You might not want to clean up your act

IT’S time to come clean: My 4-year-old son doesn’t bathe every day and sometimes wears the same clothes two days in a row. You’ll find no hand sanitizer in my purse. And in my home, children don’t have to wash their hands before meals, food that falls to the floor can be consumed if the dogs don’t get to it first, and fishing through the laundry hamper for a favorite pair of jeans is fair game.

Some people might shudder at this apparent lack of cleanliness, but proponents of the so-called “hygiene hypothesis” would applaud my family’s behavior.

These scientists believe that an excessively clean environment during early childhood can actually be harmful -- preventing exposure to germs that stimulate and strengthen the immune system.

Such exposure helps balance the two main branches of the immune system and ensure its normal maturation, they say. When babies are born, their immune systems rely primarily on one branch; after birth, however, the emphasis shifts and the other one becomes more important. Although researchers do not know exactly what triggers this change, many believe that encounters with viruses and bacteria are involved.

When the necessary exposures don’t take place during early childhood, the proper balance is never reached, and the immune system may react abnormally later in life.

Individuals with an immature immune response appear more prone to allergic conditions such as asthma, hay fever and eczema. Advocates of the hygiene hypothesis believe that reduced exposure to childhood infections may help explain the increased prevalence of allergic diseases in industrialized countries, where sanitary conditions tend to be high.

In this country, asthma rates more than doubled between 1980 and 1998, and rates of hay fever have climbed dramatically in the last several decades.

While the theory is hard to prove, several factors related to a more sterile environment are clearly associated with allergic conditions. Smaller family size, for instance, has been linked to hay fever. It’s speculated that children in small families miss out on exposure to protective germs brought home by older siblings.

Day care attendance also appears to influence a child’s risk of developing allergic conditions. A study published in the New England Journal of Medicine in 2000 found that children who enter day care before 6 months of age are less likely to develop asthma than those who are never enrolled in day care or who enter when they are older.

Several studies have found that allergic conditions are less common among children growing up on farms than among those living in more urban environments. Recently, researchers at the University of Wisconsin in Madison found that being around dogs can be protective too. Children who grew up with dogs in their homes had a dramatically lower risk of eczema (an allergic skin condition) than children raised in dog-free environments.

But the hygiene hypothesis goes well beyond allergic diseases. There’s evidence that lack of exposure to infections as a child may even increase the risk of certain types of cancer, notably Hodgkin lymphoma in young adults.

Studies show that people who attended day care or nursery school as children are somewhat protected from Hodgkin lymphoma, as are those who grew up with lots of siblings.

“If children are raised in a more protective environment, they are less likely to be exposed to many childhood infections when they are young, and when the infections are milder,” explains Dr. Wendy Cozen, an associate professor in the USC Department of Preventive Medicine.

“Acquiring one of these infections when they are older puts them at risk for developing Hodgkin lymphoma.”

Some studies suggest that underexposure to germs may contribute to heart disease as well. Last year a group of European researchers reported that viral infections early in childhood can reduce the risk of heart disease later in life. They compared 350 patients with coronary heart disease to an equal number of disease-free individuals, and found that men and women who suffered fewer viral infections as children were more likely to have heart disease as adults.

Fostering a somewhat lax attitude toward cleanliness is not to say that good hygiene doesn’t matter at all. Improvements in sanitation are responsible for saving millions of lives in this country over the last century.

It is just that even cleanliness can be taken to an extreme, and not every germ poses a threat.

It is important to keep in mind that the hygiene hypothesis is still just a theory without enough scientific evidence to call it proven. While the research in support of it is compelling and steadily increasing, it is still too early to declare that the cleanest environments -- not the dirtiest -- are the best breeding grounds for disease.

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Dr. Valerie Ulene is a board-certified specialist in preventive medicine practicing in Los Angeles. She can be reached at themd@att.net. The M.D. appears the first Monday of the month.