International Journal of Clinical Medicine, 2012, 3, 394-399
http://dx.doi.org/10.4236/ijcm.2012.35074 Published Online September 2012 (http://www.SciRP.org/journal/ijcm) 1
Asthma and Injury Risk: A Large Scale Population-Based
Study
Wenbin Liang*, Tanya Chikritzhs
National Drug Research Institute, Curtin University, Perth, Australia.
Received May 11th, 2012; revised June 16th, 2012; accepted July 18th, 2012
ABSTRACT
Purpose: Clinical data suggest that asthma impairs sleep quality and further impairs cognitive performance during the
daytime, while there is a causal relationsh ip between impaired sleep quality and injuries. Therefore asthma p atients may
have increased risk of injury, and this is supported by our recent population-based studies conducted in Australia. This
study is to investigate the effect of asthma on the risk of injury at the population level using data from the US National
Health Interview Survey collected in 2008, 2009, and 2010. Method: Data from the 2008, 2009 and 2010 National
Health Interview Surveys were combined and analyzed together. Results: US adults with current or previous asthma
had significantly greater risk of injury compared to those without asthma. The risk of injury was also significantly
higher among children with current asthma. Conclusion: This population-based study provided further evidence on the
positive association between asthma and risk of injury among both adults and children. The increased risk of injury
among asthma patients is at least partly due to impaired sleep quality and quantity caused by asthma symptoms and
asthma medications.
Keywords: Asthma; Sleep; Injury
1. Introduction
Asthma is one of the major public health problems that
affect both children and adults in many different popula-
tions among both industrialized and developing nations
[1-3]. Parallel with ur banization, the prevalence of as thma
has been increasing in all coun tries over the past 40 years.
In 2004, it was estimated that asthma affected 300 mil-
lion people worldwide, [4] and it caused around 250,000
deaths in 2007 [5]. Clinical data suggest that asthma im-
pairs sleep quality and further impairs cognitive per-
formance during the daytime, especially among those
with serious night-time symptoms [6-10]. There is a
causal relationship between impaired sleep quality and
injuries [11,12]. Moreover, it has also been shown that
asthma patients are at higher risk of fracture due to re-
duced bone mass and bruising, [13,14] a known side ef-
fect of corticosteroid medication. Our recent popula-
tion-based studies suggest that a history of asthma is as-
sociated with an increased risk of injury from various
causes and types among Australian children and adults
[15,16]. Both of our previous studies were performed on
Australian populations, and it is necessary to reassess the
associations in other regions through large scale popula-
tion-based studies.
Asthma has high prevalence in the United States [1].
The National Health Interview Surveys (NHIS) collect
information on both asthma status and injuries, this has
provided an opportunity to investigate the association
between asthma status and injuries with a large sample
representative of the US population. Data were obtained
through Internet release of the NHISs data at
http://www.cdc.gov. The present study aimed to investi-
gate the effect of asthma on the risk of injury at the
population level using data from the National Health
Interview Survey (NHIS) collected in 2008, 2009, and
2010.
2. Method
Data from the 2008, 2009 and 2010 National Health In-
terview Surveys (NHISs) were combined and analyzed
together. Details of the su rvey sampling strategy and data
collection methods have been described elsewhere [17-
19]. Briefly, the NHISs are nationally focused and con-
ducted by the National Center for Health Statistics
(NCHS), Centers for Disease Control and Prevention
(CDC). The 2008, 2009 and 2010 NHISs used the same
sample design as the 2006 NHIS survey. The NHISs
were conducted to provide comprehensive estimations of
*Corresponding a uthor.

Asthma and Injury Risk: A Large Scale Population-Based Study 395
health indictors at a national level, state stratified sam-
ples were draw from all 50 states and the District of Co-
lumbia to ensure the samples are representative at state
level [17-19]. Households were the basic unit of the
NHIS. For each selected household, if there were more
than one family residing in a household, all families in
the household were selected, and assigned a unique fam-
ily number. NHIS collected basic demographics, health
status (including injuries and poisoning incidences over
the last three months), use of health services for each fa-
mily member, and health information (including asthma
history) of one random selected adult and one random
selected child (<18 years) if there were children liv ing in
the family. Information was collected via computer-as-
sisted face-to-face interviews. Since asthma histories
were only collected from one randomly selected adult
and one child within the family (rather than every person
in the family), this study included all selected adults and
children from the three waves of the NHIS [17-19].
Defining asthma status: Both of sample adults and
sample children were asked whether they had ever been
told they had asthma by a doctor or nurse. Participants
with positive responses were further asked whether their
asthma was current. Based on the answers to these two
questions asthma status was classified into one of three
categories (without asthma; previous asthma; current).
Defining injuries and poisoning: The NHISs recorded
every reported injury which occurred in the three-month
period prior to the interview and required a medical con-
sultation (i.e. call to a poison control center; use of an
emergency vehicle or emergency room; visit to a doc-
tor’s office). The current study included all recorded in-
juries and poisoning events which occurred to the sample
adults and sample ch ildren. Demogr aphics of participan ts
extracted from the datasets and controlled for in the
analysis included: age, sex, race, marital status (adults
only), education level of the adult with the highest edu-
cation in family and ratio of family income to the poverty
threshold. Multivariate Poisson regression with robust
estimation of variance was used to investigate the asso-
ciation between asthma status and the risk of injuries,
with analyses performed separately for adults and chil-
dren. The sampling weight was used in all multivariate
analyses. Missing values of some selected family demo-
graphic variables were observed in about 10% of the total
76,669 adult participants an d 8% of th e total 31,248 child
participants. Variables with missing values were: 1)
marital status (adult only); 2) education of adult with
highest education in the family; and 3) ratio of family
income to the poverty threshold. Imputation was used to
produce estimations for the cells with missing values.
Ten imputations were produced. The first set of Poisson
models were performed while excluding observations
with missing values. Then the second set of Poisson
models were refitted with imputed estimations. All
analysis was performed with STATA® 11.
3. Results
The adult sample included 76,669 participants. The ap-
proximate crude accumulative incidence rates of adult
injury for 1) adults with current asthma; 2) adults who
previously had asthma; and 3) adults who never had
asthma, were, 222 per 1000 person-years, 153 per 1000
person-years and 111 per 1000 per son-year s respectively.
The crude relative risks were significantly different with
the risk of injury among participants with current asthma
twice as great as those who never had asthma (Table 1).
In the multivariate analysis, most of the control variables
were significantly associated with the risk of injury,
however, the effects of asthma status on the risk of injury
appeared to be largely independent of the control vari-
ables (Table 2). The multivariate relative risk estimates
for injury across the three categories of asthma status
were similar to those obtained in the univariate analysis.
The child sample included 31,248 participants. The
approximate crude accumulative incidence rates of child
injury for 1) children with current asthma; 2) children
who previously had asthma; and 3) children who never
had asthma, were, 169 per 1000 person-years, 126 per
1000 person-years and 100 per 1000 person-years
Table 1. numbers of injuries, participants and relative risk by asthma status.
Asthma status Injuries (N) Participants (N) Rate Ratio 95% CI
Adult
Never had asthma 1858 67,008 1.00
Ever had asthma 140 3670 1.38 1.15 1.65
Current asthma 333 5991 2.00 1.75 2.29
Children
Never had asthma 669 26,869 1.0
Ever had asthma 44 1402 1.26 0.93 1.71
Current asthma 126 2977 1.70 1.38 2.10

Asthma and Injury Risk: A Large Scale Population-Based Study
396
Table 2. Estimations from multivariate Poisson regression model: The association between asthma status and risk of injury in
adults.
Without imputation With imputati on
Rate Ratio 95% CI Rate Ratio* 95% CI
Asthma status
Never had asthma 1.00 1.00
Ever had asthma 1.26 1.01 1.56 1.31 1.06 1.63
Current asthma 1.81 1.54 2.13 1.86 1.59 2.17
Sex
Male 1.00 1.00
Female 0.95 0.84 1.06 0.93 0.83 1.03
Age group
18 - 24 1.00 1.00
25 - 34 0.97 0.76 1.24 0.96 0.76 1.22
35 - 44 1.05 0.80 1.37 1.06 0.81 1.39
45 - 54 1.14 0.87 1.48 1.10 0.85 1.42
55 - 64 1.18 0.90 1.54 1.19 0.92 1.55
65+ 1.30 0.97 1.72 1.26 0.96 1.66
Race
White 1.00 1.00
Black 0.79 0.66 0.95 0.80 0.67 0.95
Other 0.80 0.62 1.04 0.84 0.66 1.07
Marital status
Married or de facto 1.00 1.00
Widowed/divorced/separated 1.62 1.41 1.86 1.62 1.41 1.85
Never married 1.26 1.05 1.52 1.25 1.04 1.51
Ratio of fami l y income to the poverty t hreshold
Less than 1 1.00 1.00
1 - 1.99 0.85 0.70 1.03 0.86 0.72 1.03
2.0+ 0.88 0.75 1.03 0.90 0.77 1.06
Educational level attained for adult with highest education in the family
12 grade or less 1.00 1.00
Had some college 1.25 1.03 1.53 1.19 0.99 1.43
With a degree or above 1.26 1.02 1.55 1.19 0.98 1.44
Whether any family member had delaye d medical care in the last 12 months
No 1.00 1.00
Yes 1.35 1.19 1.54 1.36 1.20 1.54
Year of survey
2008 1.00 1.00
2009 1.06 0.92 1.22 1.05 0.92 1.20
2010 0.98 0.86 1.13 0.98 0.86 1.12
*IRR: incidence rate ratio.
respectively. Compared to participants without asthma,
the risk of injury was significantly (70%) higher for those
children with current asthma but not for those with pre-
vious asthma (Table 1). As found for the adult sample, in
the multivariate analysis, most of the control variables
were significantly associated with risk of injury, however,
the risk of injury remained significantly higher (50%) for
children with current asthma compared to those without

Asthma and Injury Risk: A Large Scale Population-Based Study 397
(Table 3). Estimations from the analysis of the original
data and estimations from the analysis with multiple im-
putatio ns we re found to be ve ry simil a r .
4. Discussion
In this large scale popu lation -based study, US adu lts with
current or previous asthma had significantly greater risk
of injury compared to those without asthma. The risk of
injury was also significantly higher among children
with current asthma. The association between asthma
status and risk of injury was largely unaffected by the
inclusion of control variables. These observations are
consistent with findings from previous studies [15,16,
20-22]. In our recent birth cohort study conducted in
Western Australia, it was observed that for both children
and adults, males with histories of hospitalizations of
asthma were at increased risk of serious injury (injuries
that required hospitalizations) [16].
Table 3. Estimations from multivariate Poisson regression model: The association between asthma status and risk of injury in
children.
Without imputation With imputation
IRR* 95% CI IRR* 95% CI
Asthma status
Never had asthma 1.00 1.00
Ever had asthma 1.08 0.74 1.58 1.08 0.75 1.57
Current asthma 1.55 1.20 2.00 1.52 1.18 1.96
Sex
Male 1.00 1.00
Female 0.83 0.69 0.99 0.81 0.68 0.97
Age group
<1 0.16 0.06 0.41 0.16 0.06 0.39
1 - 4 1.07 0.80 1.43 1.05 0.79 1.39
5 - 9 1.00 1.00
10 - 14 1.61 1.24 2.08 1.57 1.23 2.02
15 - 17 2.02 1.55 2.62 1.99 1.55 2.57
Race
White 1.00 1.00
Black 0.61 0.46 0.82 0.61 0.46 0.81
Other 0.72 0.52 1.00 0.73 0.53 0.99
Ratio of fami l y income to the poverty t hreshold
Less than 1 1.00 1.00
1 - 1.99 0.88 0.64 1.21 0.90 0.66 1.22
2.0+ 1.11 0.84 1.48 1.14 0.87 1.50
Educational level attained for adult with highest education in the family
12 grade or less 1.00 1.00
Had some college 1.63 1.12 2.38 1.72 1.19 2.49
With a degree or above 1.89 1.28 2.80 1.9 7 1.35 2.89
Whether any family member had delaye d medical care in the last 12 months
No 1.00 1.00
Yes 1.06 0.85 1.34 1.07 0.85 1.33
Year of survey
2008 1.00 1.00
2009 0.91 0.73 1.13 0.94 0.76 1.16
2010 0.88 0.71 1.10 0.92 0.74 1.13
*IRR: incidence rate ratio.

Asthma and Injury Risk: A Large Scale Population-Based Study
398
The mechanism(s) underlying this association is not
entirely clear, however, it is likely to be causal. Asth-
matic conditions and the medications used to treat
asthma (including cortico steroids) may impair th e mental
function of patients and alter their responses to hazard,
and further increase the risk of injury. Among the par-
ticipants of the current study, adults with current asthma
or previous asthma were more likely to sleep less than 7
hours every 24 hours at the time of survey compared to
those without asthma (37%, 32% and 28%, respectively,
p value of Pearson chi-square test < 0.001). Previous
research has suggested that asthma patients are more
likely to experience sleepiness during daytime hours
[7,23,24] and that asthma condition and medications for
asthma may impair sleep quality [8,25]. Moreover, there
is substantial eviden ce sugg estin g th at daytime sleep iness,
inadequate sleep or poor sleep quality is an independent
risk factor for injuries in both adults and children [11,
12,26-29]. Combining the evidence on the three rela-
tionships: 1) asthma status and injury; 2) asthma status
and its impairment on sleep quality and quantity and 3)
inadequacy of sleep and injury, indicates a direct link
between asthma status and injury mediated through im-
paired sleep quality and quantity. Interventions that im-
prove sleep among asthma patients are likely further re-
duce the risk of injury among these patients.
5. Conclusion
This population-based study provided further evidence
on the positive association between asthma and risk of
injury among both adults and children. It is highly likely
that the increased risk of injury among asthma patients is
at least partly due to impaired sleep quality and quantity
caused by asthma symptoms and asthma medications. To
reduce the risk of injury among asthmatics, interventions
are required which improve sleep quality and quantity
among patients.
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