Rapid Responses to:
|
|
Rapid Responses: Rapid Responses published:
-
![[Read Rapid Response]](/icons/misc/sectionDOWN.gif)
Why have allergic diseases been increasing worldwide over several decades? The humidity hypothesis may be the answer
- Christopher ME Rowland Payne (23 July 2009)
|
|
|||
|
Christopher ME Rowland Payne, Consultant Dermatologist The London Clinic, 5 Devonshire Place, London W1G 6HL
Send response to journal:
|
Van Bever, Lane and Common, in their Editorial, “Gene defects and allergy”(1), highlight the role of the FLG gene in the allergic (or atopic) disorders: atopic eczema, allergic rhinitis and asthma. Null (loss of function) mutations of the FLG gene may be the most important marker of the genotype that is susceptible to atopic disorders. Intriguingly, the Editorial also draws attention to the increasing worldwide prevalence of allergic disorders in recent decades. In genetic terms, one or two generations is a very short time. It is almost certainly too short a time to be able to allow a sufficient genetic change to account for the massive rise in prevalence of the atopic disorders over the last 30-40 years. The worldwide atopic epidemic is more likely to have an environmental explanation. The atopic disorders are common in westernised societies. In the underdeveloped world, they remain uncommon. A paper from Zimbabwe (2) compared the prevalence of atopic disorders in different communities. In poor, traditional rural communities, they are rare; in shanty-town life they are middling in prevalence; and in well-off town dwellers, living in western conditions, prevalence is similar to that in western countries. This African study, and similar studies from around the world, indicate that environment determines whether latent atopy becomes clinical atopic disease. Put another way, environment determines whether the atopic genotype achieves phenotypic expression. An environmental hypothesis – the humidity hypothesis (3) has been proposed. The humidity hypothesis draws attention to the relationship between improved economic circumstances and the increased availability of central heating, air conditioning, washing and bathing. Each of these better living standards disfavour humidity of the epidermis and of the mucous membranes of upper and lower respiratory tracts. The humidity hypothesis postulates that air dried by central heating or air conditioning steals moisture from the ambient air and from skin and mucous membranes and that in this dry environment, dust and other particles rise and can tickle or irritate skin and mucous membranes precipitating eczema, asthma and hay fever. Further support for the humidity hypothesis exists in a number of observations. Nebulisers and steam inhalations are standard treatments for asthma and rhinitis. Emollients enhance skin humidity and prevent attacks of atopic dermatitis. Humidification of the indoor environment improves atopic dermatitis. Going on holiday from London to the humid West Indies (so long as air conditioning is absolutely avoided) will clear even the most severe forms of atopic dermatitis in just a few days (much more swiftly than admission to hospital). The humidity hypothesis may explain the key environmental change that accounts for the worldwide atopic epidemic of the last 30 or 40 years. Christopher Rowland Payne Competing interests: None declared. 1. Van Bever H, Lane B, Common J. Gene defects and allergy. Identifying specific genotypes of allergy is a major breakthrough in patient care. BMJ 2009;339:58-59. 2. Keeley DJ, Neill P, Gallivan S. Comparison of the prevalence of reversible airways obstruction in rural and urban Zimbabwean children. Thorax 1991;46:549-53. 3. Rowland Payne CME. The atopic epidemic: why are atopic diseases more and more common? Diminished humidity may be the answer. Journal of Cosmetic Dermatology 2004;2:117-118. Competing interests: None declared |
|||