Asthma: Frequently Asked Questions
||How many Americans have ever had asthma?
||The most recently available data is from the 2004 National
Health Interview Survey (NHIS). Based on the proportion
of respondents who answered "yes" to the question: "Has
a doctor or other health professional ever told you that
you had asthma?" it is estimated that 10.5% (30.2 million)
of the US population have been diagnosed with asthma in
their lifetime. This includes 9.9% (21.3 million) of adults
18 years and over, and 12.2% (8.9 million) of children
under age 18 years. Lifetime asthma diagnosis is not a
measure of the current public health problem of
asthma-it is important to identify those with current
active symptoms as well as those whose asthma is under
control through medication or other means. Based on the
persons who also answered yes to the subsequent question:
"During the past 12 months have you had an episode of
asthma or an asthma attack?" an estimated 4.1% (11.7 million)
of the US population had a recent asthma attack. Among
children under age 18 years, 5.4% (4.0 million) had at
least one asthma asthma attack in the past 12 months.
Among adults 18 years and over, 3.6% (7.7 million) had
at least one asthma asthma attack in the past 12 months.
This is a useful public health measure because it provides
an estimate of people with asthma who are at risk of poor
health outcomes and who use more health care resources..
||How many Americans currently have asthma?
||Beginning in 2001, the prevalence of both current asthma
and asthma attack are being measured. Current asthma prevalence
is measured by asking all respondents who said that they
have been diagnosed with asthma by a health profession
the question "Do you still have asthma?" This estimate
includes people who have an asthma diagnosis and who believe
they still have the condition, but may not have had a
recent attack because of management with medication or
avoidance of asthma triggers. In 2004, 7.1% (20.5 million)
of people currently had asthma. Among children under age
18 years, 8.5% (6.2 million) currently had asthma. Among
adults 18 years and over, 6.7% (14.4 million) currently
||Is asthma increasing or decreasing?
||During the period 1980 to 1996 the prevalence of asthma
was determined from the National Health Interview Survey
question: "During the past 12 months has anyone in the
family had asthma?" The number of persons in the United
States reported to have asthma doubled between 1980 and
1996, from 7.0 million to 14.6 million persons.
In 1997, the National Center for Health Statistics
changed the way asthma prevalence is estimated. Data
before and after 1996 cannot be directly compared. First,
the asthma questions changed in 1997 to measure prevalence
of lifetime asthma diagnosis and asthma attack prevalence.
Second, the 1997 survey design was changed to make the
data more reliable. Since 1997, asthma data are gathered
for the whole NHIS sample compared to one-sixth of the
pre-1997 sample; also, second-hand or "proxy" reporting
for other household members was phased out except for
children. For the five years between 1997 and 2004 there
is no consistent pattern of an increase in prevalence
of lifetime asthma diagnosis or asthma attack prevalence.
Current asthma prevalence also remained relatively stable
from 2001 to 2004.
In summary, asthma prevalence increased from 1980 to 1996.
The new asthma prevalence measures adopted in 1997 have
remained relatively stable from 1997 to 2004.
||Why has asthma prevalence increased from 1980 to 1996?
||There have been many theories put forward to explain
the increase between 1980 and 1996, but in general, the
reasons behind rising asthma prevalence, hospitalizations,
and death rates are not well understood. It is likely
that many factors are responsible, and current theories
include: exposures to allergens, pollutants and infections;
obesity, diet and physical activity; awareness and reporting
of asthma by health care workers and the public; and use
of antibiotics. Just as it is poorly understood why asthma
increased from 1980 to 1996, it is also not well understood
why asthma attack prevalence and lifetime asthma diagnosis
have remained relatively steady from 1997 to 2004.
||Why were the NHIS questions changed?
||The survey was redesigned in 1997 to increase the reliability
of the estimates, reduce the length of the interviews,
and focus on the most important public health problems.
There was also a need to incorporate ongoing methodical
advances in the measurement of asthma and to increase
comparability across surveys. An important part of the
redesign was to eliminate "second-hand," or "proxy" reports
from one household member for other adults in the household,
as these can be inaccurate. Proxy reports are still used
for children. To further improve estimates, respondents
are now asked whether they were ever told by a doctor
or other health care professional that they had asthma.
It was also important to determine if the asthma had been
active recently by asking whether the person had experienced
an attack in the past 12 months.
||What has been the impact of the change in questions--
are current estimates better or worse than past estimates?
||The new estimates are believed to be better than past
estimates, though they have the disadvantage of not being
comparable to earlier data. The survey question asked
before 1997 was: "During the past 12 months did anyone
in the family have asthma?" This question did not require
a diagnosis by a health professional. It included "second-hand"
or "proxy" responding for adults not in the house at the
time of the survey. In addition, since the entire NHIS
sample is asked about asthma in the redesigned survey
(compared to about one-sixth of the sample before the
redesign), the prevalence estimates after 1997 are more
precise. Beginning in 1997, the NHIS includes an initial
screening question to determine who in their lifetimes
were told by a doctor or other health professional that
they had asthma (30.2 million in 2004). It then follows
up the first question by identifying those who still have
asthma (20.5 million people in 2004), and those who had
at least one attack or episode of asthma in the past 12
months (11.7 million in 2004).
Additional information on asthma prevalence and the NHIS
Childhood Asthma Prevalence Before and After the 1997 Redesign
of the National Health Interview Survey --- United States,
MMWR October 13, 2000 / 49(40);908-911.
Prevalence, Health Care Use and Mortality, 2002. NCHS Health
for Asthma -- United States, 1980-1999 MMWR Surveillance Summary,
March 29, 2002 / 51(SS-01);1-13.
Updated June 2006
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