Poisons, radiation and sunburn may all help reverse the ageing process, it has been claimed.

Small doses of "stressors" normally considered dangerous to health can actually boost the body's self-repair system, and as a side-effect preserve youth, experts believe.

Stressors having this effect might include X-rays, alcohol, or mild cases of sunburn. The phenomenon, called "hormesis", was once dismissed by most scientists but is now being taken seriously.

In recent years, it has been shown to extend lifespan in yeast, fruit flies, worms and rodents. If the results of such studies also apply to people, it means hormesis could extend average human lifespan to 90. Average lifespans in the UK are now around 75 for men and 80 for women.

Stressors seem to kick-start natural repair mechanisms, including the enzymes that patch up damaged DNA. As the repair systems fix damage normally caused by ageing, the body is rejuvenated.

The theory was described in a series of articles in the magazine New Scientist entitled "How to Live to 100.. and enjoy it". There is already indirect evidence that hormesis has positive effects on human lifespan, said New Scientist.

Between 1980 and 1988, US researchers at Johns Hopkins University in Baltimore, Maryland, tracked 28,000 nuclear shipyard workers to study the effects of low doses of radiation. To their surprise, they found the death rate of the workers was 24% lower than that of a comparison group of 32,500 shipyard workers who were not exposed to radiation.

Exercise and calorie restriction may both promote longevity partly through the stress they cause, said New Scientist. In addition, some compounds with supposed anti-ageing properties, such as vitamin E and melatonin, seemed to act hormetically in single-celled organisms. They increased life span when taken in small doses, but not large ones.

New Scientist added: "The big unanswered question is at what dose does an otherwise harmful agent become beneficial. Clearly too much radiation or poison are bad for you. However there may be a safe way to trick your body's repair mechanisms into overdrive."

Other tips for achieving a long life span included sociability, happiness, mental exercise, and moving to a "longevity hotspot" where the number of centenarians exceeds one in 10,000.