An International Comparison of Asthma Morbidity and Mortality in US Soldiers 1984 to 1988

An International Comparison of Asthma Morbidity and Mortality in US Soldiers* 1984 to 1988 David L. Ward, M.D., M.P.H.t

Asthma-related morbidity, as measured by hospital discharges, and mortality are compared for US soldiers assigned to central Europe and to the continental United States. The populations, as well as the level and type of medical care available, are quite similar in both locations and permit a purer contrast of the effect of geography than is possible by comparing indigenous populations across national boundaries. The results reveal higher discharge rates and mortality in Europe than in the United States; a peak discharge rate in the third calendar quarter in Europe

but less clearly in the United States; similar mortality in black and white subjects, in contrast to US civilian data; and declining discharge rates over time among soldiers less than 24 yr old. (Chest 1992; 101:613-20)

due to asthma, as well as the prevalence M oroftality asthma, have been reported to vary widely

patient administration systems and biostatistical activity (PASBA) of the US Army Medical Department for all active-duty soldiers under the age of 35 yr with a primary diagnosis of asthma, as classified by the ninth revision of the ICD (ICD-9, code 493) for the period from January 1984 through December 1988. The age restriction was imposed to avoid the diagnostic uncertainties attending this diagnosis in older groups. Since the bulk of soldiers are in younger groups, the effect of the age restriction was also numerically inconsequential. All discharges were assigned to the area of initial admission in the event of interhospital transfers. In the event that a soldier is admitted to a civilian hospital or dies without admission to a military hospital, the military community to which that soldier belongs notifies the local military hospital which collects summary information on the event and transmits this to PASBA. Denominators for troop strength by calendar quarter were obtained from the Defense Manpower Data Center, Monterey, Calif, by race, age, and sex groups for each geographic location.

throughout the world. 1.2 Whether this represents true differences and, if so, to what these differences are attributable are issues which are generally problematic due to the multifactorial nature of differences across national boundaries, including differences in ethnicity, environment, and diagnostic and therapeutic practices. The US Army supports large populations of soldiers and their family members in the continental United States (CONUS) and in Central Europe (Europe) and supports much smaller populations in several other regions. Demographically, these populations are quite similar; and, in addition, approximately one third of each population is reassigned to a different population each year, which assures constant and thorough mixing. Health care is provided in army hospitals and health clinics which are staffed largely by military physicians who experience a similar turnover rate. It is the purpose of this report to review observed morbidity and mortality in this demographically homogeneous population receiving identical levels of medical care in different geographic locations in order to determine the quantitative influence of the latter, both on seasonal differences and on secular trends. The contrast will be restricted to CONUS and Europe due to the large proportion of total army strength in these two locations. MATERIALS AND METHODS

Hospital discharge and mortality summary information were obtained from the individual patient data systems (IPDS) of the *From the Tenth Medical Laboratory, Landstuhl, Germany. tEpidemiology Consultant. Manuscript received April 22; revision accepted June 28. Reprint requests: Dr. Ward, 10th MedicallLlboratory, Box 14, APO New York, New York 89180

CONUS = continental United States (includes Alaska and Hawaii); GUM = generalized linear interactive modelling; ICD = International Classification of Diseases; NHDS = national hospital discharge survey; PASHA = patient administration systems and biostatistical activity; SNHANES second national health and nutrition examination survey

=

Statistical Methods

Fourfold tables were analyzed by X2 analysis and by Fisher's exact test when appropriate. The duration of hospitalization was compared by the Mann-Whitney U-test because of the inability to verify normal theory assumptions for these distributions. Poisson regression modeling' was carried out using the GLIM statistical package." Terms were kept in a model if hierarchically necessary or if their removal resulted in an increase in the scaled deviance such that the resulting p value was less than 0.05. Global comparisons of discharge rates and mortality were made by incidence density methods. 5.6 RESULTS

In order to examine seasonal differences, the data for hospital discharges were cross-tabulated into 64 cells (seasonal table) based on age, race, sex, calendar quarter, and location. The latter three were categorized in the obvious way, while the former two were categorized in an attempt to achieve balance and avoid sparse cells. Because the US Army is largely composed of whites and blacks, the racial categories are black and nonblack (ie, all other). The two age groups employed were ages less than or equal to 24 yr and CHEST I 101 I 3 I MARCH, 1992

613

Table I-Observed Discharge Frequencies by Calendar Quarter for the Entire Period, 1984-88* Europe

CONUS

>24 yr Quarter

Sex

Nonblack

F

14 (6,181) 91 (60,573) 15 (6,239) 86 (60,446) 25 (6,391) 94 (61,342) 18 (6,471) 64 (61,491)

M

F

2

M

3

F

4

M F M

>24yr

~24yr

Black 13 23 13 64 41 173 15 48

Nonhlack

(5,844) (32,810) (5,912) (32,384) (6,179) (33,028) (6,276) (32,795)

7 22 7 49 20 115 12 40

Black

(8,574) (89,310) (8,740) (89,298) (8,677) (88,850) (8,657) (89,392)

8 14 12 25 14 100 11 24

~24yr

Nonblack

(5,813) (31,310) (5,950) (31,080) (5,966) (30,606) (5,598) (30,683)

34 134 32 136 21 99 37 117

Black

(15,489) (140,336) (15,522) (140,549) (15,568) (140,354) (15,729) (140,784)

11 67 19 88 19 73 22 64

(10,889) (57,221) (11,064) (57,497) (11,235) (57,347) (11,501) (57,754)

Nonblack 31 114 22 126 20 148 32 141

(21,878) (205,485) (20,743) (198,332) (21,339) (205,837) (21,197) (201,901)

Black 11 45 6 68 15 82 16 88

(13,110) (64,518) (12,587) (62,299) (12,499) (63,741) (12,827) (62,337)

·Numbers within parentheses are population denominators in person-years.

ages greater than 24 yr. To examine differences in secular trend, a similar contingency table (secular table), with 80 cells, was constructed with a calendar year axis replacing that for quarter in the preceding table. It was not possible to model a single table incorporating both secular and seasonal effects simultaneously due to the extremely unbalanced nature of race, age, and sex groups in present army demography. The resulting contingency tables of discharge frequencies are depicted in Tables 1 and 2, with population denominators in parentheses. A Poisson regression model was fit to these tables, and the resulting estimates of discharge rates per 100,000 person-yr are seen in Tables 3 and 4. In order to facilitate the comparison of rates in the two locations for the seasonal table, Figures 1 and 2 depict the incidence density ratios (Europe/CONUS) for corresponding cells for each sex group. The final model consists of all firstorder and second-order terms, all third-order terms with the exception of age ° race ° sex and age-location-sex, and the following two 4-way interactions: age vruc e vq uar t e r-Loc at i o n and race-quarter-location-sex. The scaled deviance of the model, which has a )(2 distribution, is 12.81 on 12

degrees of freedom, p>Ool, indicating an acceptable fit. Findings from examination of the residuals were unremarkable. The pattern in these figures suggests that asthma discharge rates in Europe, compared to CONUS, are consistently higher in the older group, more so for female than male subjects. Rates in the first and fourth quarter are somewhat better for those in the younger group in Europe except for black female subjects, All age groups in Europe reveal an increasing risk ratio in the second quarter, reaching a peak in the third quarter, when rates are two to four times higher than in CONUS. The seasonal pattern is discernible in the fitted rates in Table 3 for Europe, but the pattern for CONUS is both less consistent and less pronounced. The distribution of the duration of hospitalization was distinctly nonnonnal, with marked right-skewness at both locationso The median duration was three days in both areas and was not significantly different by the Mann-Whitney rank sum test. Figures 3 through 5 depict the incidence of hospitalization by calendar year based on the model for the secular table. The final model consisted of all firstorder terms, all second-order terms except age-sex,

Table 2-0bserved Discharge Frequencies by Calendar fearCONUS

Europe s24 yr

>24yr Year 1984 1985

Sex

F M

F

M

1986

F

M

1987 1988

F

M

F

M

Nonblack 10 76 24 75 8 59 11 62 19 63

(4,523) (49,111) (4,805) (48,428) (5,126) (48,007) (5,404) (48,431) (5,423) (49,875)

Black

Nonblack

8 (3,775) 49 (27,021) 19 (4,526) 71 (26,814) 15 (5,211) 50 (25,980) 23 (5,404)

13 (6,463) 48 (71,683) 12 (6,802) 44 (70,809) 8 (7,327) 45 (72,320) 8 (7,244) 49 (73,136) 5 (6,811) 40 (68,901)

69 (25,466)

17 (5,296) 69 (25,744)

>24yr Black

14 39 8 33 12 30 8 31 3 30

(4,529) (27,474) (4,820) (24,855) (4,981) (23,683) (4,7596) (23,715) (4,597) (23,951)

Nonblack 30 (11,126) 110 (102,460) 26 (11,262) 121 (103,491) 24 (11,846) 84 (104,810) 21 (12,185) 91 (105,007) 23 (12,306) 80 (103,284)

~24yr

Black 8 47 12 58 14 49 15 65 22 73

(7,242) (41,884) (7,572) (42,454) (8,170) (42,884) (9,078) (43,788) (9,859) (43,260)

Nonblack 17 (17,841) 134 (162,058) 25 (17,007) 115 (162,342) 26 (16,144) 101 (159,168) 23 (15,619) 95 (151,649) 14 (14,463) 84 (140,534)

Black 14 62 17 57 4 50 8 60 5 54

(10,778) (54,913) (9,856) (51,179) (9,264) (48,072) (9,471) (45,898) (9,400) (43,288)

·Numbers within parentheses are population denomination in person-years.

614

Asthma Morbidity and Mortality in US Soldiers (David L Ward)

Table 3-E:tpected Disclwrge Rates per 100,000 Person-yr by Calendar Quarter Resultingfrom Seasonal RegrellSion Model Europe

CONUS

>24 yr Quarter

~24

yr

>24yr

~24 yr

Sex

Nonblack

Black

Nonblack

Black

Nonblack

Black

Nonhlack

Black

F M F M F M F M

257.80 147.04 241.72 142.16 399.84 152.32 307.56 101.00

208.04 72.72 294.08 184.08 641.76 527.88 279.24 138.68

59.08 26.80 79.16 54.96 224.08 130.04 116.64 46.88

151.76 42.08 127.96 94.56 257.24 322.32 142.24 86.44

209.28 96.60 200.68 97.36 137.00 70.32 229.44 83.76

105.36 116.24 139.04 159.32 173.24 126.48 160.68 116.92

148.92 54.72 110.16 63.12 92.20 72.04 155.28 69.40

80.28 70.48 76.40 103.36 116.32 129.36 152.16 135.52

2 3 4

Table 4-E:tpected Disclwrge Rates per 100,000 Person-yr by Calendar Year Resultingfrom Secular RegrellSion Motkl Europe

CONUS

>24 yr

>24 yr

~24yr

~24

yr

Year

Sex

Nonblack

Black

Nonblack

Black

Nonblack

Black

Nonblack

Black

1984

F M F M F M F M F M

321.09 154.12 347.56 166.82 243.88 117.07 255.04 122.41 254.29 122.05

227.55 187.96 318.47 263.05 252.14 208.28 334.35 276.21 349.66 288.86

151.24 72.59 140.64 67.50 130.31 62.54 130.45 62.61 114.45 54.93

176.31 145.62 169.05 139.66 148.54 122.70 169.49 140.00 149.96 123.89

225.17 108.07 243.71 116.97 171.02 82.09 178.85 85.84 178.31 85.58

125.75 103.88 175.99 145.39 139.35 115.11 184.80 152.65 193.26 159.64

157.06 75.38 146.05 70.10 135.32 64.95 135.46 65.02 118.85 57.04

144.29 119.19 138.35 114.30 121.56 100.43 138.72 114.59 122.75 101.40

1985 1986 1987 1988

lOR'S EUROPE / CONUS MALES ~

B,> 24

4 .5

Il

4

Q I< a:

3 .5

t:

3

w

2 .5

w

2

c U

1.5

>CIl Z

c

u zw

B,< =24

~

W,>24

III

W.<=24

~

0 .5

0

2

3

4

QUARTER

FIGURE 1. Expected incidence density ratios of hospital discharges by calendar quarter. Height of bar reflects rate in Europe relative to CONUS for male subjects. CHEST I 101 I 3 I MARCH, 1992

615

lOR'S EUROPE / CONUS FEMALES

FI(:tlRE 2. Expected incidence den sity rat ios of hosp ital discharges by calendar quarter. Height of bar reflect s rate in Europe relative to CONUS f(.r fem ale subjects.

sex ' ye a r, sex' location, and year' location, and one third-order interaction, age -race -year, Examination of the goodness-of-fit statistic, scaled deviance of 5i.12 on 55 degrees of freedom (p>O .Ol), and examination of the residuals indicated quite acceptable substanti-

ation of the model assumptions. Figure 3 depicts the combined incidence of hospitalization for CONUS vs Europe by caiendar year. There is consistent parallel movement in the absolute level of hospitalizations for the two sites, with a

TotAL EXPECTED: CONUS VS EUROPE 150 140 130 0 0

C!-

o

...0......

120

I-

110

W

c(

~

EUROPE

II1II

II:

CONUS

100 90 80

1987

1984

YEAR FI(:t 1R E

616

3. E xpect ed total hospit al discharge rate s by calendar year for Europe and CONUS . Aslhma Morbidity and Mortality in US Soldiers(David L. ~rd)

WHITE MALES CONUS AND EUROPE ~

EUR W,M,>24

180

lim

16 0 0 0

CON W,M, >24

14 0

~

q

...

1 20

I-

10 0

0 0

EUR W,M,<=24

III

<,

w

< a:

CON W,M,<=24

80 60 40

FIG URE

4. Expec ted hospital dischar ge rates for Europe and CO NUS for white male subjec ts.

fleeted in tho se white subjec ts olde r than 24 yr; but in both sexes, rate s for tho se less than thi s age show a consistent gradu al de cre ase ove r the study period; and, furthermore, note the similarity of rates between CONUS and Europe in the younge r category. Figure 5 for black male subjec ts and a similar one for black

relative ly constant 30 to 40 per cent higher incide nce in the latte r. Th e remaining figures dem onstrate that there is considerable heterogen eit y among sex, race , and age groups. Figure 4 for white male subjects and a similar one for white female subjects (not shown) reveal that the total rate observed previously is re-

BLACK MALES CONUS AND EUROPE ~

EUR B,M,

3 00

> 24

WJ

2 80

CON B,M , >24

260

~

0 0

240

0 0

...

220

EUR B,M,<=24

<,

2 00

oe(

180

III CON B,M,<=24

C!. W I-

a:

160 14 0 120 100

1984

1985

1986

1987

1988

YEAR FIG URE

5. Expected hospital d ischa rge rates for Eu rope and CO NUS for black male subjec ts. CHEST I 101 13 I MARCH. 1992

617

female subjects (not shown) reveal that black subjects are worse off in Europe for all ages, with no discernible trend in those less than 24 yr but with a very noticeable increasing trend in both sexes for those in the older group. During this same interval, there were ten deaths attributable to asthma. Six of these occurred in Europe, whereas four occurred in CONUS. Of these, two deaths in CONUS and one in Europe occurred in individuals greater than 34 yr of age and are not included in subsequent calculations. The respective denominators are 3,852,913 and 8,397,890 person-yr, giving incidence densities of 0.13 and 0.024 per 100,000 person-yr of observation. The exact binomial probability of observing five or more deaths, out of seven total, in Europe, based on the fraction of total population time, is 0.035. Applying the rates in CONUS to the population time in Europe indicates that about three of those deaths may be attributable to exposure to the European environment, or an etiologic fraction of 77 percent. Similarly; there were 1,277 total hospital discharges in Europe, compared to 1,938 in CONUS. The incidence densities are 33.14 and 23.08 per 100,000 person-yr, respectively; and using the normal approximation to the binomial, the difference is highly significant (p0.1). The time distribution was signficantly different (p<0.05, FET), with both deaths in CONUS occurring in the first half of the interval, while all deaths in Europe occurred in the last half: In fact, three of the five deaths in Europe occurred over two years in one of 11 hospital catchment areas. Population data for such areas are unavailable, but this suggests that clustering cannot be ruled out. Fifty percent of 818

the deaths in Europe occurred in the third quarter, and 83 percent occurred in the second and third quarters combined. The figures for CONUS are 25 and 50 percent, respectively The difference is not significant, but it is consistent with the seasonal pattern of hospital discharges observed previously. DISCUSSION

This investigation was prompted by the concern expressed in several military communities in Europe that the incidence of asthma and exacerbations of asthma was greater than one would expect based on CONUS experience. This concern was not focused exclusively on the active-duty subjects but included all family members; however, this investigation was restricted to soldiers because, regrettably, the Army does not maintain demographic information on soldiers' families, thereby precluding the determination of epidemiologic denominators for this group, which includes all of the children and most of the adolescents who are known to experience prevalence and incidence rates for asthma that are as high, or perhaps even a bit higher, than in the age group examined here. The pattern exhibited by the comparative hospital discharge rates is complex and, indeed, somewhat surprising in that older groups are the most affected by living in Europe. Age is confounded with the duration of time in the country, and so this may simply reflect the cumulative effect of exposure. Data bearing on the length of time served in the present assignment are not available from hospital discharge summaries used in this analysis. One potential bias is that soldiers with severe asthma are less likely to be assigned overseas and may be retired from active duty for medical reasons. The net effect of the former should be to increase the frequency of hospitalizations and deaths in CONUS vis-a-vis Europe, so that, if anything, the relative incidence in Europe has been underestimated. This may also explain the relatively lower discharge rates in younger soldiers in Europe, except during the summer peak, since the population of soldiers assigned overseas would be expected to be healthier than those not so assigned. Hospitalizations exhibit a sharp peak during the summer months in Europe, with rates doubling or even tripling in some groups. The effect is exaggerated somewhat in the incidence density ratios because rates in CONUS remain relatively constant or even decline a bit in the summer. The mortality pattern is also consistent with increased risk during this quarter. This suggests some seasonal change in environmental conditions such as air quality, (eg, air pollutants or aeroallergens) as likely candidates to explain this phenomenon, although it has not been possible to explore this up to the present time. This stands in Asthma MorbidItyand MortaIty in us SoldIers (Devid L KtW)

contrast to data for Denmark as reported by Pedersen and Weeke,7 who failed to detect any appreciable seasonal variation in the percentage of consultations for asthma in all age groups, either with or without symptoms, in general practice. Additionally, they could not demonstrate any correlation between aeroallergen or air pollutant concentrations and the frequency of consultations for asthma, although a large effect was demonstrated for allergic rhinitis. This discrepancy could reflect environmental differences in Denmark, which is located north of Germany, whereas most American soldiers live in southern Germany; or it may indicate that extrinsic factors play a larger role in this population than in the Danish population. Woolcock) reported comparative mortality for the United States and West Germany as of 1980. The allages mortality in the former was 0.2/100,000 population and 0.41100,000 in the latter. The US age-specific mortality for those between the ages of 20 and 40 yr was similar to the all-ages mortality. The US figure is approximately ten times higher than the results reported herein; whereas for soldiers serving in Europe, mortality is only three times less than that reported for local nationals. A more recent estimate of US mortality is given in a report by Evans et al," in which the age-specific mortality for ages 15 to 34 yr is 0.4/ 100,000 for calendar year 1982, a rate more than 16 times higher than the value reported herein. The lower rates observed in this study most likely represent the "healthy-worker" effect, in that persons with severe asthma are less likely to seek or qualify for military service. The absence of children in the current data set should have the effect of increasing estimated mortality, if anything, since children exhibit uniformly lower rates than adults. H The race-specific and sex-specific rates for the same year reveal that mortality is much higher in black subjects (I.l to 1.5) and of a similar magnitude for male and female subjects, with no apparent difference in rates for those above and below 24 yr of age. This contrasts with the findings reported here, in which mortality is lower in black subjects, in those under 25 yr, and in male subjects, although the latter is less pronounced in Europe than in CONUS. The reasons for these discrepancies are far from clear but may reflect the fact that soldiers are not simply a random sample of the general US population. There is a further paradox in that male subjects, exhibiting lower overall mortality, display the greatest increase in Europe compared to CONUS. Although the denominators are large, total deaths are relatively few, suggesting that the estimates reported herein may be somewhat unstable, precluding definitive analysis at the present time. In both locations, discharge rates are highest for female soldiers, with white female subjects having

consistently higher rates than black female subjects except during the summer months. On the other hand, black male subjects have higher rates than white male subjects except during the first calendar quarter, although this effect is more pronounced in Europe than in CONUS. Comparative data for the general US population are reported by Evans et al" for calendar year 1983 based on the National Hospital Discharge Survey (NHDS). In these data, discharge rates were highest for black subjects in all age and sex groups and for female subjects of all races for those older than 15 yr. Sherman et a19 reported that female subjects were nearly three times as likely to experience hospitalization as male subjects in a follow-up study of children and adolescents, a finding which may reflect poorer pulmonary function in female asthmatic subjects. In all groups, discharge rates among soldiers are lower than comparable civilian figures, in some cases up to a factor of ten. Nevertheless, there is approximate correspondence of risk groups between those identified in the civilian data cited and those reported herein. Differences in respiratory pathogens between the two locations or differences in susceptibility to infection among the defined groups may influence the results observed during winter months. The lack of discrimination of seasonal differences in the NHDS data may also account for some of the observed differences with rates observed among soldiers. Data from the seconed national health and nutrition examination survey" (SNHANES) indicate that in the age range examined herein, the prevalence of asthma is greatest in male subjects and black subjects, although the NHDS data reflect the fact that severity, as measured by hospital discharge rates, does not always follow prevalence. The data on soldiers presented herein suggest that seasonal effects are important to consider when comparing demographic groups because average annual relationships may be less representative of disease activity than season-specific rates, due to the marked interseasonal variation in these rates. This is especially true when extrinsic factors are important determinants of the frequency and severity of exacerbations of asthma. The parallel movement of discharge rates apparent in Figure 3 would seem to indicate that in addition to

individual and microenvironmental determinants of

acute, severe asthmatic exacerbations, there are also global ones, or at least intercontinental ones, as well. The 25 percent decline in discharge rates observed among young white subjects could represent a cohort effect, in that these individuals are all recent accessions compared to the older groups, many of whom were recruited in the late 1970s and early 198Os, when the army experienced severe recruiting difficulties and, consequently, had lower entry standards than CHEST I 101 I 3 I MARCH, 1992

619

those established more recently. Overall poorer health may be a concomitant feature of lower standards, which would be reflected in hospital discharge rates for older soldiers. There is some visible decline in rates in young black subjects as well, but it is smaller in magnitude and less clearly consistent. All demographic groups at both locations exhibit some improvement in the last half of the study period, compared to the first half, except for older black subjects, who are worse off in later years, particularly in Europe. This is not inconsistent with data from the NUDS reported by Evans et al" for the immediately preceding five years. In these data, discharge rates for nonwhite subjects increased by 38 percent from 1979 to 1983, while those for white subjects increased by 25 percent. In fact, the rates observed herein for black subjects in CONUS in 1988 are similar in absolute magnitude to those reported for 1983 in NUDS data cited previously. On the other hand, rates for white subjects are better than the national average for those in CONUS and worse, to nearly the same extent, in Europe. In conclusion, it is clear that asthma-related morbidity, as reflected by the need for hospitalization, and mortality are elevated among those US soldiers living in Europe, compared to those living in CONUS. These data also strongly suggest that there is an environmental difference, with a pronounced seasonal pattern between the two locations which accounts for this, and it therefore seems reasonable to suspect that assigning asthmatic soldiers, and presumably their family members as well, to Europe will exacerbate their disease and increase their risk of death, possibly directly related to seasonal factors. Two possible public

820

health approaches to this dilemma would be to further characterize the environmental agent(s) responsible or to alter assignment policies to preclude sending families affected by asthma to Europe. The former approach mayor may not lead to successful interdiction of the burden of disease, whereas the latter may adversely affect soldiers' careers. The decision is, of course, a value-laden one which ultimately must reach a compromise between the health of individuals and the need to have large numbers of soldiers deployed abroad as part of national policy. REFERENCES 1 Woolcock AJ. Worldwide differences in asthma prevalence and mortality. Chest (suppl), 1986; 9O:40-5S 2 Cookson J8. Prevalence rates of asthma in developing countries and their comparison with those in Europe and North America. Chest (suppl) 1987; 91:97-103S 3 Aitken M, Anderson D, Francis B, Hinde J. Statistical modelling in GLIM. Oxford, England: Oxford Science Publications, 1989:217-50. 4 Baker RJ, NeIder JA. The GLIM system: release 3. Oxford, England: Numerical Algorithms Group, 1978 5 Kleinbaum DG, Kupper LL, Morgenstern H. Epidemiologic research: principles and quantitative methods. Belmont, Calif: Lifetime Learning Publications, 1982:284-88 6 Miettinen OS. Theoretical epidemiology: principles of occurrence research in medicine. New York: John Wiley and Sons, 1985:137-52 7 Pedersen PA, Weeke EW Epidemiology of asthma in Denmark. Chest (suppl) 1987; 91:107-14S 8 Evans R, Mullally DI, Wilson ~ et ale National trends in the morbidity and mortality of asthma in the US. Chest (suppl) 1987; 91:65-74S 9 Sherman CB, Tosteson TO, Tager IB, Speizer FE, Weiss ST. Early childhood predictors of asthma. Am J Epidemiol 1990; 132:83-95

Asthma Morbidityand Mortally in US Soldiers (Dtwid L KiIrd)