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What is the “hygiene hypothesis?”

Last updated on November 21st, 2021

The hygiene hypothesis is a theory that attempts to explain the increased prevalence of allergy and asthma in affluent, industrialized nations. It also strives to elucidate factors that are responsible for the development of asthma in individuals.

The British epidemiologist David Strachan advanced the hypothesis in 1989 after studying the health records of approximately 17,000 British children. The hypothesis proposes that the rising prevalence of asthma and allergic diseases parallels the decreasing prevalence of infections in childhood. Over the last 100 years, urbanization, advances in public health, improved sanitation, and the adoption of cleaner living environments, along with the introduction of antibiotics, have all lead to reduction in infectious illnesses in children.

During the same period of time, the occurrence of asthma and the allergic diseases has increased. The hygiene hypothesis links the two observations. The hypothesis suggests that the reduced exposure to “dirty” environments and to infectious agents at a specific point in childhood leads to less stimulation of certain parts of the growing child’s immune system. Changes consequently fail to take place in the maturing immune response and, in turn, predispose that child to an increased risk of developing allergies or asthma.


Epidemiological studies of large groups of people both with and without asthma have noted that two environmental factors seem to protect against the development of asthma: attendance at a day-care facility before the age of 1 year and the presence of a dog in the family home before birth and onward. Both factors support aspects of the hygiene hypothesis. Early exposure to “dirty,” less sanitary environments appears to lead to a lower occurrence of asthma in that specific, exposed population.

Research continues in the area of the hygiene hypothesis. Is there an age at which a child’s immune system needs to be “stimulated” in a specific way, by certain environmental agents or viruses, in order for asthma not to develop? If such were the case, specific interventions or medications could be developed to modify a child’s risk of asthma. The hygiene hypothesis remains controversial, and at present it represents an intriguing theory that is far from definitive.

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