Q Is there any validity to the popular claim that one can "sweat out toxins?" If so, what "toxins" can the body sweat out?

A The body does appear to sweat out toxic materials - heavy metals and bisphenol A (BPA), a chemical found in plastics, for instance, have been detected in sweat. But there is no evidence that sweating out such toxins improves health.

"The claims for the benefits of saunas and other sweat-inducing treatments are not backed by science," said Dr Harriet Hall, a retired family physician and former Air Force flight surgeon, who edits the website Science-Based Medicine and is a co-author of Consumer Health: A Guide To Intelligent Decisions.

The concentration of metals detected in sweat is extremely low. Sweat is 99 per cent water. The liver and kidneys remove far more toxins than sweat glands.

People who have dangerously high levels of heavy metals in their body will need prescription medication, not sweating, to get rid of them, Dr Hall noted. For everyone else, "we can rely on our liver and kidneys to do all the 'detoxifying' our body usually needs".

It is also unclear whether the minuscule amounts of toxins that can be measured in sweat actually indicate a health concern.

No one will ever be able to conduct a large enough study to link such low levels of chemicals with health problems, said professor of chemistry Joe Schwarcz at McGill University in Montreal. "It's not 'figure-outable'," he said.

And removing tiny quantities of toxins does not necessarily mean there will be any health benefit.

Prof Schwarcz compared it to someone sitting in a bathtub worrying about drowning. Removing a dropperful of water from the tub will theoretically reduce the risk - because the chance of drowning is lower in less water - but getting rid of so little water will be effectively meaningless.

So does it matter that people excrete small amounts of toxins in their sweat? "The fact is, nobody really knows," Prof Schwarcz said.

NYTIMES