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