JULY 30, 2004

Climate, geography affect asthma and eczema risk

Breathe easier livin' the high life. But if you're itching for a change, head for the tropics

Asthma can be pretty capricious, getting revved up by changes in season, temperature or humidity. Allergy specialists have long suspected that geography may also affect asthma rates. Their suspicions have at last been confirmed by an international study headed by Dr Stephan Weiland of the University of Ulm in Germany. The researchers found that climate and geography influence rates of both asthma and atopic eczema.

Published in the July issue of Occupational and Environmental Medicine, the study was based in 146 centres in 56 countries on six continents. The 670,000 subjects aged six to seven and 13 to 14, and their parents answered a written questionnaire.

For asthma, the main question was: "Have you (or your child) had wheezing or whistling in the chest in the last 12 months?" Similar questions were included to chart flare ups of allergic rhinoconjunctivitis and atopic eczema as well. The frequency of these symptoms was then compared to local climate conditions, such as the annual mean temperature and humidity, the annual variation and estimates of indoor humidity for buildings kept at a temperature of 20°C. The researchers analyzed the data, controlling for known variables, particularly national differences in asthma rates.

The researchers found that each 10% increase of indoor humidity brought with it a 2.7% rise in asthma risk. The most likely explanation for this effect is that dust mites -- a major allergen -- and mould thrive in very humid conditions.

A clear correlation between low altitude and asthma symptoms was also discovered. In Western Europe, each 100m of elevation brought a 0.88% decrease in risk of asthma. Interestingly, countries with colder average temperatures had higher asthma scores while those with fewer very cold winter months, such as Sweden, had lower rates of asthma than countries with low but steady average temperatures like Britain. On a worldwide scale, however, the investigators did not find an association between asthma and the annual averages of outdoor temperature and relative humidity.

Eczema showed clear geographic variations, with higher latitudes and lower average temperatures bringing extra cases. Higher estimated indoor humidity appeared to reduce eczema rates, likely because low humidity dries out skin, increasing the risk of eczema.

Although there's little anyone can do to change the climate, these findings suggest that adjusting indoor humidity could have a positive impact on asthma and eczema. The authors say that their research will have more relevance in the future when the world's climate may be very different than it is today. This research, they argue, may "have implications for the assessment of potential health effects due to climate change."



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