The Hygiene Hypothesis: Is Cleanliness Next to Sickliness When it Comes to Asthma and Allergies?
By Christina Elston

Twelve-year-old Micah had his first and only ice cream as a baby. He was 10 months old, and his grandmother let him have just a taste from an older brother’s dish. “He turned bright red, broke out in hives, and began this wet wheezing, like he was drowning,” recalls his mother, Vicky Enriquez. Fortunately, there was a hospital just a few blocks away.

Today, Enriquez knows that Micah – but not her other three children – is allergic to milk, eggs and soy products. He also has asthma. The family now specializes in reading food labels, and has an inhaler in every car. They know which fast-food restaurants use egg in their hamburger buns and have an understanding with their local pizza parlor about the true meaning of “no cheese.”

The U.S. Centers for Disease Control and Prevention (CDC) reports that there are more kids like Micah every year – kids who have allergies, asthma or both. But asthma and allergy rates aren’t the same everywhere.

“It’s highest in our ultra-urban modernized cities,” says Dr. Andrew Liu, an associate professor in pediatric allergy and clinical immunology at the National Jewish Medical and Research Center. Dr. Liu is one of several researchers trying to find out why.

There’s a growing body of evidence that the increase in allergy and asthma could have something to do with our modern tendency to like things super clean. Known as the “Hygiene Hypothesis,” this theory suggests that all this sanitation keeps us from being exposed to natural substances that help us develop normal, healthy immune systems.

“Think about being born,” says Dr. Stanley Goldstein, an allergist who has written about the Hygiene Hypothesis for the American Academy of Allergy, Asthma and Immunology. “Everything is sterile.” Dr. Goldstein points out that even after infants leave the hospital, the environment is still fairly sterile at home. Even in inner-city conditions, with eight people living in a small apartment, “that is still a more sterile environment than being on a farm,” he says.

Exposures and Lower Risks

Studies have shown that exposure to farm animals or pets, other children, and even illness can be linked with lower allergy and asthma rates.

For example, the occurrence of fevers during infancy have been linked to decreased likelihood of developing allergies later on. In the February 2004 issue of Journal of Allergy and Clinical Immunology researchers reported that half of the children they studied who had not experienced a fever during their first year of life showed allergic sensitivity by the age of 7. Of those who had only one fever, 47 percent became allergy prone. But among those who suffered two or more fevers in infancy, only 31 percent developed allergies.

Studies have also pointed to a link between rural living and lower asthma and allergy risk. Goldstein notes that prior to the reunification of Germany, there were more East German children growing up in large families on farms and they had fewer allergies than their counterparts in the more urban and modernized West Germany. After unification, East Germany developed a more western culture with less farming and more sterile conditions, and the prevalence of allergies went up.

A study highlighted in the American Journal of Respiratory and Critical Care Medicine in 2000 found a similar link for asthma. Among 1,200 rural Canadian teenagers, those who grew up on farms were 40 percent less likely to have asthma than those who did not.

Animals in the home, contrary to popular belief, may also help. A National Institute of Allergy and Infectious Diseases (NIAID) study reported in a 2002 found that children raised in a house with two or more dogs or cats during the first year of life were less likely to develop allergic disease than those raised without pets. Those with pets were 66 to 77 percent less likely to have any allergic antibodies to common allergens – including dust mites, ragweed and grass.

Like exposure to animals, exposure to other children – whether those are siblings in the home or fellow daycare kids – seems to be beneficial. A study reported in the New England Journal of Medicine in 2000 found that the presence of older siblings in homes and children’s attendance at daycare centers during the first six months of life was associated with lower risk of development of asthma.

And a Danish study published in April in the British Medical Journal found that having three or more siblings, attending daycare, having pets and living on a farm all reduced the risk of allergic illness among infants

Still Just a Theory

Allergists and researchers are quick to point out that the Hygiene Hypothesis is far from proven fact, and that it is too soon for them to make specific recommendations.

Dr. Jeffrey Kishiyama, a leading allergist and president of the California Society of Allergy, Asthma and Immunology, says that the hypothesis is mainly based on studies that look at subjects’ history. Because these studies aren’t performed in a lab, it is difficult to isolate specific causes.

“In retrospective studies, you can identify an association, but you can’t really assign or prove any cause-and-effect,” Dr. Kishiyama explains.

Pediatric allergist Dr. Richard Wasserman says the NIAID pet study failed to take into account whether the result was due to breathing in pet allergens, fecal material from pets in the carpet that might contact children’s skin or fecal material from indoor pets finding its way into the mouths of infants.

In addition, not all studies support the hypothesis. “There’s evidence that points the other way,” says allergist Dr. Richard Barbers. “Asthma is a very complex disorder. Genetics, environment and development can all play a role.”

The development of allergies is also not a simple process, according to Kishiyama. Genetic factors, level of exposure to allergens, and exposure to bacterial by-products could all be involved. “It may take as many as three different things occurring at the same time before you develop allergies,” he says. Because of this, it will take large-scale studies to firmly establish cause and effect of these factors. Researchers are also studying the possible role of diesel particles in the air in urban areas, and diet, Kishiyama says.

How Clean Is Clean?

Still, it is hard to discount the evidence that our ultra-clean urban living isn’t really helping. Newborns do need clean conditions to protect them against life-threatening infection, Goldstein says. But many parents go overboard. Following your doctor’s orders is essential, but boiling baby’s bottles or using only bottled water usually isn’t necessary.

“You have to be reasonable, but you don’t have to be totally sterile,” Goldstein says.

Sometimes bacteria is good, says allergist Dr. John Costa. “Your gut is filled with bacteria, but it is part of your body’s normal system of functioning,” Dr. Costa explains. “Just because it’s a bacteria, doesn’t necessarily mean it’s doing something bad.”

Though Costa considers the Hygiene Hypothesis only a partial explanation for the increase in allergies and asthma, he also sees no need for our antibacterial obsession. “There is no proof of any harm,” he says. “But I think there’s equally not much proof of any benefit.”

When you remove harmless bacteria, you can leave a void that will be filled by something more harmful, because no surface will stay bacteria-free for long, Costa says. “It’s not possible to eliminate all bacteria.”

style="FONT-FAMILY: Verdana; FONT-SIZE: 10pt">And even if you could keep household surfaces bacteria free, it wouldn’t keep your family from getting sick. “Risk of infection comes overwhelmingly from the people you have contact with, not from other things,” says Dr. Wasserman.

Keep Washing Those Hands
style="FONT-FAMILY: Verdana; FONT-SIZE: 10pt">Hand-washing reduces the risk of transmitting many viral illnesses, and won’t wash away your allergy protection. Why? Because researchers suspect that only pieces of bacteria are needed to set the immune system properly. Called endotoxin, these micro-organisms are found in specific bacteria, and are potent long after the bacteria itself is dead.

In addition, researchers believe protective exposures happen early in life, while the immune system is still developing.

“Our immune system is learning a lot about the environment from the moment of birth,” says Liu, who compares the process to the way the brain learns, which is more intense in early childhood. Endotoxin exposure later in life hasn’t been shown to be very helpful.

Keep the Cat?
style="FONT-FAMILY: Verdana; FONT-SIZE: 10pt">Having pets at home still isn’t considered helpful to children who already have allergies or asthma. But if your kids don’t suffer from allergies or asthma, even experts not sold on the idea that pets decrease the risk of allergy say there’s no conclusive proof they do any harm.

style="FONT-FAMILY: Verdana; FONT-SIZE: 10pt">“As an allergist, I’ve become more pet-friendly,” says Liu. “I no longer tell people they need to get rid of their pets if they want to keep their children from becoming allergic.” When Liu measured dust samples from various homes, he found evidence for his position. “The main indicator of more endotoxin is pets,” he says.

Don’t Demand Antibiotics
style="FONT-FAMILY: Verdana; FONT-SIZE: 10pt">One thing many doctors like about the Hygiene Hypothesis is that it offers one more reason to steer parents away from asking for unnecessary antibiotics.

style="FONT-FAMILY: Verdana; FONT-SIZE: 10pt">Overuse of antibiotics in infants and young children needlessly exposes children to side-effects and encourages the growth of bad germs, Wasserman says. “And you may actually be hurting your child in terms of allergy or asthma.”

The American Academy of Pediatrics has moved to discourage antibiotic overuse, especially in instances of middle ear infection, and the CDC is working to emphasize the appropriate indications for prescribing antibiotics.

Someday, A Vaccine?

If doctors ever do make concrete recommendations based on the Hygiene Hypothesis, they probably won’t be suggesting lifestyle changes. “It is very difficult for physicians to induce lifestyle changes in their patients,” Wasserman notes.

Instead, conclusive proof that the theory is true might result in an asthma and allergy vaccine. There is already at least one U.S. study involving a vaccine made up of bacterial by-products, Kishiyama says.

But researchers are still in need of some definite answers. “What do you have to be exposed to, and how early?” are still crucial unknowns, Goldstein says. Researchers must follow subjects from early infancy and through early adulthood, so it will be some time before definite conclusions are reached.

Meanwhile, we watch, wait and pass the tissues. “We’re honing in on it,” says Dr. Barbers, “and that’s the good news compared to 10 years ago.”


• The American Academy of Allergy Asthma & Immunology – Offers pollen counts, allergy reports and referrals to allergists. A service of the National Institutes of Health that lists information on clinical trials of new vaccines and other treatments.

• The National Jewish Medical and Research Center – Lists patient information, referrals and research studies.


  • What’s New in Our Understanding of Allergies?

  • Examining the Childhood Asthma Epidemic

    Christina Elston is a freelance writer, an author of books on children’s health and a former editor with United Parenting Publications.
    September 2004

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