Asthma: epidemiology, etiology and risk factors

Asthma is one of the most common chronic conditions affecting both children and adults, yet
much remains to be learned of its etiology. This paper evolved from the extensive literature
review undertaken as part of a proposal for a longitudinal birth cohort study to examine risk
factors for the development of allergy and asthma in early childhood.

Although genetic predisposition is clearly evident, gene-by-environment interaction probably

explains much of the international variation in prevalence rates for allergy and asthma.
Environmental factors such as infections and exposure to endotoxins may be protective or
may act as risk factors, depending in part on the timing of exposure in infancy and childhood.
Some prenatal risk factors, including maternal smoking, have been firmly established, but
diet and nutrition, stress, use of antibiotics and mode of delivery may also affect the early
development of allergy and asthma. Later in childhood, putative risk factors include exposure
to allergens, breastfeeding (which may initially protect and then increase the risk of
sensitization), family size and structure, and sex and gender. In adulthood, recurrence of
childhood asthma may be just as common as new-onset asthma, which may have an
occupational basis. A better understanding of these risk factors may eventually lead to
opportunities for primary prevention of asthma.

Methods

This paper arose from an extensive literature review undertaken in developing the Canadian
Healthy Infant Longitudinal Development (CHILD) study, a multicentre national
observational study that is currently in progress. The study, which will eventually recruit
5000 pregnant women, has the aim of determining the environmental, host, genetic and
psychosocial risk factors for development of allergy and asthma in children. Although not a
systematic review, the examination of epidemiologic risk factors in the development of
asthma presented here began in 2004 with a search of MEDLINE, using the Medical Subject
Heading (MeSH) terms “asthma,” “longitudinal” and “cohort study.” One of us (P.S.)
reviewed the abstracts of all studies identified in the search, excluding those without at least
one objective outcome measure and those in which the primary outcome measure was not
asthma. Studies examining the same outcome measure were tabulated but not combined,
since most did not consider exactly the same outcome at the same age. We then performed
specific searches to fill gaps in the information gathered via the original search, specifically
nutrition, sex and gender effects, and novel environmental exposures. The review was
updated in July 2008.

Although the present article includes some references to adult asthma, its primary focus is the
epidemiology of and risk factors for this condition in children. A more extensive summary of
the literature review for the Canadian Healthy Infant Longitudinal Development study has
been published elsewhere.1

Epidemiology of asthma: an overview

The recent substantial increase in the reported prevalence of asthma worldwide (Figure 1) has
led to numerous studies of the prevalence and characteristics of this condition. 2 Foremost
among these are 2 major international initiatives that have collected data using validated
questionnaires, one among children, the International Study of Asthma and Allergies in
Childhood,3 and the other among young adults, the European Community Respiratory Health
Survey.4 Follow-up investigations for both of these studies 5,6 have examined temporal trends
within and across populations. During a mean of 7 years following phase I of the
International Study of Asthma and Allergies in Childhood, which in most participating
countries was conducted between 1991 and 1993, the prevalence of asthma was stable or
decreased in some areas of the world but increased substantially in many other areas,
especially among children 13–14 years of age (Figure 2).5

Figure 1

Changes in prevalence of diagnosed asthma (A) and asthma symptoms (B) over time among
children and young adults. Reproduced, with permission, from Eder W, Ege MJ, von Mutius
E. The asthma epidemic. N Engl J Med 2006;355:2226–35. Copyright 2006 Massachusetts
Medical Society.2
Figure 2

Annual changes in worldwide prevalence of asthma symptoms among children 6–7 years old
and 13–14 years old, over a mean of 7 years following phase I of the International Study of
Asthma and Allergies in Childhood (which in most participating countries was conducted
between 1991 and 1993). Blue triangles identify locations where prevalence was reduced by
at least 1 standard error (SE) per year, green squares identify locations where there was little
change in prevalence (i.e., change of less than 1 SE per year) and red triangles identify
locations where prevalence increased by at least 1 SE per year. Reproduced from The Lancet,
Vol. 368, Asher MI, Montefort S, Bjorksten B, et al.; ISAAC Phase Three Study Group.
Worldwide time trends in the prevalence of symptoms of asthma, allergic rhinoconjunctivitis,
and eczema in childhood: ISAAC Phases One and Three repeat multicountry cross-sectional
surveys. Pages 733–43, copyright 2006, with permission from Elsevier.5

Cross-sectional population-based studies such as these are highly dependent on recognition of

symptoms, so they do not necessarily reflect the true heterogeneity of asthma. However, a
wide variation in prevalence rates has been documented: studies of both children and adults
have revealed low prevalence rates (2%–4%) in Asian countries (especially China and India)
and high rates (15%–20%) in the United Kingdom, Canada, Australia, New Zealand and
other developed countries.3–6
Observations of migrating populations7 and of Germany after reunification8 have strongly
supported the role of local environmental factors, including allergens but likely many
lifestyle factors as well, in determining the degree of expression of asthma within genetically
similar populations. A recent analysis of data from the International Study of Asthma and
Allergies in Childhood, comparing data from Vancouver, Canada, with data from centres in
China, showed significant differences in prevalence rates between children of similar genetic
ancestry living in different environments, with evidence for an effect of duration of residence
in the new environment.9 Prevalence rates for asthma among children 13–14 years old were
lowest for Chinese children born and studied in China, intermediate for Chinese children who
had migrated during their lifetime to Canada and highest for Chinese children who had been
born in Canada. In addition, the prevalence rate for the third of these groups was still lower
than among non-Chinese children in the same environment. Together, these results strongly
suggested gene-by-environment interactions.

Local and national studies have also provided insights into the epidemiology of exacerbations
of asthma. For example, epidemics of asthma exacerbations in Barcelona, Spain, were
eventually linked to exposure to atmospheric soybean dust released during cargo handling at
the local port.10 The highly predictable annual epidemic of asthma exacerbations in school-
age children in the northern hemisphere every September, peaking some 17 days after the
return to school, appears to be predominantly driven by seasonal rhinovirus infection,
probably compounded by other risk factors for asthma exacerbations, including reduction in
use of asthma controller therapy over the summer months, exposure to seasonal allergens and
possibly the stress of returning to school.11,12

Complementing these cross-sectional studies are longitudinal epidemiologic studies in a

variety of populations and countries, which have allowed examination of risk factors
predicting the development, persistence, remission or relapse of childhood asthma. One such
population-based birth cohort study in Dunedin, New Zealand, which had a high retention
rate, examined outcomes of childhood asthma at age 26 years. 13 Female sex, airway
hyperresponsiveness in mid and later childhood, and sensitization to house dust mites were
all significantly and independently related to the likelihood of persistence of childhood
asthma to early adulthood. Early age of onset of wheezing symptoms was predictive of
relapse after remission, as were airway hyperresponsiveness and allergy to house dust mites.
That study and others have clearly demonstrated the tracking of characteristics of asthma
from childhood to adulthood, including severity and impairment of lung function.

Etiology of and risk factors for asthma

Asthma comprises a range of heterogeneous phenotypes that differ in presentation, etiology

and pathophysiology. The risk factors for each recognized phenotype of asthma include
genetic, environmental and host factors. Although a family history of asthma is common, it is
neither sufficient nor necessary for the development of asthma.14

The substantial increases in the incidence of asthma over the past few decades and the
geographic variation in both base prevalence rates and the magnitude of the increases support
the thesis that environmental changes play a large role in the current asthma epidemic.
Furthermore, environmental triggers may affect asthma differently at different times of a
person’s life, and the relevant risk factors may change over time.
Short-term studies of risk factors may suggest a lower likelihood of asthma, whereas the
same factors may be associated with greater risk if follow-up is more prolonged. This pattern
may relate to overlap between different wheezing phenotypes in early childhood, only some
of which persist as asthma in later childhood and adulthood. Because of this phenomenon, we
examine here the risk factors for persistent asthma at different ages, specifically the prenatal
period, infancy, childhood and, briefly, adulthood.

Genetics

Family and twin studies have indicated that genetics plays an important role in the
development of asthma and allergy,15 likely through several genes of moderate effect (i.e.,
genes associated with relative risks in the range of 1.2–2).16,17

Genome-wide linkage studies and case–control studies have identified 18 genomic regions
and more than 100 genes associated with allergy and asthma in 11 different populations. In
particular, there are consistently replicated regions on the long arms of chromosomes 2, 5, 6,
12 and 13. Association studies of unrelated individuals have also identified more than 100
genes associated with allergy and asthma, 79 of which have been replicated in at least one
further study.18 A recent genome-wide association study19 identified a new gene, ORMDL3,
that exhibited a highly significantly association with asthma (p < 10−12) (for single nucleotide
polymorphism rs8067378, odds ratio 1.84, 95% confidence interval 1.43–2.42) a finding that
has now been replicated in several populations.20,21

Extensive heterogeneity in the genetic basis of asthma, and in gene-by-environment

interactions, is likely. Failure to identify and precisely quantify environmental exposures and
their timing may account for some of the difficulty that researchers have had in replicating
genetic associations.

Prenatal risk factors

Risk factors in the prenatal period are multifactorial. Assessment is complicated by the
variety of wheezing conditions that may occur in infancy and childhood, only some of which
evolve to classical asthma.

Prenatal tobacco smoke

Prenatal maternal smoking has been consistently associated with early childhood wheezing, 22–
25
and there is a dose–response relation between exposure and decreased airway calibre in
early life.26,27 Prenatal maternal smoking is also associated with increased risks of food
allergy,24 cytokine responses in the cord blood28,29 and concentrations of nitric oxide in
exhaled air in newborns.30 Studies have shown a clear prenatal effect of smoking; this effect
is increased when combined with postnatal smoke exposure.

Diet and nutrition

Observational studies examining prenatal nutrient levels or dietary interventions and the
subsequent development of atopic disease have focused on foods with anti-inflammatory
properties (e.g., omega-3 fatty acids) and antioxidants such as vitamin E and zinc. Several
studies have demonstrated that higher intake of fish or fish oil during pregnancy is associated
with lower risk of atopic disease (specifically eczema and atopic wheeze) up to age 6 years. 31–
33
Similarly, higher prenatal vitamin E and zinc levels have been associated with lower risk of
development of wheeze up to age 5 years. 34–36 However, no protective effect against the
development of atopic disease in infants has been shown for maternal diets that excluded
certain foods (e.g., cow’s milk, eggs) during pregnancy.37–40 The authors of 2 recent
studies41,42 reported an inverse relation of maternal vitamin D levels with wheeze in early life,
but no relation with atopy or symptoms in later life.

Stress

A number of animal models have suggested that prenatal maternal stress acts through
regulation of the offspring’s hypothalamic–pituitary–adrenal axis to decrease cortisol levels,
which may affect the development of an allergic phenotype. Although there is a correlation
between caregiver stress early in the infant’s life and higher levels of immunoglobulin E in
the infant43–45 and early wheezing,46 no studies to date have shown an association with
asthma.47,48

Antibiotic use

The association between prenatal antibiotic treatment and subsequent development of atopic
disease has been examined in 2 ways: with treatment as a dichotomous predictor (i.e., any
antibiotic use) and by number of courses of antibiotics during pregnancy. Longitudinal cohort
studies examining any antibiotic use showed a greater risk of persistent wheeze and asthma in
early childhood49,50 and a dose–response relation between number of antibiotic courses and
risk of wheeze or asthma.49,51

Mode of delivery

Development of atopy was 2 to 3 times more likely among infants delivered by emergency
cesarean section,29,52–56 although no such association occurred with elective cesarean
section.29,52,53,56–59 Potential reasons for these findings include maternal stress and differences
in the infant’s gut microflora associated with different modes of delivery.

Risk factors in childhood

Phenotypes of asthma

Although some 50% of preschool children have wheezing, only 10%–15% have a diagnosis
of “true” asthma by the time they reach school age.13,60 Commonly described phenotypes in
early infancy and childhood are transient wheezing, nonatopic wheezing, late-onset wheezing
and persistent wheezing.61 Only transient wheezing in early infancy has been well
characterized, with decreased airflow rates on pulmonary function testing at birth, 56,60,62 onset
of wheezing within the first year and resolution by mid-childhood with no lasting effects on
pulmonary function.

The other 3 phenotypes have been described primarily by age of onset in cohort studies, and
their genesis in early infancy is largely unknown. The majority of children with persistent
wheezing (in whom asthma will subsequently be diagnosed) experience their first symptoms
before age 3. By 3 years, they have abnormal lung function that persists to adulthood, 13,60,61
and by adolescence, most have atopy. Of children with nonatopic and late-onset wheezing,
some experience remission, whereas others experience persistent symptoms and atopy.63
Distinguishing among these different phenotypes in early childhood is critical to
understanding the role of risk factors and their timing in early infancy.

Breastfeeding

The influence of breastfeeding on the risk of childhood atopy and asthma remains
controversial. The following represents observational data accumulated to date. Some studies
have shown protection,64–66 whereas others have reported higher rates of allergy and asthma
among breastfed children.67,68 A meta-analysis69 and several individual studies66,70 showed that
exclusive breastfeeding for at least 3 months was associated with lower rates of asthma
between 2 and 5 years of age, with the greatest effect occurring among those with a parental
history of atopy. One of the difficulties in interpreting these data lies in differentiating viral-
associated wheeze in childhood from development of atopic asthma. In a longitudinal birth
cohort study, breastfeeding was associated with a higher risk of atopic asthma in later
childhood, with the greatest in fluence occurring among those with a maternal history of
atopy.67,68,71

The influence of avoiding nutritional allergens during breastfeeding is also controversial. In

some studies, exclusion of milk, eggs and fish from the maternal diet was associated with
decreased atopic dermatitis in infancy,72,73 but other studies found no association. 40,74,75 Studies
following children to 4 years of age have demonstrated no effect of maternal dietary
restriction during lactation on the subsequent development of atopic diseases, including
asthma.76

Lung function

Decreased airway calibre in infancy has been reported as a risk factor for transient
wheezing,60 perhaps related to prenatal and postnatal exposure to environmental tobacco
smoke.26,27 Furthermore, the presence of airways with decreased calibre has been associated
with increased bronchial responsiveness and increased symptoms of wheeze. 26 Several studies
have suggested an association between reduced airway function in the first few weeks of life
and asthma in later life.62,77 The magnitude of the effect of this risk factor in isolation (i.e.,
without concomitant allergy) is unclear; perhaps individuals with smaller airways require less
stimulus (i.e., airway inflammation) before symptoms become apparent.

Children with wheezing (and diagnosed asthma) persisting to adulthood have a fixed
decrement in lung function as early as age 7 or 9 years. 13,78 Recent studies of preschool
children have documented abnormal lung function in children with persistent wheezing as
young as age 3 years.61 However, some infants in whom persistent wheezing develops have
normal lung function shortly after birth, which suggests a critical period of exposures within
the first few years of life, before the development of these persistent abnormalities in
expiratory flows.60,79 In contrast, infants who exhibit early transient wheezing have decreased
airflow shortly after birth.60,80 Maternal smoking with in utero nicotine exposure has been
correlated with this type of lung dysfunction, 26,27,60 but the effects of other exposures have
been less well studied.

Family structure

Family size and the number and order of siblings may affect the risk of development of
asthma. The hygiene hypothesis posits that exposure of an infant to a substantial number of
infections and many types of bacteria stimulates the developing immune system toward
nonasthmatic phenotypes.81,82 This may be exemplified in the real world by large family size,
whereby later-born children in large families would be expected to be at lower risk of asthma
than first-born children, because of exposure to their older siblings’ infections.

Although this theory has been supported by some studies of allergy prevalence, 83,84 it has been
partially refuted by recent studies of asthma prevalence suggesting that although large family
size (more than 4 children) is associated with a decreased risk of asthma, birth order is not
involved.85,86 Furthermore, doubt has been cast on simplistic renditions of this hypothesis, in
that infections per se cannot explain some epidemiologic patterns (e.g., prevalence rates for
allergy and asthma are high in some South American countries, where exposures to infection
are higher than in some countries with lower rates of asthma 3). In addition, not only allergic
but also autoimmune and other chronic inflammatory diseases are increasing,87 a trend that is
difficult to explain by the hygiene hypothesis alone, since allergic and autoimmune diseases
are associated with competing immunologic phenotypes.

Socio-economic status

Children of parents with lower socio-economic status have greater morbidity from asthma, 88–
92
but findings with respect to the prevalence of asthma are mixed. 93–97 Such results may
depend both on how socio-economic status is measured and on the specific outcome
examined. Some studies have reported associations of lower socio-economic status with
greater airway obstruction and symptoms but not with a diagnosis of asthma. 91,92 Whether
socio-economic status is as relevant to the incidence of allergy and asthma as it is to the
expression, severity and management of these diseases re mains unclear. Parental stress has
also been prospectively associated with wheezing in infancy,46 and family difficulties have
been linked to asthma.48,98 Children whose caregivers report high levels of stress and who
have difficulties parenting are at greatest risk for asthma.99

Antibiotics and infections

The use of antibiotics has been associated with early wheezing and asthma in several
studies,47,100,101 One suggested mechanism for this association is immunologic stimulation
through changes in the bowel flora, but Kummeling and associates 100 found no coincident
increase in eczema or atopy, despite increased wheezing rates, which would argue against
this mechanism. Greater antibiotic use might also represent a surrogate marker for a higher
numbers of infections (perhaps viral) in early life.

Viral infections of the lower respiratory tract affect early childhood wheezing. Whether lower
respiratory tract infection promotes sensitization to aeroallergens causing persistent asthma is
controversial: childhood viral infections might be pathogenic in some children but protective
in others.102–106 Infants of mothers with allergy or asthma have a relatively persistent
maturational defect in Th1 cytokine synthesis in the first year of life, which may play a role
in the development of persistent or severe viral infections. 107 Severe viral infection of the
lower respiratory tract in genetically susceptible infants who are already sensitized to inhalant
allergens may lead to deviation toward Th2 responses promoting asthma. It is unclear
whether these effects of lower respiratory tract infection are virus-specific (e.g., respiratory
syncytial virus, rhinovirus) or whether synergistic exposures to allergens can induce asthma
even in individuals who are not genetically susceptible. Interactions of genes with
environmental exposures (including allergens, air pollution, environmental tobacco smoke
and diet) modulate the host response to infections. 108,109 It remains controversial whether the
occurrence or timing of childhood infection is pathogenic or protective for the development
and long-term outcome of asthma and allergy and of nonallergic wheeze phenotypes. This
controversy relates in part to small sample size, cross-sectional analysis, lack of precise case
definition and incomplete microbial assessment in studies of this phenomenon.110,111

Respiratory infections in early childhood are associated with early wheezing, 109 but it is
unclear whether infection alone has a role in the development of persistent asthma. Repeated
lower respiratory tract infection may affect infants who are already at risk for asthma because
of family history or atopy.63,112 Severe infection with certain viruses such as respiratory
syncytial virus106 and rhinovirus113 may play a role in persistent wheezing, although other
studies have suggested no effect. 114 Considered as a proxy for viral infections, daycare
attendance is associated with greater incidence of early wheeze but lower incidence of
persistent wheeze.115

Allergic sensitization

Total serum immunoglobulin E level, a surrogate for allergen sensitivity, has been associated
with the incidence of asthma.116 High levels of immunoglobulin E at birth were associated
with greater incidence of both atopy117–119 and aeroallergen sensitivity but not necessarily
asthma. However, sensitization to aeroallergens, particularly house dust mite, cat and
cockroach allergens, is well documented as being associated with asthma.

Immune responses in the developing infant and young child may affect the development of
asthma. For example, impairment in interferon γ production at 3 months was associated with
a greater risk of wheeze. 115 Immaturity in neonatal immune responses may promote the
persistence of the Th2 immune phenotype and development of atopy,120 but an association
with persistent asthma is as yet unproven. More recent work has focused on the role of the
innate immune system in handling and presentation of antigens and suggests that
polymorphisms in Toll-like receptors121,122 may play a greater role than previously recognized
in the development of the skewed immune responses associated with persistent asthma.

Exposure to environmental tobacco smoke

Postnatal exposure to environmental tobacco smoke, especially from maternal smoking, has
been consistently associated with respiratory symptoms of wheezing. 22,26,56 Exposure to
environmental tobacco smoke also consistently worsens asthma symptoms and is a risk factor
for severe asthma.123,124

Exposure to animals

Although several studies have demonstrated a lower risk of development of atopy and asthma
with exposure to farm animals in early life, the findings of studies of the influence of
exposure to domestic cats and dogs have been inconsistent. 125,126 In some studies, exposure to
cats was associated with a greater risk of allergic sensitization, 127 whereas other studies
showed a lower risk.128,129 Exposure to dogs may be protective not only against the
development of specific sensitization to dog allergen 127,128 but also against other sensitization
(e.g., to house dust mites) and asthma. Other studies of exposure to dogs have suggested that
protection against wheezing may be mediated by high levels of endotoxin.130
Gene-by-environment interactions

The effects of gene-by-environment interactions in asthma are complex. In some cases the
genes code for enzymes that detoxify inhaled agents (e.g., glutathione transferase genes and
environmental pollution), whereas in other cases, the exposures may have a more direct effect
on gene expression via epigenetic mechanisms, such as DNA methylation or histone
modification. Epigenetic modification of DNA is believed to be responsible for the
phenotypic differences that develop over time between monozygotic twins.131 It has been
suggested that it is principally through epigenetic modification of DNA that lifestyle and
chemical exposures affect susceptibility to diseases.132 Nutrition and diet (e.g., folic acid,
vitamin B12), smoking, exposure to microbial products, maternal stress and maternal care are
potential factors influencing fetal genetic expression, and a further window for epigenetic
modification in early life may allow environmental factors to modify a child’s genome with
the potential to cause or prolong allergy and asthma. Further work is needed to verify and
understand these risks.

Sex and gender

Sex affects the development of asthma in a time-dependent manner. Until age 13–14 years,
the incidence and prevalence of asthma are greater among boys than among girls. 133–142
Studies through puberty139,143–155 have shown a greater incidence of asthma among adolescent
and young adult females 133–135,156,157 and a greater proportion of males with remission of
asthma.136–140 Before age 12, boys have more severe asthma than girls, 142 with higher rates of
admission to hospital.158–165 In contrast, adult females have more severe asthma than males,
with more hospital admissions,161,166,167 slower improvement,120 longer hospital stays161 and
higher rates of readmission.168 Most authors have attributed these changes in prevalence and
severity to events of puberty,140,141 although mechanisms for differences between the sexes
have not been established.

In childhood, airway hyperresponsiveness is more common and more severe among males; 169
however, airway hyperresponsiveness increases in females during adolescence, 170,171 such that
by adulthood it is both more common and more severe among adult women. 154,155,172–174
Similar findings have been reported from studies of atopy, which is more common in males
before age 13;175 during adolescence, the rate of new-onset atopy is higher among
females,176,177 so that by young adulthood the prevalence of atopy is almost equal.

The influence of some environmental risk factors such as allergens may be modified by sex.
In one study of adults, 18% of women with asthma, but only 2.3% of men with asthma, had
normal results on common tests related to atopy (negative skin prick tests, immunoglobulin E
< 100 IU/mL and eosinophilia < 5%), 178 which suggested different disease mechanisms
between the sexes. Interactions have been found between maternal and paternal history of
atopy, breastfeeding and sex of the child in terms of the risk of asthma and atopy. 71 Finally,
the influence of obesity on the development of asthma is greater among women than among
men and has not been shown to be influenced by caloric intake or physical activity. 179,180
Some have suggested that the relation between obesity and asthma may be causal, given the
consistency, temporal association and dose–response relationships reported in the
epidemiologic literature, but the mechanisms remain to be elucidated.181
Adult-onset asthma

Asthma in adults may have persisted from childhood, may have occurred as a relapse of
earlier childhood asthma (whether or not recalled by the individual) or may be true adult-
onset asthma with no symptoms in earlier life. 182–184 New-onset asthma in adulthood may have
environmental (especially occupational) causes with or without allergen sensitization. 185–187
Although adult asthma may develop in relation to specific drug treatments (e.g., β-blockers,
nonsteroidal anti-inflammatory drugs) or, in women, the use of hormone replacement
therapy,188 occupational exposure to sensitizing agents or irritants is more common.

Occupational asthma

Asthma related to workplace exposures has been documented in many occupational settings.
Commonly associated occupations and exposures include car painting (isocyanates),
hairdressing (various chemicals), domestic and commercial cleaning (cleaning solutions),
health care professions (latex) and baking (flour dust), among many others.189

The relation between exposure to substances in the work-place and new-onset adult asthma
was explored among 6837 participants with no previously reported asthma symptoms in
phase I of the European Community Respiratory Health Study.187 Exposure to substances
known to cause occupational asthma was associated with a higher risk of asthma overall
(relative risk [RR] 1.6, 95% confidence interval [CI] 1.1–2.3) and of asthma defined by
airway hyperresponsiveness (RR 2.4, 95% CI 1.3–4.6). Of common occupations, nursing was
associated with the highest risk of occupational asthma (RR 2.2, 95% CI 1.3–4.0, p = 0.007),
whereas exposure to an acute inhalation event, such as fire, mixing of cleaning agents or a
chemical spill, was associated with an even higher risk (RR 3.3, 95% CI 1.0–11.1, p = 0.05).
The population attributable risk of occupational exposure for adult asthma in that study
ranged from 10% to 25%.

Other risk factors for adult asthma

Smoking tobacco190 or marijuana191,192 may give rise to symptoms suggesting asthma,

although symptoms of cough and sputum production, suggesting chronic bronchitis, are more
common. As in childhood, the differential diagnosis should include other forms of airway
inflammation and other causes of intermittent dyspnea and wheezing, such as cardiac failure.
However, new-onset asthma can occur at any age, without prior illness or concomitant
disease. Atopy as a risk factor for asthma is less common with increasing age, 193 but
occasionally it is the dominant trigger. Air pollution may affect adult asthma, but more often
it is a factor worsening pre-existing asthma rather than a cause of incident asthma.194–196

Conclusions
Many cross-sectional studies have confirmed increases in the incidence and prevalence of
asthma over the past 2 to 3 decades, but much remains unknown as to the fundamental
immunologic, genetic and environmental mechanisms underlying the development of this
condition and its increased expression, especially in the developed world. Nonetheless, some
risk factors have now been clearly and consistently identified. For instance, avoidance of
maternal smoking in pregnancy and in the early postpartum period can be strongly
encouraged, as can avoidance of known occupational sensitizers. In contrast, previous advice
to avoid animals and to breastfeed as long as possible to reduce the risks of asthma has been
challenged by more recent studies. It is likely that detailed studies of gene-by-environment
interactions and of epigenetics will eventually untangle the inconsistencies among the many
putative exposures and outcomes. Although there are indications that the increase in asthma
has reached a plateau, at least in countries with the highest prevalence rates, much of the
epidemiology and many of the risk factors for asthma remain to be adequately explained.
Reduction in risk, and perhaps even true primary prevention of asthma, remains elusive but is
a key goal of asthma management.

Key points

 The prevalence of asthma varies widely around the world, probably because of gene-
by-environment interactions.

 Prenatal risk factors for asthma may include maternal smoking, diet and nutrition,
stress, use of antibiotics and delivery by cesarean section.

 Childhood risk factors for asthma may include allergic sensitization, environmental
tobacco smoke, exposure to animals, breastfeeding, decreased lung function in
infancy, family size and structure, socio-economic status, antibiotics and infections,
and sex and gender.

 Occupational exposures constitute a common risk factor for adult asthma.

This article is the first in a 7-part case study series that was developed as a knowledge
translation initiative of the Canadian Thoracic Society Asthma Committee. The series aims to
educate and inform primary care providers and nonrespiratory specialists about the diagnosis
and management of asthma. The key messages presented in the cases are not clinical practice
guidelines but are based on a review of the most recent scientific evidence available.
Financial support for the publication of this series has been provided, in part, by the Canadian
Thoracic Society.

Acknowledgement
The authors thank Dr. Peter Paré, iCapture Centre, University of British Columbia, for his
invaluable assistance in editing and contributing to the genetics sections of this manuscript.

Footnotes

This article has been peer reviewed.

Competing interests: Piush Mandhane has received speaker’s honoraria from Merck Canada
and the Edmonton Thoracic Society. None declared for Padmaja Subbarao and Malcolm
Sears.

Contributors: All authors contributed to the development and editing of the publication, and
all approved the final version submitted for publication.
Funding: The Canadian Thoracic Society has received funding to facilitate the knowledge
translation activities of the CTS Asthma Committee from AstraZeneca Canada,
GlaxoSmithKline Inc., Merck Frosst Canada and Novartis Pharmaceuticals. None of the
sponsors played a role in the collection, review, analysis or interpretation of the scientific
literature or in any decisions regarding the key messages presented in the case studies.

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