As the number of people with
soars, medical scientists are trying to figure out why. Most experts say that exposure to dust mites, cockroaches, and similar environmental contaminants may lead to asthma. But another theory suggests that modern improvements in public sanitation, vaccination, and medical treatment have contributed to the increasing rates of asthma.
The Hygiene Hypothesis
Proponents of the "hygiene hypothesis" contend that infections at a young age help the immune system mature. They feel that vaccinations and prompt treatment of infections with antibiotics may thwart this process, as may overzealous cleaning practices aimed at removing bacteria and viruses from the environment.
"Infections prime the immune system and may have a reciprocal effect of lessening the likelihood of developing asthma and allergies," says Andrew Liu, MD, at the National Jewish Medical and Research Center in Denver, Colorado.
But not all experts agree with the hygiene hypothesis. "The cleanliness or hygiene hypothesis is far from proven," says Darryl Zeldin, MD, head of clinical studies at the National Institute of Environmental Health Sciences.
Evidence Behind the Hypothesis
Several population studies have shown an association between people's living environments and allergies. For example, children who grow up on a farm, live in large families with older siblings, or attend day care from an early age seem less likely to suffer from allergic conditions.
In addition, allergies and asthma are more common in developed nations, such as the United States, Western European nations, Australia, and New Zealand, whereas fewer people suffer from these conditions in developing countries, such as Eastern European nations, China, and India, where contagious diseases occur more frequently and children get more infections.
But population studies alone do not prove the hypothesis. The best way to determine if infectious exposures afford preventive benefits is to enroll infants in long-term clinical trials, something that's not likely to happen without safety assurances.
The Role of Endotoxins
Dr. Liu recently investigated whether exposure to bacteria is associated with a protective effect against asthma and allergies. He measured the level of endotoxins—substances in the cell walls of common bacteria—in the environment. Endotoxins, which are difficult to destroy, are found in dust, soil, pets, and the digestive system. The study, published in the medical journal
, found that children whose
did not show any allergen sensitization were more likely to live in homes with higher endotoxin levels. Allergen sensitization is a risk factor for allergy development.
According to Dr. Liu, if environmental endotoxins do affect immune system development, then exposure to bacteria may have two separable and opposite effects: the prevention of allergy and the promotion of infection.
Dr. Zeldin, on the other hand, believes that exposure to endotoxins will increase allergy and asthma risk.
"We think endotoxin is a culprit, not a protective agent," he says. "It may be that exposure to some of the bacteria that gives rise to endotoxin might lead to immune alterations that can be nonallergic or protective, but that's not the current thinking by most people."
People in both camps agree that endotoxins can trigger an allergic response in people who already have been sensitized to it. "Once you have asthma and allergies, endotoxins make them worse," Dr. Liu says. "It's a tremendous paradox that…endotoxins should be protective, and yet, once you have the disease, clinically they make it worse."
What's a Person to Do?
The hygiene hypothesis, which refers to public, not personal, hygiene, needs further testing before any public health recommendations can be made. Twentieth century indoor plumbing, antibiotics, and other public hygiene advances have dramatically reduced infant deaths, and none of these researchers suggests returning to the days of outhouses or banishing vaccines. Further developments in asthma prevention and treatment will hopefully shed more light on the validity of this hypothesis.
Meanwhile, other scientists pursue the dust connection. Researchers at the National Institute of Environmental Health Sciences recently announced results of a study that found high levels of dust mite allergens in approximately 45% of US homes. While it is clear that these allergens contribute to worsening asthma symptoms in susceptible people, the extent to which they alone cause a person to develop asthma in the first place is still widely debated.
Previous studies have found children exposed to higher levels of dust mites, pets, and other allergens had a greater chance of developing asthma and more severe asthma later in life. "…The things that are in indoor dust are not good for asthma," says Dr. Zeldin. "And a lot of those things can lead to allergic sensitization and development of asthma." Others argue that these studies do not reliably differentiate between the development of new onset asthma and the worsening of symptoms in previously asthmatic patients.
Either way, it is clear that people with asthma and allergies should avoid substances that trigger attacks. Several experts recommend keeping the house as dust-free as possible, covering bedding with plastic, protective covers, and vacuuming frequently with a machine equipped with a HEPA filter.
"To decrease the prevalence of asthma," Dr. Zeldin says, "one of the things people can do is clean up the indoor environment."
Eder W, Ege MJ, von Mutius E. The asthma epidemic.
N Engl J Med.
Gereda J, Leung D, Thatayatikom A, et al. Relation between house-dust endotoxin exposure, type 1 T-cell development, and allergen sensitisation in infants at high risk of asthma. Lancet. 2000;355:1680-1683.
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