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3 PATTERNS OF ASTHMA MORBIDITY AND Mortality Epidemiology is the study of the distribution of a disease and its risk factors in a population. Epidemiologists attempt to under- stand the causes of a disease by studying its occurrence across time, place, and persons. Studying the observed differences will identify risk factors contributing to the development and activity of a disease. Once these risk factors are understood, interventions can be developed to ameliorate their effects. As we see in this chapter, asthma is not homogeneously spread throughout the population, nor has it remained static in terms of its burden on the U.S. population. These differences are reviewed to help the reader gain a better understanding of the importance of and in- terplay between the environment and genetics in asthma. Because the epidemiology of asthma is difficult to cover in such a brief space, the reader seeking further information is directed to a num- ber of more extensive reviews (Manning et al., 1998; Weiss et al., 1993). THE BURDEN OF ASTHMA Asthma is an important health problem in the United States. In the 1995 National Health Interview Survey (NHIS), approxi- mately 15 million individuals identified themselves as asthmat- ics, with approximately 5 million being under age 18 (Benson and 67
68 CLEARING THE AIR Marano, 1998~. The Centers for Disease Control and Prevention (CDC) estimated that there were about 17.3 million people with the illness in 1998 (Rappaport and Boodram, 1998~. Despite these high numbers, asthma is a relatively minor cause of mortality in the United States. Asthma was listed as the underlying cause of death in 5,667 out of 2.3 million deaths in the United States dur- ing 1996 (U.S. Vital Statistics, 1996~. In contrast, the morbidity bur- den is much greater. During 1996 there were 474,000 asthma hos- pitalizations, of which 195,000 occurred in children less than 15 years of age (Graves and Kozak, 1998~. In addition, there were approximately 11.9 million medical visits for asthma, of which 9.1 million occurred in physicians' offices, 0.9 million in hospital out- patient departments, and 1.9 million in emergency rooms (Schappert,1998~. The economic burden of asthma was estimated to range from $5.8 billion to $6.2 billion in the early 1990s (Smith et al., 1997; Weiss et al., 1992~. Asthma was estimated to account for approximately 1% of U.S. health care costs in the mid-1980s (Weiss et al., 1992~. MORTALITY Asthma mortality data are readily available from the U.S. Vi- tal Statistics System. The U.S. asthma mortality rate of 2.13 deaths per 100,000 population in 1996 is low compared to as many as 7 to 9 deaths per 100,000 population in other parts of the world the highest rates being noted in New Zealand, West Germany, and Norway (Sears, 1991~. Although the number of deaths from asthma is not large, an understanding of their causes is important since asthma mortality is considered preventable. Asthma mor- tality primarily affects adults, with approximately 67% occurring at or after 45 years of age. Males tend to have higher asthma death rates than females until about age 25, after which females have the higher rates for the rest of the life span. African Americans have consistently higher asthma death rates than whites. The dif- ference is greatest in the younger group approximately 10 times higher in 1- to 4-year-olds and decreases with age until the ratio is only 1.2 times greater at age 85 and over (Figure 3-1~. Little difference in asthma mortality rates is seen when the United States is divided into four census regions (U.S. Vital Statistics, 1995,
PATTERNS OF ASTHMA MORBIDITY AND MORTALITY 20 18- 16- 14- o ° 12 - o - o o 10 - Q 8- 6- 4- 2- O- 69 White · Black .~. .,^ / i, , I / ~1 l l 1 W * -- ~---~ --by -- - '' 1 1 1 - 5-9 10-14 15-19 20-24 25-34 35~4 45-54 55-64 65-74 75-84 285 Age (years) FIGURE 3-1 Asthma mortality by race, 1995. SOURCE: Anderson et al., 1997. 1996~. However, numerous studies conducted in urban areas have much higher asthma mortality in areas characterized by high lev- els of poverty and minority populations (Carr et al., 1992; Lang and Polansky, 1994~. Asthma mortality has been increasing in the United States since the late 1970s, although in the past several years the increase appears to have slowed down. No group has been spared this increase. UTILIZATION OF HEALTH CARE SERVICES Data on asthma health care utilization patterns in the United States are available from two national surveys the National Hos- pital Discharge Survey and the National Health Care Survey. In contrast to mortality, approximately 37°/O of asthma hospitaliza
70 CLEARING THE AIR lions occur before 15 years of age. Rates are highest in the young (less than 5 years of age), with a second, lower peak in the group age 65 or more (Graves and Kozak, 1998~. In the young, hospital- ization rates are higher for males, where as among adults, females have higher rates (Skobeloff et al., 1992~. Hospitalization rates are elevated in urban areas with high levels of poverty and/or mi- nority populations (Carr et al., 1992; Gergen and Weiss, 1990; Gottlieb et al., 1995~. Readmissions can account for up to approxi- mately 20% of hospitalizations, with the readmission rate being higher in the younger age group (Goldring et al., 1997~. Hospital- izations show a regional variation in the United States, with the highest rates in the Northeast and the lowest in the West. How- ever, this variation is true of hospitalizations in general not just for asthma (Graves and Kozak, 1998~. Asthma hospitalizations have primarily been rising in the United States in the under-15 age group, especially among those under 5 years of age. African- American children appear to have a higher rate of increase than white children do (Gergen and Weiss, 1990~. Somewhat less detail is available on outpatient care. Emer- gency room (ER) visits have been tracked in the United States only since 1992. Similar to hospitalizations, African Americans and women have higher rates. The rates for ER visits are the high- est in the very young but, unlike hospitalizations, do not increase in the older age groups. Ambulatory care visits for asthma in phy- sicians' offices are only slightly higher in the young versus older ages. The most striking difference is the low number of visits in the 15- to 34-year age range. African Americans and women have slightly higher rates. The number of office visits for asthma in- creased from 4.6 million in 1975 to 10.4 million in 1993-1994 (Manning et al., 1998~. PREVALENCE The prevalence of asthma is highly dependent on the defini- tion chosen. The prevalence of asthma varied almost threefold, from 3.6 to 9.5°/O, when various combinations of physician diag- nosis and wheezing were used to define asthma among children age 3 to 17 who participated in the second National Health and Nutrition Examination Survey (NHANES II) from 1976 to 1980
PATTERNS OF ASTHMA MORBIDITY AND MORTALITY TABLE 3-1 Prevalence of Asthma by Type of Diagnosis Type of Diagnosis Percentage Ever diagnosed with asthma by physician Current physician diagnosis of asthma Self-reported wheezing Ever diagnosed with wheeze by physician Current physician diagnosis of wheeze 7.0 3.6 5.3 9.5 6.7 SOURCE: Gergen et al., 1988. Adapted with permission from Pediatrics, 81:1-7, 1988. 71 (see Table 3-1) (Gergen et al., 1988~. Despite this dependence on definition, certain patterns appear consistently in the age distri- bution of asthma. Most childhood asthma begins before age 5 (Gergen et al., 1988~. A review of medical records from the Mayo Clinic found that the highest incidence of "asthma" occurred in the first year of life, with incidence rates falling throughout child- hood and reaching their lowest levels among adults (Yunginger et al., 1992~. More males develop asthma during childhood, while the prevalence in females surpasses that in males during adoles- cence. After adolescence, males and females have equivalent rates when a broad range of ages is examined (Figure 3-2) (Benson and Marano, 1998; Turkeltaub and Gergen, 1991~. Asthma prevalence is found to vary with race or ethnicity and urban location. In national surveys, African Americans reported higher rates of asthma and a higher rate of increase over the past decade (see Figure 3-3) (Benson and Marano, 1998; Gergen et al., 1988; Turkeltaub and Gergen, l991~. Hispanics reported wide dis- crepancies in prevalence: Mexican-American children in the Southwest reported some of the lowest rates of asthma in the United States while Puerto Rican children living on the East Coast of the United States reported some of the highest rates (Carter- Pokras and Gergen, 1993~. Other data suggest that racial differ- ences may reflect, at least in part, diagnostic acquisition rather than true differences in disease prevalence. A survey of 9- to 11- year-old school children in Philadelphia, Pennsylvania, found that white and African-American children reported the same
72 60 50 o - ~ 40- Q 35 30 20 CLEARING THE AIR ~ Male -I- Female ,," a. 1980 1981 -1983 1984-1986 1987-1989 1990-1992 1993-1994 Year FIGURE 3-2 Asthma prevalence by gender, 1980-1994. SOURCE: Mannino et al., 1998. 60 55 50 0 45- - ~ White - ~ - Black _______ Q 40- ~ , ~35- · ;=) 30 20 ~,. 1980 1981 -1983 1984-1986 1987-1989 1990-1992 1993-1994 Year FIGURE 3-3 Asthma prevalence by race, 1980-1994. SOURCE: Mannino et al., 1998.
PATTERNS OF ASTHMA MORBIDITY AND MORTALITY 73 prevalence of persistent wheezing, while among children with wheeze, the African-American children were more likely to have received the diagnosis of asthma (Cunningham et al., 1996~. The NHIS has reported the prevalence of asthma to be higher in urban areas. More focused studies have shown that the preva- lence of asthma can be just as high in less populated areas of the United States for example, Los Alamos, New Mexico (Sporik et al., 1995) compared to highly populated areas such as the Bronx, New York (Crain et al., 1994~. Regional data from the 1996 NHIS show asthma prevalence to be highest in the Northeast and low- est in the Midwest and South (Benson and Marano, 1998~. State- specific asthma prevalence rates have been estimated by multi- plying age-, race-, sex-, and region-specific asthma prevalence rates calculated from the NHIS with the state's demographic com- position. The 1998 state-level estimates are provided in Table 3-2. California, New York, and Texas were found to have the greatest number of asthmatics. The state-specific prevalence rates ranged from 5.8 to 7.2%, leading the authors to conclude that the demo- graphic composition of states had minimal influence on estimated asthma prevalences (Rappaport and Boodram, 1998~. Based on data from the NHIS, the self-reported prevalence of asthma increased 75°/O between 1980 and 1994 (Manning et al., 1998~. All groups are thought to be affected by the increase in asthma prevalence. However, a study of the Mayo Clinic patient population reported that between 1964 and 1983, the incidence of asthma increased only in the 1- to 14-year age group, with no change in other age groups (Yunginger et al., 1992~. Epidemiologic investigations of asthma prevalence have dem- onstrated wide variability both between and within different countries. The International Study of Asthma and Allergy in Childhood (ISAAC) has shown that asthma-like symptom preva- lence varies almost thirtyfold, from 1.6% in Indonesia to 36.8% in the United Kingdom. Generally, more developed Western coun- tries have higher rates of asthma, while less developed countries have lower rates (ISAAC, 1998~. The ISAAC project found that 12-month prevalence rates of wheeze were greater than 25% in the United Kingdom, New Zealand, Australia, Ireland, Canada, Peru, Costa Rica, Brazil and the United States, while Indonesia, Albania, Romania, Georgia, Greece, China, and Russia had
74 CLEARING THE AIR TABLE 3-2 Forecastecl Estimates of Self-Reportecl Asthma Prevalence by State, 1 998 State Region or Number Estimated of Cases Prevalence (%) 95% Confidence Standard Interval (%) Error (%) Northeast Connecticut215,900 6.6 5.6-7.5 7.2 Maine80,300 6.4 5.4-7.4 7.8 Massachusetts401,000 6.5 5.6-7.5 7.2 New Hampshire78,500 6.6 5.5-7.6 7.8 New Jersey540,400 6.7 5.7-7.6 7.2 New York1,236,200 6.8 5.8-7.8 7.3 Pennsylvania800,900 6.6 5.6-7.5 7.2 Rhode Island64,400 6.5 5.5-7.4 7.3 Vermont39,500 6.5 5.5-7.6 7.8 Total3,241,200 6.7 5.7-7.6 7.3 Midwest Iowa190,100 6.6 5.6-7.6 7.5 Illinois795,200 6.7 5.7-7.6 7.5 Indiana398,400 6.7 5.7-7.7 7.3 Kansas174,900 6.7 5.7-7.6 7.3 Michigan642,300 6.7 5.7-7.7 7.5 Minnesota318,600 6.7 5.8-7.7 7.1 Missouri362,300 6.1 4.7-7.4 11.3 Nebraska112,100 6.7 5.7-7.7 7.4 North Dakota43,600 6.7 5.7-7.6 7.3 Ohio748,200 6.7 5.7-7.6 7.4 South Dakota51,000 6.7 5.8-7.7 7.3 Wisconsin350,800 6.7 5.7-7.7 7.2 Total4,187,600 6.6 5.6-7.6 7.4 South Alabama280,500 6.0 4.8-7.1 9.5 Arkansas162,600 5.9 4.9-6.9 6.9 District of Columbia31,400 5.9 3.6-8.2 19.7 Delaware44,300 5.9 4.9-6.9 8.5 Florida863,900 5.8 4.9-6.8 8.0 Georgia458,700 6.0 4.9-7.2 9.7 Kentucky232,800 5.9 4.9-6.9 8.2 Louisiana265,500 6.1 4.8-7.3 10.5 Maryland307,300 6.5 5.6-7.5 7.2 Mississippi167,900 6.1 4.7-7.4 11.3 North Carolina447,200 5.9 4.9-7.0 8.9
PATTERNS OF ASTHMA MORBIDITY AND MORTALITY TABLE 3-2 Continuecl 75 Region orNumber Estimated 95%Confidence Standard Stateof Cases Prevalence (%) Interval (%) Error (%) Oklahoma191,700 5.8 4.8-6.7 7.9 South Carolina228,600 6.0 4.8-7.2 10.1 Tennessee328,300 5.9 4.9-6.9 8.3 Texas1,175,100 6.0 5.0-7.0 8.2 Virginia403,400 5.9 4.9-6.9 8.6 West Virginia108,600 5.8 4.9-6.8 8.2 Total5,697,800 5.9 4.9-7.0 8.8 West Alaska42,500 6.7 5.7-7.7 7.7 Arizona316,200 6.9 6.0-7.9 6.9 California2,268,300 7.1 6.1-8.0 6.8 Colorado283,700 7.1 6.1-8.0 6.8 Hawaii73,100 6.0 4.1-7.8 15.3 Idaho86,100 6.7 5.7-7.8 7.6 Montana61,600 6.6 5.7-7.6 7.4 Nevada125,700 7.2 6.3-8.1 6.4 New Mexico121,800 6.8 5.8-7.8 7.2 Oregon225,900 6.9 5.9-7.8 6.9 Utah141,200 6.7 5.6-7.8 8.1 Washington391,900 6.9 5.9-7.8 6.8 Total4,172,400 7.0 6.0-8.0 7.0 U.S. total17,299,000 6.4 5.5-7.5 7.8 NOTE: Persons were considered to have asthma if asthma had been diagnosed by a physician at some time in their life and they had reported symptoms of asthma during the preceding 12 months. SOURCE: Rappaport and Boodram, 1998. 12-month prevalence rates of less than 5°/0. Figure 3-4 illustrates the range of country-specific rates. SEVERInr Based on the data shown above, one could conclude that the severity of asthma has increased. However, asthma mortality,
76 UK New Zealand Australia Ireland Australia Canada Peru Costa Rica Brazil USA Paraguay Uruguay Panama Kuwait South Africa Malta Finland Lebanon Kenya Germany France Japan Thailand Sweden Hong Kong Philippines Belgium Austria Iran Argentina Estonia Nigeria Spain Chile Singapore Malaysia Portugal Uzbekistan Oman Italy Pakistan Larvia Poland Algeria South Korea Morocco Mexico Ethiopia I ndia Taiwan Russia China Greece Georgia Romania Albania I ndonesia CLEARING THE AIR ·. · .~ ~ - FIGURE 3-4 Prevalence of asthma symptoms (percentage) from writ- ten questionnaires in the ISAAC da- tabase. SOURCE: ISAAC Steering Committee (1998~. 0 5 10 15 20 25 30 Prevalence of Symptoms (%)
PATTERNS OF ASTHMA MORBIDITY AND MORTALITY 77 health care utilization, and the like are as much a reflection of asthma control as intrinsic severity (Cockcroft and Swystun, 1996~. Asthma control reflects the access to appropriate health care, the use of medications, and environmental controls. Data from a number of sources imply that the increase in asthma is not due solely to an increase in its severity. Data collected on self- reported asthmatics participating in the Child Health Supple- ments of the NHIS a nationally representative health survey- found that, in comparing 1988 with 1981, a number of markers of asthma impact had decreased: the percentage of asthmatics re- porting fair or poor health status, the number reporting 30 or more days in bed over the past year, the rate of extreme behavior problem scores, and the number of school days missed. The re- ductions were similar in white and African-American children (Weitzman et al., 1992~. Repeat surveys among 7- and 8-year-olds in the London borough of Croy~on during 1978 and 1991 reported significant increases in the 12-month prevalence of attacks of wheezing or asthma, the one-month prevalence of wheezing epi- sodes, and night waking, but not the prevalence of individuals reporting five or more attacks per year. In addition, there was a decrease in the number reporting 10 or more school days missed per year due to wheezing and any days in bed or restrictions in activities (Anderson et al., 1994~. TRENDS IN RISK FACTORS The changing epidemiology of asthma provides the opportu- nity to compare the changes in the reported asthma risk factors to determine the role these are playing. Data are available on a num- ber of asthma risk factors during the period of the observed in- crease in asthma. Outdoor air pollution does not appear to play an important role in the increase in asthma in the United States since pollutant levels have been decreasing during the time of asthma increase. Between 1987 and 1996, the ambient concentra- tions of a number of pollutants decreased: nitrogen dioxide by 10%, ozone by 15%, and sulfur dioxide by 37°/O; also, between 1988 and 1996, ambient particulate matter (PM~o) decreased by 25% (U.S. EPA, 1998~. Similarly, cigarette smoking does not appear to be driving the increase in U.S. asthma rates since cigarette smok
78 CLEARING THE AIR ing decreased during the time of the well-documented asthma increase. Between 1979 and 1994, the percentage of active smok- ers dropped from 33.5 to 25.5% among individuals 18 years of age or more (Fingerhut and Warner, 1997~. As discussed elsewhere in this report, there is little good information on allergen levels in the environment. A longitudinal study reporting that exposure to house dust mites was associated with the development of asthma found no change in the household levels of mites over the 10-year period of the study (Sporik et al., 1990~. The rate of breast feed- ing reported by some (Oddy et al., 1999) to protect against the development of asthma has remained stable in the United States since the early 1980s. In 1981-1983,58.1% of babies in the United States were reported to be breast-fed in the National Survey of Family Growth. Although the rate fluctuated a few percentage points during the intervening years, during 1993-1994 the rate was still 58.1% (Fingerhut and Warner, 1997~. There is some evidence that infections early in life could be protective for asthma and allergies (Martinez, 1994~. However, this runs counter to data suggesting an increase in childhood asthma incidence at a time when the use of day care increased (from 13% in 1994 to 29.4% in 1997 according to a 1998 U.S. Cen- sus Bureau estimate [1998~), given that day care attendance is as- sociated with an increase in infections (Nafstad et al., 1999~. The relationship of infections to asthma is clearly complex. Although they may exacerbate asthma Johnston et al., 1995), their role in the development of or protection from asthma is controversial (Kramer et al., 1999; Sporik et al., 1990~. The role of infectious agents in asthma exacerbation and development is addressed in greater detail in Chapter 5. TWIN, ADOPTION, AND MIGRANT STUDIES Twin studies have long been used to determine the relative importance of environment and heredity in disease development. A study of 13,888 Finnish twin pairs age 18 to 70+ years reported a heritability estimate of 35.6% (Nieminen et al., 1991~. Surpris- ingly, when examined by gender, the heritability estimate was 67.8% for females and 0°/0 for males. In contrast, a report on 5~864 Norwegian twins studied at 18-25 years of age reported a 75%
PATTERNS OF ASTHMA MORBIDITY AND MORTALITY 79 heritability estimate (Harris et al., 1997~. No separate estimates were given by gender. In a preliminary report on the develop- ment of asthma among adopters, the presence of asthma in the adoptive parents was found to be a risk factor, implying that the home environment was important in the development of asthma (Smith et al., 1998~. Other empirical evidence for the role of the environment in the development of asthma comes from the study of migrant populations, which allows one to compare the development of asthma between two relatively homogeneous groups of individu- als living in different environments. For example, a study of chil- dren living on Tokelau, an island in the Pacific Ocean, and chil- dren of Tokelau origin living in New Zealand found that the children living in New Zealand had about double the asthma prevalence of those living in Tokelau. This was true even when the children were new immigrants to New Zealand (Waite et al., 1980~. SOCIOECONOMIC STATUS VERSUS RACE OR ETHNICITY Poverty and minority status are consistently reported to be related to increased asthma morbidity and mortality (Carr et al., 1992; Gottlieb et al., 1995~. Thus far, research has not provided evidence that asthma is intrinsically a different disease among the various racial or ethnic groups in the United States. However, is asthma more prevalent in poor communities? Although asthma prevalence rates have been reported to be very high in poor ur- ban areas with a high percentage of minority residents, such as the Bronx, New York (Crain et al., 1994), equally high rates have been reported in less urban areas with low levels of poverty and minority residents, such as Los Alamos, New Mexico (Sporik et al., 1995~. According to asthma prevalence data for poor and non- poor children participating in the 1988 Child Health Supplement of the NHIS, there was only approximately 10% more asthma among poor children than non-poor children (Halfon and Newacheck,1993~. The previously mentioned differences in utili- zation and mortality are much greater. Thus, the differences in utilization appear to more clearly reflect differences in the control of asthma. It is clear that access to high-quality medical care can
80 CLEARING THE AIR greatly decrease the impact of asthma: the variation in hospital rates for asthma among three cities in the United States was found to be related to the quality of primary care received (Homer et al., 1996~; almost 80% of ER visits for asthma were found to be poten- tially preventable with better self-management training (Wasilewski et al., 1996~. In a managed care setting, the use of an action plan was associated with reduced asthma hospitalizations and emergency room visits (Lieu et al., 1997~. The quality of asthma care received by some poor and minority individuals is clearly inadequate (Finkelstein et al., 1995; Kattan et al., 1997~. ASTHMA RATES IN GER~NY A NATURAL EXPERIMENT The reunification of Germany in the early l990s offered a chance to compare allergic diseases between two genetically simi- lar populations living under vastly different and changing envi- ronmental conditions. A cross-sectional study found that 9- to 11- year-old children in West Germany reported more asthma, and allergic rhinitis, and had higher rates of allergen skin test reactiv- ity and bronchial hyperreactivity (BHR). The East German chil- dren reported higher rates of bronchitis (von Mutius et al., 1994~. A second cross-sectional study was done in the same East Ger- many city four years later, during which time pollution levels had dropped and prosperity had increased. The allergic skin test reac- tivity and reported rates of hay fever were found to have in- creased, but the prevalence of asthma and BHR had not (von Mutius et al., 1998~. Somewhat different results were reported when two cross-sectional surveys (1992-1993 and 1995-1996) were carried out among 5- to 7-year-olds in East Germany. Self- reported allergies increased and bronchitis decreased after unifi- cation, while the level of allergen sensitivity as measured by RAST (radioallergosorbent test) did not change. The authors concluded that the differences were due to changes in physician diagnostic practices and/or reporting patterns by parents (Heinrich, 1999~. The lack of change in asthma in both of these reports could repre- sent either the lack of importance of these environmental changes to the development of asthma or an indication that there is a rela- tively brief period of vulnerability early in life when these types of environmental influences can have an effect.
PATTERNS OF ASTHMA MORBIDITY AND MORTALITY REFLECTIONS ON THE TRENDS 81 There is no question that the prevalence of, health care utili- zation for, and morbidity from asthma have been reported to be increasing in the United States. The crucial issue is whether the observed changes represent a true increase or merely a better recognition of previously undiagnosed asthma. Better recognition has the potential to greatly increase the reported prevalence of asthma, since numerous population-based studies have shown that asthma is underdiagnosed in all age groups (Banerjee et al., 1987; Speight et al., 1983~. One review of the reported increase in asthma prevalence concluded that much of the increase could be explained by changes in labeling and information bias, since few studies have employed objective measures to diagnose asthma (Magnus and laakkola, 1997~. However, one of the few studies that employed an objective measure (peak flow) to define asthma reported an increase in prevalence over a 15-year time span (Burr et al., 1989~. Markers of utilizations are subject to their own set of biases. Practice patterns have been found to vary greatly from one region to another for many diseases not just asthma (Wennberg and Gittelsohn, 1982~. Asthma hospital admission rates are influenced by many factors other than disease preva- lence or severity. For example, higher hospitalization rates are found in areas with increased numbers of available hospital beds (Goodman et al., 1994), and it has been suggested that the major- ity of children hospitalized for asthma could have been treated in outpatient settings (McConnochie et al., 1999~. Lower socioeconomic status is clearly linked with increased levels of asthma morbidity and mortality. Asthma is closely asso- ciated with other manifestations of atopy such as immunogIobu- lin E (IgE) levels and allergen skin test reactivity (Burrows et al., 1989~. However, in the United States, allergen skin test reactivity increases with increasing socioeconomic status (Gergen et al., 1987~. Even mortality statistics should be interpreted with caution. The accuracy of the coding of death certificates for asthma de- creases with age dropping from an accuracy of more than 97°/O in those under age 35 to 50°/O in the oldest age groups. (Sears et al., 1986~. Chronic obstructive pulmonary disease (COPD) can be mis
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