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Global Trends in Asthma
Although asthma patterns vary throughout
the world, considerable increases in both the
prevalence of asthma and its severity have
occurred globally over recent decades (Bach
2002; Isolauri et al. 2004; Pearce et al. 2000).
Because this rise has been far too rapid to
implicate any genetic basis for change, various
environmental factors and lifestyle factors have
been proposed, and most recently the “hygiene
hypothesis” has been explored extensively
(Bach 2002) as an explanation for increased
asthma prevalence. In this commentary, we
propose an additional explanation: that a sig-
nificant proportion of the increase in both
asthma prevalence and its severity is the result
of anthropogenic climate change.
Evidence for the global increase in the
burden of asthma has come from studies of
incidence, prevalence, and morbidity. Asthma
prevalence appears to have increased since the
early 1960s (Beasley 2002), with the rise in
asthma prevalence occurring among both chil-
dren and adults (Beasley et al. 2000) and in a
wide range of countries with differing lifestyles
(Beasley 2002). Over a similar period, the
prevalence of other atopic disorders, such as
allergic rhinitis, atopic eczema, and urticaria,
has also increased, once again throughout the
world (Bach 2002; Beasley 2002). Figure 1
shows increasing prevalence of asthma in sev-
eral locations. Although different diagnostic
definitions have been used in different loca-
tions, each location shown is internally consis-
tent, and each shows an increase in asthma
prevalence.
Studies of hospital admissions and surveys
of symptoms of severe asthma indicate
increased asthma morbidity since the early
1960s (Beasley 2002; Kao et al. 2001; Kerr
2002), particularly in young children (Beasley
2002). This increase in asthma morbidity can
not be completely explained by an increase in
readmissions, diagnostic transfer from related
disease categories, or changes in medical prac-
tice (Beasley 2002).
In contrast with the overall trend of a rise
in asthma over several decades, a few studies
have reported an apparent leveling off or even
a decline of asthma in recent years. Robertson
et al. (2004) reported a 26% decline in the
prevalence of reported wheeze between surveys
conducted in 1993 and 2002 in Melbourne,
Australia, among children 6–7 years of age.
The study also found a reduction in emer-
gency department visits and hospital admis-
sions, which may be due at least partly to
improved asthma management. Interestingly,
the same survey found a 31% increase in
allergic rhinitis, which is commonly linked to
asthma, a 55% increase in eczema, and an
increase in those taking regular steroid medi-
cation among those with frequent wheeze. A
United Kingdom study comparing reported
wheeze in 12- to 14-year-olds in 1995 and
2002 (Anderson et al. 2004) also found a
decrease in reported prevalence of wheezing,
and a decline in frequency and severity of
attacks, but the proportion reporting that they
had ever had asthma increased by 26% (aller-
gic rhinitis by 8%). Both of these studies infer
trends from only two time points. Although
this is not uncommon where good data are
scarce, care is required in their interpretation.
Fleming et al. (2000) examined weekly
general practitioner returns in England and
Wales between 1989 and 1998. After a peak
in 1993–1994, there appears to have been a
gradual decline in new asthma presentations
to general practitioners, which is mirrored by
a similar decline in acute bronchitis presenta-
tions. This study benefits from having con-
tinuous (weekly) data available over a decade
rather than two points in time. Nevertheless,
the observed decline occurred only over
4–5 years, which is a relatively short time in
a half-century of overall observed increase.
Recent milder winters (perhaps due to climate
change) may have contributed to this decline,
although some decrease was observed in other
seasons (Fleming et al. 2000). Furthermore,
although new presentations declined, there
was no reduction in the use of bronchodila-
tors, and the use of inhaled steroids increased
during the same period.
If the plateaus in asthma prevalence
recently observed in these studies are real and
prove to be sustained, this could be an indica-
tion that saturation point has been reached in
some locations. With heritability estimated
to be up to 75%, there is likely to be some
genetic component in the etiology of asthma.
Recent plateaus may reflect that the propor-
tion of the population genetically more sus-
ceptible to developing asthma may already
have done so. Further, several co-occurring
factors may promote a decline in asthma,
exerting perhaps converse pressure protecting
against asthma—for instance, increases in the
number of children attending child care.
There has been a sustained focus on identi-
fying the causative environmental factors of
the overall trend to increasing asthma preva-
lence and morbidity (Sunyer et al. 1999); how-
ever, these environmental factors are still
unknown (Nolte et al. 2001). Some environ-
mental factors previously proposed to explain
the increased global prevalence of asthma
include increased air pollution (D’Amato et al.
2000; Rios et al. 2004), changed diet (Ellwood
et al. 2001; Hijazi et al. 2000; Seaton and
Environmental Health Perspectives
•
VOLUME 113 |NUMBER 8 |August 2005
915
Address correspondence to P.J. Beggs, Department of
Physical Geography, Division of Environmental and
Life Sciences, Macquarie University, New South
Wales 2109, Australia. Telephone: 61-2-9850-8399.
Fax: 61-2-9850-8420. E-mail: paul.beggs@mq.edu.au
Many thanks to N. Pearce for providing asthma
prevalence data, and to the National Centre for Epi-
demiology and Population Health review group for
their very helpful comments: C. Blumer, R. D’Souza,
K. Glass, J. Harris, R. Lucas, L. Strazdins, and
R. Woodruff.
The authors declare they have no competing
financial interests.
Received 4 November 2004; accepted 20 April 2005.
Commentary
Is the Global Rise of Asthma an Early Impact of Anthropogenic Climate
Change?
Paul John Beggs1and Hilary Jane Bambrick2
1Department of Physical Geography, Division of Environmental and Life Sciences, Macquarie University, New South Wales, Australia;
2National Centre for Epidemiology and Population Health, Australian National University, Canberra, Australian Capital Territory, Australia
The increase in asthma incidence, prevalence, and morbidity over recent decades presents a
significant challenge to public health. Pollen is an important trigger of some types of asthma, and
both pollen quantity and season depend on climatic and meteorologic variables. Over the same
period as the global rise in asthma, there have been considerable increases in atmospheric carbon
dioxide concentration and global average surface temperature. We hypothesize anthropogenic cli-
mate change as a plausible contributor to the rise in asthma. Greater concentrations of carbon
dioxide and higher temperatures may increase pollen quantity and induce longer pollen seasons.
Pollen allergenicity can also increase as a result of these changes in climate. Exposure in early life
to a more allergenic environment may also provoke the development of other atopic conditions,
such as eczema and allergic rhinitis. Although the etiology of asthma is complex, the recent global
rise in asthma could be an early health effect of anthropogenic climate change. Key words: aero-
allergens, anthropogenic climate change, asthma, carbon dioxide, phenology, pollen, temperature.
Environ Health Perspect 113:915–919 (2005). doi:10.1289/ehp.7724 available via http://dx.doi.org/
[Online 20 April 2005]
Devereux 2000; Sigurs et al. 1992), and
increased prevalence of maternal smoking
(Lødrup Carlsen 2002; Ulrik and Backer
2000). Further potential explanatory factors
come under the hygiene hypothesis, which
proposes that greater risk of atopy results from
altered challenges to the immune system in
early life—particularly reduced infections—
and the consequent development of a bias
towards T-helper type 2 immune response over
T-helper type 1 (Strachan 2000a). Specific
factors proposed under the hygiene hypothe-
sis include changed immunization practices
(Portengen et al. 2002; von Mutius 1998),
changed living conditions and increased
exposure to indoor allergens (Kaiser 2004),
increased use of antibiotics (Cohet et al.
2004; Droste et al. 2000; Foliaki et al. 2004),
and reduced exposure to endotoxins (Eder
and von Mutius 2004; Eduard et al. 2004;
Gehring et al. 2001). These factors may
explain some of the increase in predisposition
to atopy, but any effects on asthma prevalence
and morbidity could be compounded by
changing pollen profiles. Furthermore, studies
examining the hygiene hypothesis have not
been entirely consistent, with some showing
no effects of these exposures on subsequent
development of asthma (von Hertzen and
Haahtela 2004). Inconsistencies may result
from different social trends that alter expo-
sures in more than one direction. Although
some trends may have reduced potentially
protective exposures (increased urbanization,
reduced family size, and increased maternal
work stress during pregnancy), others (e.g.,
greater use of formal child care) may increase
the protective exposures.
Climate change provides an additional
plausible explanation for both increasing
asthma susceptibility and increasing severity
observed over several decades. There are many
types of asthma, and we seek to explain only
the component of the increase that is allergic
asthma, particularly pollen-induced asthma.
This is likely to be a significant proportion of
asthma cases. For example, Grossman (1997)
suggests that up to 78% of people with asthma
also suffer from allergic rhinitis. Furthermore,
although the hypothesis that trends in air pol-
lution have been major determinants for the
rise in prevalence of asthma and allergic dis-
ease in recent decades is now generally dis-
proved (Charpin et al. 1999; Strachan 2000b),
air pollution is likely to have its own effects on
pollen production (D’Amato et al. 2001).
Anthropogenic Climate Change
Human activities have led to increases in
atmospheric carbon dioxide concentration
and consequent changes in climate [Inter-
governmental Panel on Climate Change
(IPCC) 2001]. Before the advent of the
Industrial Era (circa 1750), atmospheric CO
2
concentration had been 280 ± 10 ppm for sev-
eral thousand years (Prentice et al. 2001). It has
risen since then, with the mean annual concen-
tration recorded at the Mauna Loa Observatory
in Hawaii in 2002 at 373 ppm (Keeling and
Whorf 2003) (Figure 2). The increase over this
period has not been linear. The Mauna Loa
data show an 18% increase in the mean annual
concentration since the start of the records in
1959, when it was 316 ppm (Keeling and
Whorf 2003). This suggests that approximately
two-thirds of the increase in atmospheric CO
2
concentration since the Industrial Era has
occurred over the last 50 years or so. These
increases in CO
2
and other greenhouse gases
have enhanced the greenhouse effect, resulting
in global warming and other changes to cli-
mate. The global average surface temperature
has increased by 0.6 ± 0.2°C since the late
19th century, with much of this warming
occurring during two periods, 1910–1945 and
1976–2000 (Albritton et al. 2001). The IPCC
has stated that “most of the observed warming
over the last 50 years is likely to have been due
Beggs and Bambrick
916
VOLUME 113 |NUMBER 8 |August 2005
•
Environmental Health Perspectives
Figure 1. Changes in asthma point prevalence observed since 1956. The locations used different diagnos-
tic criteria, but these were consistent within each study location. Different studies for the same nation are
distinguished by a, b, c, and d. Data from Pearce et al. (2000).
USA (a)
USA (b)
USA (c)
Wales
Australia (a)
Australia (b)
Australia (c)
Australia (d)
Canada (a)
Canada (b)
Canada (c)
England (a)
England (b)
England (c)
England (d)
England + Wales (a)
England + Wales (b)
Finland
France
Germany
Israel
Italy
Japan
New Zealand (a)
New Zealand (b)
Papua New Guinea
Scotland (a)
Scotland (b)
Sweden (a)
Sweden (b)
Tahiti
Taiwan
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
Year
50
45
40
35
30
25
20
15
10
5
0
Prevalence (%)
Figure 2. Three-monthly seasonal average atmos-
pheric CO2concentration recorded at Mauna Loa
Observatory, Hawaii, from March 1958 through
December 2002 [data from Keeling and Whorf
(2003)]. Highest CO2occurs in the northern hemi-
sphere spring [March, April, May (MAM)]. Because
this is peak growing season for northern hemi-
sphere plants, the higher spring CO2levels may fur-
ther magnify the impact on plant growth and pollen
production. DJF, December, January, February;
JJA, June, July, August; SON, September, October,
November.
1956
1960
1964
1968
1972
1976
1980
1984
1988
1992
1996
2000
Year
380
370
360
350
340
330
320
310
CO2 concentration (ppm)
DJF
MAM
JJA
SON
to the increase in greenhouse gas concentra-
tions” (Albritton et al. 2001). In addition to
these already observed changes, human influ-
ences will continue to change atmospheric
composition (including increasing CO
2
) and
climate throughout the 21st century and
beyond (Albritton et al. 2001).
Some researchers have proposed that global
climate change is likely to have an effect in the
future on asthma (Longstreth 1991). We sug-
gest that some of the observed increase in
asthma could be due to climate change that has
already occurred.
Elevated CO2and Climate
Change Impacts on Pollen
The balance of evidence strongly suggests
that a significant impact of climate change is
already discernible in animal and plant popu-
lations (Root et al. 2003), as well as commu-
nities and ecosystems (Walther et al. 2002).
For example, analysis of data from the Inter-
national Phenological Gardens in Europe
(a network of sites covering 69–42° N and
10° W–27° E) has shown that spring events,
such as flowering, have advanced by 6 days,
and that autumn events have been delayed by
4.8 days, compared with the early 1960s
(Menzel and Fabian 1999).
There is now also considerable evidence
of impacts of climate change on aeroallergens,
particularly pollen (Beggs 2004). First, it appears
that plants produce a greater quantity of pollen
under these changed climatic conditions.
Experimental studies have found substantial
increases in pollen production resulting from
exposure to increased CO
2
concentration,
including from levels equivalent to preindustrial
CO
2
to current concentrations (Wayne et al.
2002; Ziska and Caulfield 2000) (Figure 3).
Other studies have examined trends in pollen
amount over the latter decades of the 1900s
and found increases to be associated with local
rises in temperature (Corden and Millington
2001; Spieksma et al. 1995). Second, there is
some evidence of significantly stronger aller-
genicity in pollen from trees grown at increased
temperatures (Ahlholm et al. 1998; Hjelmroos
et al. 1995). The association between changes
in temperature and pollen allergenicity is under
investigation and is likely to vary across plant
species. Third, changes in climate appear to
have altered the temporal and spatial distri-
bution of pollen. For example, some studies
have found that trends toward earlier pollen
seasons are associated with local warming
over the latter decades of the 1900s (Emberlin
et al. 2002; Fitter and Fitter 2002), and
recent reports have concluded that the dura-
tion of the pollen season is extended in some
species (Huynen and Menne 2003). Finally,
several studies have examined other attributes
of allergenic plants, which have also been
responsive to CO
2
concentration and/or tem-
perature increases (e.g. Menzel 2000; Wulff
and Alexander 1985). These latter studies pro-
vide indirect evidence of impacts of climate
change on pollen aeroallergens.
Impacts of Climate Change
on Asthma
The links between aeroallergens and allergic
diseases such as asthma are well established
(Burge and Rogers 2000; Nolte et al. 2001). It
is feasible that faster plant growth, earlier plant
maturity, and longer growing season, plus ear-
lier pollen season, increased season duration,
and increases in both pollen quantity and
allergenicity have already had an impact on
asthma, reflected in the global rise in asthma
prevalence and increased severity of episodes.
Climate change might readily explain more
than just increased morbidity among those with
the condition; it could also be a candidate for
increasing the initial susceptibility to asthma
and hence the prevalence of the condition.
Exposure to allergens in infancy is thought to
sensitize individuals to asthma (Pearce et al.
2000) and other atopic conditions such as
eczema and allergic rhinitis. Björkstén and
Suoniemi (1981) found, for example, that
exposure to more intense pollen seasons in early
infancy increased the likelihood of later devel-
opment of allergy. Therefore, increases in
pollen quantity and extended pollen seasons
due to climate change may lead to both an
increase in the development of the condition
and greater morbidity among those who have it
(Figure 4).
Although future impacts of climate change
on human health have received considerable
and increasing amounts of attention since the
mid-1990s, few studies have documented
human health impacts already evident. A
study of the El Niño–Southern Oscillation and
cholera in Bangladesh from around 1900 to
2001 may provide the first evidence that warm-
ing trends over the last century are already
affecting human health (Patz 2002; Rodó
et al. 2002). The World Health Organization
(WHO) has identified climate change as a
major environmental risk to health (WHO
2002). The WHO estimated that climate
change was responsible for approximately
2.4% of diarrhea cases (worldwide), 6% of
malaria cases (in some middle-income coun-
tries), and 7% of dengue fever cases (in some
industrialized countries) in 2000. In total, cli-
mate change was estimated to be responsible
for 0.3% of deaths and 0.4% of disability-
adjusted life years (WHO 2002).
The potential for future climate change to
have an impact on asthma and other allergic
diseases has been recognized for some time
(Curson 1993; IPCC 2001), mostly a result of
the well-established link between climate and
many aeroallergens and aeroallergen-producing
organisms, on the one hand, and air pollution
on the other. However, that climate change
could alter the burden of noncommunicable
diseases such as asthma has received far less
attention than have potential impacts on infec-
tious vector-borne and diarrheal diseases.
It is somewhat easier to establish direct
links between changing climate and the burden
of these infectious diseases, where etiology is
fairly well established. In contrast, the etiology
Is the rise of asthma an impact of climate change?
Environmental Health Perspectives
•
VOLUME 113 |NUMBER 8 |August 2005
917
Figure 3. Percentage change in pollen quantities
(
Ambrosia artemisiifolia
L.) produced under differ-
ent concentrations of atmospheric CO2. Increase in
pollen quantity is shown for CO2concentrations
equivalent to preindustrial levels (280 ppm),
through 1950s levels, to 2000 levels and to potential
future levels.
a
Data from Ziska and Caulfield (2000).
b
Data from Wayne
et al. (2002).
Change in pollen quantity (%)
720
680
640
600
560
520
480
440
400
360
320
280
CO2 concentration (ppm)
0 20 40 60 80 100 120 140
b
a
a
2002 level
1958 level
Figure 4. Schematic diagram of the relationship
between global climate change and the rise in
asthma prevalence and severity, via impacts of
climate change on plant and pollen attributes. ↑,
increase.
Human activities
Atmospheric CO2
Temperature
Earlier pollen season
Pollen season duration
Pollen allergenicity
Altered distribution
Plant growth, germination,
biomass
Pollen quantity
Exposure to allergens
↑
Sensitization to allergens
Incidence of allergic
asthma
Asthma prevalence
↑
↑
Frequency
of asthma
episodes
Severity
of asthma
episodes
Burden of asthma
↑↑
↑
↑
↑
↑
↑
↑
↑
↑
of asthma is complex and not well understood.
The complexity of asthma and the greater
urgency that tends to be ascribed to communi-
cable disease may explain why little attention
has to date been paid to the effects of climate
change on asthma. Furthermore, that climate
change impacts on asthma may already be evi-
dent has previously not been explored.
Recent urban–rural comparisons in asthma
and pollen provide a useful analogy to the
global climate change hypothesis presented
here. Using the existing CO
2
and temperature
gradient between rural and urban areas, Ziska
et al. (2003) showed that the higher CO
2
con-
centrations and air temperatures of urban areas
were associated with differences in ragweed
(Ambrosia artemisiifolia). The urban plants of
this established allergy-inducing species pro-
duced significantly more pollen than the plants
in rural areas. Greater aeroallergen levels in
urban areas may have contributed to the higher
childhood asthma prevalence of urban areas.
These local effects provide some support for
pollen quantity as a significant contributor
to asthma prevalence. Weinberg (2000) con-
firms the urban–rural differences in childhood
asthma prevalence, but has shown a much nar-
rower gap, suggesting an accelerated rise in
aeroallergen level in rural areas associated with
global increases in atmospheric CO
2
concen-
tration and temperature.
Conclusions
Seeking evidence of early effects of climate
change on human health has been identified
as a major challenge for scientists studying
climate change and health (Woodward and
Scheraga 2003). The detection of health effects
of climate change is necessary as evidence
underpinning national and international poli-
cies relating to measures to protect public
health, such as the mitigation of greenhouse
gas emissions (Wilkinson et al. 2003) and,
given that we are already committed to some
climate change, strategies for adaptation.
Asthma is etiologically complex, with
numerous contributing factors and interactive
effects within the causal web, many of which
are modified by climate. The changing global
climate compounds this complexity. There is
some evidence to suggest that asthma preva-
lence—but not severity—may have plateaued
in some countries very recently. However, it
is too early to determine whether this leveling
off will be sustained. Either way, the hypothe-
sis that the global rise of asthma is an early
impact of anthropogenic climate change still
stands. Further, this climate change hypothe-
sis does not conflict with the hygiene hypoth-
esis, but adds an additional possibility to the
mix; each may contribute to the observed rise
in asthma.
Specific hypotheses relating asthma to cli-
mate change must be developed and rigorously
tested. To tease out the climate change–asthma
relationships, it will be necessary to distinguish
between an increase in the prevalence of asthma
and an increase in the morbidity, incidence,
and burden of disease, because climate change
may contribute to both a rise in prevalence and
increased severity. Furthermore, because of the
variation in prevalence throughout the world,
studies will need to address patterns at national,
subnational, and local scales. Proof that recent
climate change has had an adverse impact
on asthma will come only from the accumula-
tion of studies focused on this topic. For com-
parisons of the impact of global climate change
between areas and over time, international defi-
nitions of asthma, such as that developed by the
International Study of Asthma and Allergies in
Childhood (Asher et al. 1995), should be used
to establish baselines and to measure trends.
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