Some scientists who have observed the dramatic rise in the incidence of childhood asthma in recent years have developed an intriguing theory: They believe it may be partly caused by the increasingly sanitary conditions of modern life. That may sound paradoxical -- dirt is bad, right? -- and it contradicts everything we are taught about "germs," starting in childhood, when we were forced to abandon any treat that spent even a moment on the ground. Americans have become so germ phobic that parents whose child has a minor cough often feel obliged to withdraw from social events; meanwhile, grocery store shelves are packed with antibacterial soaps, tissues, wipes and other germ-killing products.

But supporters of the hygiene hypothesis, as it is called, believe that shunning dirt -- and the bacteria that comes with it -- may in effect confuse the developing immune systems of our children. Humans evolved to deal with bacteria that were ubiquitous in the environment of our early ancestors and to deal with many illnesses now banished by antibiotics and vaccines. According to proponents of the hygiene hypothesis, without exposure to these bacteria, our immune systems overreact. The result: increased rates of a variety of immune disorders -- including allergies, inflammatory bowel diseases, Type 1 diabetes and asthma -- in much of the developed world. These illnesses have not increased at the same rate in the underdeveloped world, where ultracleanliness remains a luxury few can afford.

In the case of asthma and allergies, proponents of the hygiene hypothesis believe that when children are raised in germ-free environments and are given antibiotics, their immune systems do not receive the early "priming" they require, and so do not develop properly. The result in many children is an inappropriate response to relatively benign substances like pollen and cat hair.

A highly publicized study by scientists in Europe found that children raised around farm animals had significantly lower rates of asthma and allergies than children from cleaner households. Researchers, who published their findings in the New England Journal of Medicine, believe that the farm children's early exposure to bacteria from farm animals strengthened their immune systems.

Recent findings by researchers at the Medical College of Georgia in Atlanta seem to bolster the hygiene hypothesis as well. The study concluded that exposure to two cats or dogs during the first year of life reduced a child's risk of developing allergies by 75 percent. The children raised with pets were not only less reactive to pet dander, they were less vulnerable to other potential allergens, such as pollens, grasses and mold.

• Cleaning Up Our Air
• Return to 'Why Johnny, Julianna and Jose Can't Breathe'

back to top