Toxins like radon and even DDT may have beneficial effects at very low doses.

Ursula Sautter | May 2008 issue
A glass of red wine after dinner a couple of times a week can help prevent heart disease; a nightly magnum of the stuff will corrode your liver. This is pretty much accepted wisdom. But what isn’t yet widely understood is that in very small doses, an array of substances considered toxic could potentially be good for you.
Take cadmium. This soft, blueish-white heavy metal, frequently used in batteries, is a known carcinogen. But studies on rodents suggest that in small doses, it can reduce cancer rates. Or consider mercury, a neurotoxin in fluorescent bulbs and thermometers; in tiny amounts, it has an antioxidant effect and can protect against cell damage.
The theory behind these phenomena is hormesis, a term derived from the Greek word hormo, “to excite.” The mechanism behind it is simple: A small dose of a toxin excites a stress reaction in the body, causing it to bolster its defences against the invader. This idea has been around since at least the 16th century, when Renaissance physician Paracelsus observed, “It’s the dose that makes the poison.”
In the 18th century, the German pioneer of homeopathy, Samuel Hahnemann, had the same notion. He believed remedies for diseases could be created by ingesting minute amounts of substances that cause symptoms of the disease.
It may not be surprising that some potentially harmful natural substances may have healthy effects at low doses. After all, the same principle underlies vaccinations, in which a tiny dose of, say, polio is administered so the immune system can build up resistance to it.
What’s more surprising is that the same may be true of man-made chemicals like dichloro-diphenyl-trichloroethane (DDT) and dioxins. These toxic substances can cause serious environmental and health problems, but research suggests that in small doses they can have positive effects. Hormesis “could be a major revolution across broad reaches of the biomedical world,” says toxicologist Edward Calabrese of the University of Massachusetts at Amherst, who found thousands of instances of the phenomenon in the scientific literature. “It would also be very important for drug discovery.” Here’s a look at some of the bad stuff that, in tiny amounts, might be good for you.

Radiation

The conventional view is that the higher the dosage of radiation, the greater the risk of cancer. Yet evidence is mounting that shows exposure to low-dose radiation can be a boon. For example, research into the long-term effects of the atomic bombs dropped on Nagasaki and Hiroshima in 1945 has found no indication that the attacks caused genetic damage. In fact, the frequency of leukemia among survivors exposed to relatively low-dose radiation was abnormally low.
A similar effect is seen in less dramatic cases of exposure. Reviewing scientific literature, Myron Pollycove, professor emeritus of laboratory medicine and radiology at the University of California at San Francisco, concluded in a 2007 article in the U.S. medical journal Dose-Response that “[p]opulations living in high background radiation areas and nuclear workers with increased radiation exposure show lower mortality and decreased cancer mortality than the corresponding populations living in low background radiation areas and nuclear workers without increased radiation exposure. … Both studies of cancer in animals and clinical trials of patients with cancer also show, with high statistical confidence, the beneficial effects of low-dose radiation.”
The impact of exposure to radon is a case in point. An odourless, tasteless and invisible radioactive gas produced from the natural breakdown of uranium in water, soil and rock, it’s often detected in building materials and causes thousands of lung cancer deaths per year. Yet several scientific studies suggest low levels of the element can decrease mortality from this type of cancer too. In 1993, researchers from Osaka found that residents of Misasa, Japan, where radon is plentiful, develop lung cancer less often than those in surrounding suburbs. A 1995 study conducted at the University of Pittsburgh also showed a correlation between higher radon levels and decreasing mortality rates from lung cancer.
Reviewing these and other papers, scientists at the Institute for Nuclear Science and Techniques in Hanoi, Vietnam, concluded in the 2006 International Journal of Low Radiation that “radon has been proven to decrease the lung cancer mortality rate at the low level.”
The secret behind the possible beneficial effects of low-level ionizing radiation—and other kinds of hormesis—appears to be cellular stress adaptation. Bombarded by radioactive substances, cells muster their defences to fend off damage.

Arsenic

The notoriously toxic metalloid arsenic, found in the groundwater of some regions, is a known carcinogen. But recent studies suggest that when it’s consumed in minute concentrations, arsenic can reduce the risk of cancer. Geneticists from the Kolkata-based Indian Institute of Chemical Biology found that people who regularly ingest tiny amounts of the substance in drinking water are less likely to develop aberrant cells in their lymphocytes than people who aren’t exposed at all.
A 2004 study of bladder cancer mortality in the U.S. produced comparable results. A team of scientists from the School of Biological and Chemical Sciences at Deakin University in Burwood, Australia, and the New York University School of Medicine found a “protective effect against oxidative stress and DNA damage” when they treated human skin cells with small amounts of arsenite, one of the most common forms of arsenic.

DDT

DDT, a potential carcinogen, was banned in most countries and for most uses back in the 1970s, but traces of this pesticide (as well as others) can still be found in our food and our bodies. There is laboratory evidence that DDT boosts tumour growth in large doses. Research also suggests that at lower doses the substance may reduce abnormal tissue growth.
In 2005, pathologists at the University Medical School in Osaka, Japan, reported in Toxicology and Applied Pharmacology that DDT fed to rats in low doses inhibited the development of lesions in their livers. The researchers concluded this could be “related to changes in metabolizing enzymes, cell communication and DNA damage and its repair.”

Dioxins

Dioxins can be found in substances as diverse as herbicides, wood preservatives and car exhaust. It was the poison that almost killed Ukrainian politician Victor Yushchenko in 2004, leaving his face pocked and scarred. Trace amounts of dioxins routinely enter our bodies through the consumption of meat, fish, eggs and dairy products. One of the most harmful dioxins, TCDD, has been found to cause cancer in laboratory animals.
The same 1976 study that showed high doses of TCDD caused cancer in rats also demonstrated an anti-cancer effect when the chemical was administered in small amounts. More evidence of this effect was published in 2005, when a group at the National Public Health Institute in Kuopio, Finland, wrote in Dose-Response, “Moderately low exposures to dioxins could even decrease the risk of [soft-tissue sarcoma].” This is still a hypothesis, researchers stress, not a proven fact.
Hormesis has its critics, who argue that the potential benefits of low levels of toxins are due to random statistical fluctuations. Even if some substances do turn out to have positive effects in low doses, they point out, that shouldn’t distract from their harmful effects at higher doses. Indeed, no one suggests that popping low-dose dioxin pills—or exposing yourself randomly to any other toxin—is a good thing.
“It is also possible that some of these low-dose effects are in fact detrimental as well,” warns Dave Eaton, director of the Center for Ecogenetics and Environmental Health at the University of Washington in Seattle. “For example, low levels of DNA damage may well activate certain DNA repair pathways that increase the extent and possibly the efficiency of DNA repair. But it is still possible that some of the low-level DNA damage escapes repair and is ultimately detrimental. Thus I don’t think one can assume that all ‘low-dose’ responses are without potential harm.”
Still, if further research confirms the potential benefits of hormesis, Paracelsus’ old observation may acquire a new relevance.
Ursula Sautter is a freelance journalist living in Bonn, Germany.