Can You Be Too Clean?

If you're feeling guilty about leaving your baby in germ-ridden daycare, letting the dog slobber all over her, and not keeping your house spotlessly clean all the time, you can stop worrying. Though your little one may have a few more colds now than her less hygiene-compromised peers, she may be less likely to suffer chronic problems with asthma and allergies later, according to the latest research.

In this century, there has been a huge increase in the numbers of children with asthma and allergies worldwide, particularly in developed countries. In the United States, asthma is now the most common chronic disease of childhood, affecting nearly 5 million children and adolescents. Reports show a 75 percent increase in asthma prevalence from 1980 to 1994, with an astounding 160 percent rise in children under the age of four, according to the Centers for Disease Control and Prevention.

In 1989, an epidemiologist writing in the British Medical Journal suggested that the rise in allergies may be due to declining family sizes and higher standards of cleanliness that provide young children with less exposure to germs. This, in turn, is thought to give their still-developing immune systems less practice fighting off intruders. The result, as the theory goes, is that the under-challenged immune system wants to be used, so it's primed to see harmless substances like dust and pollen as dangerous invaders, leading to allergies and asthma.

Recent studies show that there may be some truth to this idea, which has come to be known in medical circles as the "hygiene hypothesis." In 1997 a study of almost 12,000 families in England and Scotland found that the more children a family had, the less the incidence of asthma. A May 2000 study in the American Journal of Respiratory and Critical Care Medicine reported that among almost 1,200 teenagers in Canada, those who grew up on farms were 40 percent less likely to have asthma than their urban and suburban counterparts.

And in a more comprehensive study in the August 24, 2000 New England Journal of Medicine, scientists at the University of Arizona followed a group of 1,035 children from infancy until they were as old as 13. They found that among babies under six months, those who had older siblings or were in daycare were more likely to have asthma symptoms such as wheezing. But after the age of six, these same children were 40 percent less likely to be suffering from asthma.

"What do siblings, animals, and daycare have in common?" asks Dr. Thomas M. Ball, assistant professor of Clinical Pediatrics at the University of Arizona and one of the authors of the study. It sounds inelegant, but the answer is probably contact with trace amounts of feces. Ball speculates that what actually causes that decrease in asthma later isn't actually the number of infections a baby has, but rather the amount of contact she has with endotoxins, which are substances that are given off by bacteria when they die. Feces are loaded with them.

But the important thing to note from Ball's study is that the window of opportunity for affecting the developing immune system seems to occur during the first year of life. Research has shown that a baby's immune system begins preparing for microbial onslaught even before birth, with the placenta acting as a filter that lets through small amounts of innocuous allergens and microbes. Babies, it seems, are born ready to have their immune systems challenged. So don't stress too much about the various bugs your baby may be encountering at daycare or from the family pet. They may be the best thing for her in the long run.