Dangerous Levels of Toxins Miscalculated 

HELEN R PILCHER / Nature 13feb03

Potential pollutants and poisons may be beneficial in low doses.

We may be putting too much effort into cleaning our environments.

The levels at which potentially toxic substances such as mercury and lead are classified as dangerous may have been miscalculated, two US scientists are warning. Risk assessments and regulations on safe limits for these substances in medicine and the environment may have to be rethought, they warn1.

There are safe levels below which potential pollutants and poisons may actually be beneficial, say Edward Calabrese and Linda Baldwin of the University of Massachusetts in Amherst. For the past 30 years, cancer-causing chemicals and X-rays have been viewed largely as dangerous whatever their level.

"The field of toxicology has made a terrible blunder," says Calabrese. "A lot of high-powered people need to take the time to explore this."

For example, dioxins, which are industrial by-products that at certain doses can cause cancer, can actually reduce tumour growth in some species. Similarly, small amounts of the toxic trace metal cadmium can promote plant growth.

"What we call 'toxic chemicals' is a misnomer," says cell biologist and UK government advisor Anthony Trewavas from Edinburgh University. "Mild chemical stress is beneficial."

Having identified over 5,000 similar examples, Calabrese and Baldwin are among a growing number of researchers who feel that the hazardous nature of toxic substances has been overstated. The levels used in studies are not comparable to those normally experienced by humans, Calabrese says. "This provides an interesting challenge for the clinical and pharmaceutical industries as they develop new medicines."

The point of toxicological testing is to determine the drug exposure at which undesired effects are observed UK Medicines Control Agency

Britain's Medicines Control Agency (MCA) is more cautious. "The point of toxicological testing is to determine the drug exposure at which undesired effects are observed," a spokesperson said, adding that the new criticisms are, "unlikely to change the way in which the MCA or other regulatory agencies conduct product risk-benefit analysis."

The debate also raises the question of how clean our environment really needs to be. Some argue that billions of dollars are being wasted ridding the world of substances that are dubbed 'hazardous', when low levels could actually be a good thing.

"We don't need to spend large amounts of money on removing chemicals from the environment," says Trewavas. "Food contains lots of natural chemicals that are as damaging as synthetics. We consume lots of these all the time without harm. The public need re-educating in this."

But convincing people that 'safest' is a more meaningful description of risk than 'safe' and 'dangerous' is notoriously difficult.

References

Calabrese, E.J. & Baldwin, L.A. Toxicology rethinks its central belief. Nature, 421, 691 - 321, (2003).