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OCR Page 1 of 45171
Hypothesis
Increase in asthma: a more toxic environment or
a more susceptible population?
Anthony Seaton, David J Godden, Katrina Brown
Serial prevalence studies have shown that
cided with the industrial revolution of the 18th
childhood asthma is becoming more common.
and 19th centuries,¹⁶ and it is not surprising
In the UK two recent studies have found that
that the attention of researchers should have
the overall prevalence of episodic wheeze and
focused on air pollution - both outdoors and
of diagnosed asthma has doubled over the last
indoors - as a potential cause of increased
two decades. 12 In these studies the prevalence
asthma prevalence. With regard to outdoor
of hay fever was also shown to have increased
pollutants, there is no doubt that many of these
by a factor of three to four, while that of
- including sulphur dioxide, ozone, sulphuric
eczema had doubled. In children in New
acid and oxides of nitrogen - can induce bron-
Zealand and Australia asthma prevalence has
choconstriction in susceptible individuals in
also doubled over the same period. 34 The sit-
the laboratory setting. 17 There is epidemiologi-
uation in adults is less clear, and data for the
cal evidence from the USA¹⁸ and from the
UK are not available, but most evidence from
UK19 that increases in ambient photochemical
other countries suggests that a similar increase
oxidants may be associated with respiratory
is occurring. Thus, although Yunginger et als
symptoms and increased bronchodilator use in
found no evidence of an increase in the inci-
asthmatic subjects. However, air pollution in
dence of asthma in the adult population of
general in the UK has lessened substantially
Rochester, Minnesota during the years 1964-
since the smogs of the 1950s with reduced
83, analysis of data from conscripts to the
burning of coal in cities, and there has been a
Finnish and Swedish defence forces reveals a
well documented fall in levels of smoke and
sixfold increase in asthma prevalence at call-up
sulphur dioxide. Ozone is primarily a rural
examinations between 1966 and 1989 in Fin-
pollutant and the limited evidence available
land,6 and a 47% increase between 1971 and
suggests that urban levels have not altered
1981 in Sweden.⁷ In Busselton, Australia⁸ the
since the early 1970s. 20 There is some evidence
prevalence of diagnosed asthma in adults aged
of a rise in background levels of oxides of
18-55 increased from 9% to 163% between
nitrogen in rural situations since the 1970s, but
1981 and 1990, and in Manitoba, Canada,
peak urban levels in cities do not appear to
physician diagnosed asthma was found to be
have risen. 2021 While these data over an ade-
increasing in all age groups between 1980 and
quate time and range of sites are sparse, there
1990.9 As well as changes over time, increases
is no evidence of the significant rises in any
in the prevalence of asthma have also followed
pollutants that would have been necessary
Department of
"westernisation" of a society¹⁰ and migration
were they to have been responsible for the
Environmental and
from subsistence societies to economically de-
increase in asthma and hay fever. In support of
Occupational
veloped countries.
Medicine
this conclusion, no association between expo-
Seaton
It is generally agreed that the observed in-
sure to particulates, nitrogen dioxide, or sul-
crease in the prevalence of asthma is not
phur dioxide and prevalence of asthma was
Department of
Medicine and
simply a consequence of improved diag-
found in the American six cities study, 2223 nor
Therapeutics
nosis¹³¹⁴ or of diagnostic transfer. 15 A change
in a more recent study2⁴ which compared
Godden
in genetic susceptibility of the population to
schoolchildren in the heavily polluted city of
University Medical
the development of asthma is unlikely in the
Leipzig and the cleaner city of Munich; there
School. Foresterhill,
time period over which the observed increase
was significantly more hay fever and rhinitis in
Aberdeen AB9 2ZD
has occurred. A more probable explanation is
Munich children, while the prevalence of
that the increase is due to the effects of life in
Rowett Research
asthma and airway hyperreactivity did not
Institute, Greenburn
the industrialised world resulting in an in-
differ significantly between the two popula-
Road. Aberdeen
crease in exposure to exogenous factors which
tions.
AB!9SB
K
may induce asthma, a reduction in host resist-
The major component of indoor air pollu-
Brown
ance, or a combination of both mechanisms.
tion is cigarette smoke and there is abundant
Lenn: to:
Among exogenous factors which have been
evidence that exposure to the constituents of
implicated in the increase in asthma are out-
cigarette smoke in utero and in childhood can
Learned to authors
Received 14 May 1993
door pollutants, indoor pollutants, cigarette
increase the risk of developing allergy and
Revised version received
August 1993
smoke, and allergen exposure. Factors altering
respiratory symptoms. 25-30 Again, however, it
host resistance have received less attention.
September 1993
is difficult to attribute the considerable in-
November 1993
publication
It has been suggested recently that a similar
crease in asthma to changes in parental smok-
increase in the prevalence of hay fever coin-
ing. In the UK there has been a decline in the
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