No evidence of a "safe" threshold with environmental toxins

Four of the most widespread environmental toxins - lead, trihalomethanes (found in drinking water), ionizing radiation from indoor radon gas, and tobacco smoke - can cause serious damage to health even at very low levels, say researchers in the open access journal PLoS Medicine.

What this means, say the researchers Donald Wigle of the University of Ottawa and Bruce Lanphear of Cincinnati Children's Hospital Medical Center, is that there are simply no safe levels of exposure to these toxins and they must be "virtually eliminated to protect human health."

Children can suffer brain damage from being exposed to very low levels of lead, they say. Although the US Centers for Disease Control and Prevention does not recommend public health or medical action unless the blood lead level of children exceeds 0.48 micromoles/liter, several longitudinal studies of children found inverse relationships between IQ and blood lead levels over a range extending below 0.48 micromoles/liter. These studies found no evidence of a "safe" threshold.

The Canadian government has concluded that the human lifetime cancer risk associated with drinking water containing trihalomethanes at 100 micrograms per liter (the current Canadian trihalomethane drinking water guideline) would be negligible. But recent research showed that there was an excess bladder cancer risk in men exposed to trihalomethanes at levels above one microgram per liter compared to less exposed men (the excess lifetime risk was about seven per 1000). This excess risk, say Wigle and Lanphear, is "much higher than those usually designated as negligible."

The researchers say that both radon and environmental tobacco smoke can damage health at very low levels. A recent expert committee concluded that the most plausible relationship between ionizing radiation and cancer was a linear relationship with no safe threshold, while studies have shown that even low level exposure to passive smoking can reduce fetal growth.

"The public depends on decision makers, scientists, and regulators to restrict exposure to widespread toxins that have known or suspected serious potential health effects," say Wigle and Lanphear.