How much has asthma prevalence increased during the last 20 years?
The Centers for Disease Control report that the prevalence of asthma in children up to 17 years of age has increased by 5% per year each year from 1980 to 1995 (1).
Similar increases are being reported from more developed or industrialized countries all around the world, including recently in Latin American countries (2). The form of asthma that is increasing is the allergic form.
What caused this increase?
Many scientists have theories about what has caused this dramatic increase in asthma prevalence.
Until recently, most theories have focused either on changes in housing conditions (such as more sealing of modern houses) or increased sensitization of children to allergens. However, the magnitude of the increase is so great that it appears increasingly unlikely that such factors could provide an explanation.
Genetic factors are also not thought to be the cause of this increase. Genetic factors contribute to the onset of asthma, and asthma tends to run in families, genetics do not change quickly. So, genetic factors are not thought to have caused the recent increase in asthma.
Sensitivity to allergens is the single risk factor that best predicts development of asthma (3). However, this sensitivity is not likely to have changed to the degree necessary to explain the increase (4).
Exposure to environmental tobacco smoke at an early age is considered to be a risk factor for development of asthma in children (5), though unlikely as well to have caused the large increase in prevalence. Exposure to outdoor air pollutants has been found in some studies to be associated with development of asthma (7)(8)(9). Again, the magnitude of the increase is so great that most scientists do not think the outdoor pollutants are responsible.
Exposure to indoor allergens is widely considered to be a risk factor for the development of asthma (6), perhaps very early in chilhdood, but even for this possible cause, not all studies support it (4). A recent study in England that looked at whether an increase in asthma and wheezing could be explained by indoor, home exposures found that neither changes in smoking in the home, cooking on gas stoves, nor pet ownership was sufficient to cause the change in prevalence (13).
Other theories for causes for the increase in asthma have been suggested including
Lack of severe or repeated infectious during childhood (10, 11);
Obesity and/or lack of physical exercise (12);
Decreased family size;
Changes in diet.
Scientific opinion has not converged into a consensus about which, if any, of these factors have caused the increase in asthma. Studies are often conflicting.
1. CDC. Measuring childhood asthma prevalence before and after the 1997 redesign of the national health interview survey - United States. Mortality and Morbidity Weekly Report 2000; 49:908-911.
2. Mallol J, Solé D, Asher I, Clayton T, Stein R, Soto-Quiroz M. Prevalence of asthma symptoms in Latin America: the International Study of Asthma and Allergies in Childhood (ISAAC). Pediatric Pulmonology 2000; 30:439-44.
3. Simpson BM, Custovic A, Simpson A, Hallam CL, Walsh D, Marolia H, Campbell J, Woodcock A. NAC Manchester Asthma and Allergy Study (NACMAAS): risk factors for asthma and allergic disorders in adults. Clinical and Experimental Allergy 2001; 31:391-9.
4. Crater SE, Platts-Mills TA. Searching for the cause of the increase in asthma. Current Opinion in Pediatrics 1998; 10:594-9.
5. Gold DR. Environmental tobacco smoke, indoor allergens, and childhood asthma. Environmental Health Perspectives 2000; 108 Suppl 4:643-51.
6. Litonjua AA, Carey VJ, Burge HA, Weiss ST, Gold DR. Exposure to cockroach allergen in the home is associated with incident doctor-diagnosed asthma and recurrent wheezing. Journal of Allergy and Clinical Immunology 2001; 107:41-7.
7. Guo YL, Lin YC, Sung FC, Huang SL, Ko YC, Lai JS, Su HJ, Shaw CK, Lin RS, Dockery DW. Climate, traffic-related air pollutants, and asthma prevalence in middle-school children in taiwan. Environmental Health Perspectives 1999; 107:1001-6.
8. Ramadour M, Burel C, Lanteaume A, Vervloet D, Charpin D, Brisse F, Dutau H. Prevalence of asthma and rhinitis in relation to long-term exposure to gaseous air pollutants. Allergy 2000; 55:1163-9.
9. McDonnell WF, Abbey DE, Nishino N, Lebowitz MD. Long-term ambient ozone concentration and the incidence of asthma in nonsmoking adults: the AHSMOG Study. Environmental Research 1999; 80:110-21.
10. Settipane RJ, Settipane GA. IgE and the allergy-asthma connection in the 23-year follow-up of Brown University students. Allergy and Asthma Proceedings 2000; 21:221-5.
11. Illi S, von Mutius E, Lau S, Bergmann R, Niggemann B, Sommerfeld C, Wahn U. Early childhood infectious diseases and the development of asthma up to school age: a birth cohort study. British Medical Journal (Clinical Research Ed.) 2001; 322:390-5.
12. Platts-Mills TA, Carter MC, Heymann PW. Specific and nonspecific obstructive lung disease in childhood: causes of changes in the prevalence of asthma. Environmental Health Perspectives 2000; 108 Suppl 4:725-31.
13. Butland BK, Strachan DP, Anderson HR. The home environment and asthma symptoms in childhood: two population based case-control studies 13 years apart. Thorax 1997; 52:618-24.
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