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Asthma:  Frequently Asked Questions

Q1.   How many Americans have ever had asthma?
A1. 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..

Q2. How many Americans currently have asthma?
A2. 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 had asthma.

Q3. Is asthma increasing or decreasing?
A3. 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.

Q4. Why has asthma prevalence increased from 1980 to 1996?
A4. 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.

Q5. Why were the NHIS questions changed?
A5. 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.

Q6. What has been the impact of the change in questions-- are current estimates better or worse than past estimates?
A6. 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 redesign:

Measuring 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.

Asthma Prevalence, Health Care Use and Mortality, 2002. NCHS Health Estats

Surveillance for Asthma -- United States, 1980-1999 MMWR Surveillance Summary, March 29, 2002 / 51(SS-01);1-13.



Updated June 2006


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