Low level stressors are slight stresses to one or more of your body's systems or organs. These stresses can be caused by anything that would be harmful to your health in a large dose, for example X-rays, sunburn, carcinogens (cancer-causing chemicals), other toxins, high temperatures, heavy excercise and the like. Virtually everyone will tell you that these things are bad for your health, and they are right - a large dose of X-rays can kill within a few weeks, prolonged heavy excercise will dramatically shorten your lifespan and so on. But research has shown that for some of these, the exposure/risk relationship is not linear. In other words, exposure at high levels is harmful, but at low levels it may not be, or may even be beneficial.
The effect is called hormesis, and it is well known in the drug industry. Research has shown that some drugs which stimulate at low doses suppress at high doses (paracetamol, asperin), and vice versa. Some scientists now think that other stresses can act in much the same way, by stimulating your natural repair and defense systems and so increasing their effectiveness. You may have heard that people whose immune systems are not stressed are often sickly, whereas country dwellers who are exposed to a range of bacteria enjoy enviable health and freedom from allergies. It is now thought that other body systems work in the same way - a "use it or lose it" idea. A simple analogy is callousing. If your hand's skin is not stressed, it remains soft and pliable, and relatively easy for a thorn to enter. If you rub that spot on and off for a couple of weeks with a rock, the skin will thicken in response and sharp objects will have a much harder time getting through it. You may not want callouses on your hands, but people who play guitars without a pick need them to prevent their skin wearing through. Other organs and systems respond in the same way. Using your mind a lot will sharpen your perception, excercise strengthens your muscles and stimulates your heart and lungs to work more efficiently. So small doses of toxins, radiation and other stressors can build up the repair systems to be more robust and even reduce the effects of aging. Animals have shown important increases in longevity as a result of caloric restriction, which is now thought to operate as a low level stressor.
But there's no need to go on a near-starvation diet if you want to live longer. Just enjoy life a little closer to the edge.
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