A marked increase in the incidence of allergies and atopic disorders is evident in developed countries, beginning in the 1960s and steadily progressing throughout the rest of the twentieth century.
The hygiene hypothesis was proposed as an explanation for this trend. It suggests a link between the incidence of allergies and factors such as family size, hygienic standards and exposure to microbial substances.
Skin allergy test. Image Copyright: nito / Shutterstock
Hygiene Hypothesis and “Old Friends” Hypothesis
The hygiene hypothesis was first introduced in 1989 by Strachan, who observed a link between smaller family size or hygiene standards and increased risk of allergies. There had been some observational research in this area prior to 1989, such as a large study of more than 17,000 British children in 1958 that suggested an inverse relationship between allergic diseases and the number of older siblings.
The presence of microbes is believed to assist in the function of the human immune system and play a protective role against allergies. Due to the considerable changes in sanitation standards introduced in the industrial revolution, exposure to some microbes that would otherwise boost the immune system was diminished. This was believed to result in compromised function of the immune system and an increase in the incidence of allergies.
However, there were some aspects that fail to be explained by the hygiene hypothesis. In 2003, Graham Rook developed the “old friends” hypothesis as a replacement for hygiene hypothesis to explain some of these aspects. Notably, the “old friends” hypothesis places an emphasis on the ancient microbes that were present throughout human evolution, rather than childhood infections that reduced in incidence greatly throughout the same time period.
The rates of allergic rhinitis, also known as hay fever, and atopic sensitization in East German children increased significantly following the reunification of the country, which led to questions about the impact of Western lifestyle on the incidence of hay fever.
The prevalence of asthma has risen by approximately 1% annually on a consistent basis from about 1980, and allergic asthma is thought to cause the majority of this increase, particularly among children. Some recent research appears to show a slowing of the trend of atopic disease as it reaches a plateau.
The prevalence of eczema has also increased along with the other allergic diseases and is currently 10% of all children in the United States, as high as 18% in some states. This is in stark contrast to prevalence rates of 3.1 to 6.4% in the 1970s.
The prevalence of food allergies has also been on the rise in recent decades, which may be linked to the hygiene hypothesis. In 2008, the Center for Disease Control and Prevention released a report that notes an 18% increase in food allergies in children less than 18 years of age in the preceding 10 years. Additionally, children with food allergies were more likely to be affected by another atopic disease such as asthma.
Research has indicated that 8% of children have an allergy to a food, and 30% of these have multiple food allergies and more than a third have a history of severe allergic reactions. The most common food allergies are peanuts, eggs, milk and shellfish. However, early exposure to these foods is currently being investigated as a method to reduce the development of food allergies.