HCA image Millions of people suffer from the sneezing and wheezing of allergies and asthma , diseases that have suddenly become epidemic in some parts of the world. Once thought simply bothersome and best managed by over-the-counter antihistamines, doctors now understand that allergies are serious disorders that may demand advice from a physician. But what causes allergies and asthma? And why the dramatic rise in these two diseases over the past decades?

What are allergies?

Allergies are an overreaction of the immune system to substances that under normal circumstances would cause no reaction in most individuals. The results of this immunologic overreaction are symptoms such as sneezing, wheezing, coughing, and itching.

Today, allergies are the sixth leading cause of chronic disease in the US, with an annual cost exceeding $18 billion. They are believed to complicate and even handicap the lives of nearly 50 million children and adults.

What is asthma?

Asthma is a disease in which the airways become blocked or narrowed, causing shortness of breath, breathing trouble, and other serious symptoms. In the case of a severe asthma episode, a person may need emergency treatment to restore normal breathing.

It is estimated that more than 17 million people in the US have asthma and that severe episodes result in nearly half a million hospital stays each year.

What is the hygiene hypotheses?

In 1989, David Strachan proposed the hygiene hypothesis. He suggested that the rising incidence of allergic disease, such as asthma or allergies, was actually linked to reduced exposure to germs through declining family sizes, more limited exposure to animals, and higher general standards of cleanliness. Strachan reasoned that repeated exposure to microbes at an early age, for example as a result of having siblings, owning a pet, living on a farm, or attending day care, actually helped our immune systems to properly adapt so they would not overreact to routine environmental stimuli, such as potential allergens.

According to this hypothesis, in the absence of this repeated exposure, our under-stimulated, developing immune system would then be over stimulated by harmless substances like pollen, resulting in the development of allergies.

Since Strachan’s 1989 report, the Center for Disease Control and Prevention (CDC) has reported that the prevalence of asthma in the US has doubled from approximately 6.8 million in 1978 to more than 15 million in 1998. Additionally, about 5% of the US population has asthma, with the highest percentage of these being children between the ages of 5-14.

What does the evidence say?

Over the years, some research has backed up Strachan’s hypothesis, and many scientists have come to believe that too hygienic an environment may set the stage for allergic disease later in life.

Possibly the most famous and clear cut example of the hygiene hypothesis comes from comparing the prevalence of allergies in the East and West German populations before and after unification. Before unification, East Germany had more children growing up on farms and in larger families than West Germany; the population also had much lower rates of allergies and asthma than West Germany. After unification, however, when East Germany developed a more western culture, its rates of allergies and asthma increased to the degree that they now resemble those of West Germany. East Germany also had a quite highly developed health care system, so it is perhaps less likely that the observed differences in asthma diagnosis were due to different patterns of medical diagnosis rather than true differences in disease prevalence.

A more recent study, published in the May 2004 British Medical Journal sought to discover why some people who are exposed to microbes develop infectious diseases while most do not. The researchers theorized that an association likely existed between the diagnosis of an infectious disease and the subsequent development of allergies. They compared the prevalence of eczema (a type of allergic disease also known as atopic dermatitis) in children who had been repeatedly diagnosed with infectious diseases, such as colds and otitis media, to children who had been repeatedly exposed to environmental microbes without developing these infections. They found that while early and repeated microbial exposure decreased a child’s risk of developing allergic diseases later in life, repeated infectious diseases actually increased this risk.

These findings support the hygiene hypothesis as it applies to microbial exposure, but contradict it when an infectious disease develops. They also appear to contradict the results of a study published in the Journal of Allergy and Clinical Immunology. In that study, researchers found that not only were children who had fevers early in life less allergy prone, but the more fevers they had, the more allergy resistant they became.

What can A concerned parent do?

The hygiene hypothesis is still merely that: a hypothesis. It may eventually teach us a great deal about why allergic diseases occur, or it may (like many other hypotheses) turn out to be simply wrong. A 2008 article, entitled “The end of the hygiene hypothesis?”, argued that asthma rates have begun to decline in most industrialized countries even though few children now live in less hygienic environments. The author argues that at most only a modest proportion of asthma cases are likely to be affected by early life infections. For now the hypothesis may or may not have relevance for scientists, but it has no certain implications for parents. Childhood continues to have risks. Among these risks are allergies, but also trauma and, occasionally, fatal injuries from animal-related bites or other injuries. Clearly, more research is needed before we solve the riddle of our increased rates of allergic disease.