers studying its impact in tiny doses have discovered that it amounts to an extra dollop of estrogen, the female sex hormone - linked to everything from enlarged prostate glands and erectile dysfunction among men to increased risk of breast cancer.
Now, the question of whether tiny chemical residues should be alarming is about to get its first serious investigation - from the U.S. Food and Drug Administration. This represents a major change for the agency, which, for nearly half a century, has said BPA in small exposures is harmless, allowing it to spread to countless products, from DVDs to the plastic lining inside nearly every food and beverage can.
About 200 academic studies have linked low exposures of BPA to conditions including obesity, diabetes, cancer (breast, prostate and uterine), cardiovascular disease and asthma. Meanwhile, research financed by industry has found no such health threats.
As a result, FDA Commissioner Margaret Hamburg says, "the current literature cannot yet be fully interpreted for biological or experimental consistency, or for relevance to human health."
To settle the issue, the U.S. government has earmarked $30-million of stimulus spending for a crash-course study, lasting 18 to 24 months.
Health Canada, meanwhile, has declared BPA toxic. It banned baby bottles made from it two years ago. Yet it calls the ban a precaution to protect infants, and still allows BPA as an unlisted additive in practically all canned food or beverages, even though it is on the country's list of most dangerous substances.
