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Occupational Medicine Advance Access originally published online on August 25, 2008
Occupational Medicine 2008 58(7):480-484; doi:10.1093/occmed/kqn096
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© The Author 2008. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Does exercise cause asthma?

Ido Katz1,2, Shlomo Moshe2,3, Michael Levin4, Dan Slodownik5 and Yaron Yagev6

1 The Israel Lung Association, University of Tel Aviv, Tel Aviv, Israel
2 The Sackler School of Medicine, University of Tel Aviv, Tel Aviv, Israel
3 Occupational Medicine Department, Maccabi HealthCare Services, Holon, Israel
4 Occupational Medicine Department, Maccabi HealthCare Services, Netanya, Israel
5 The Department of Dermatology, Hadassah Medical Center, Jerusalem, Israel
6 Occupational Medicine Department, Maccabi HealthCare Services, Beer-Sheva, Israel

Correspondence to: Shlomo Moshe, Head of Occupational Unit, 43 Geulim St, Holon, Israel. Tel: +972 3 6530054/56/15; fax: 972 3 6521031; e-mail: moshe_sh{at}mac.org.il


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Background The Israel Defence Forces needed to define the correlation between physical activity and asthma.

Aim To determine whether combat unit (CU) soldiers are more susceptible to exercise-induced asthma (EIA) than other military units.

Methods A follow-up study of recruits with normal clinical and pulmonary function tests over a period of 30 months after having been assigned to CU, maintenance units (MU) or clerical tasks (CT). The participants chosen had already been subjected to additional tests 6 weeks after induction to eliminate any cases of active asthma.

Results Out of 799 subjects, 125 developed asthma during the follow-up. Twenty-one per cent of those in the CU developed asthma against 15% in the MU and 5% in the CT. The relative risks for newly diagnosed asthma were 3.7 for CU/CT (P < 0.001), 2.7 for MU/CT (P < 0.001) and 1.4 for CU/MU (P < 0.05). EIA was observed as the only manifestation of asthma in 32% of the soldiers posted in CU compared to 13 and 11% in MU and CT, respectively.

Conclusion The increased risk of EIA in CU compared to MU and CT may indicate that any one or all the factors associated with CU service conditions could contribute to this increased risk of uncovering the mild cases of asthma, especially EIA, that had been overlooked up to the time of induction into the army.

Keywords      Exercise-induced asthma; military service; military units; pathogenesis


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Asthma is one of the most common chronic diseases of childhood and adolescence [1–4]. Exercise is one of the most common precipitants of acute asthma. Exercise-induced asthma (EIA) is a condition in which vigorous physical activity triggers acute airway narrowing in people with heightened airway reactivity [5]. The prevalence of exercise-induced symptoms in patients with asthma has been reported to range from 40 to 90% [5]. The reasons for this wide range are related to exogenous factors, such as differences in the intensity of the exercise, lack of uniformity in the methods used to detect the response and failure to standardize the environmental variables which influence the magnitude of the obstruction [6,7]. EIA is seen most frequently in children and young adults because of their high levels of physical activity; however, the condition can occur at any age [5].

It is generally accepted that exercise can provoke an acute asthmatic attack in asthmatic individuals. However, it is not considered that exercise per se will cause normal individuals to develop asthmatic symptoms. Considerable attention has focused on asthma and athletics. The frequency with which asthma had previously been reported in various groups of athletes has been reported to range from 6 to 14% [8–11]. However, no control populations were included in these studies and it is not known whether the prevalence of asthma in athletes is the same as in the general population as athletes may deliberately not report complaints. There is a discrepancy between the frequency of reported respiratory disturbances among highly trained athletes without a known history of asthma compared to those who had previous complaints which ranges from 7 to 24% [8–12]. Overall interpretation of the evidence suggests that there is little difference in the frequencies either of asthma or of exercise-induced bronchoconstriction between athletes and non-athletes [12–14].

In our previous study [15], we evaluated various problems of asthma in a population of young adults, including the impact of different occupations, using a computerized database. In the current study, we concentrated on a specific population of new recruits aged 18–20 years old who complained of dyspnoea at the time of their induction examination but had no clinical or laboratory evidence of obstructive airways disease prior to beginning their service. We retrospectively examined their clinical records in order to establish a connection between their occupation and the clinical manifestation of asthma that they had developed.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflicts of interest
 References
 
All 17-year-old Israeli nationals are obliged by law to appear at the Israel Defence Forces (IDF) recruiting office for medical examination prior to their compulsory military service at age 18. In the current study, we included all the recruits to the IDF between 1 January 1992 and 31 December 1994 (>95% of the Jewish male population aged 18 years). We made use of the IDF’s database which included follow-ups on all the soldiers 6 weeks after their induction into the IDF (the period of basic training) in order to eliminate cases of asthma and/or bronchial hyperactivity overlooked at the time of the initial examination.

There was no intention at the time of data collection for further applications of this information. As researchers employed outside the defence establishment, we received the approval for using these data from the chief medical officer of the IDF. These steps should ensure the anonymity and confidentiality of the study.

Subsequently, the subjects posted in combat units (CU), maintenance units (MU) and clerical tasks (CT) were followed up for 30 months noting newly diagnosed asthma or recrudescence or worsening of known asthma that required a change in asthma category and often a change in duty assignment.

Medical history was obtained from the family physicians for most conscripts. During the examination, the subjects were asked specifically whether they had ever been diagnosed as having asthma and whether they had ever suffered from recurrent wheezing, nocturnal cough, cough or wheeze after exertion. Conscripts (or their family physician) who gave a positive reply to any of these questions were referred for a second examination by a trained respiratory physician. Subjects were instructed not to take any medication for asthma on the day of examination. The second examination included further detailed history, physical examination and spirometry at rest. All subjects except those with overt clinical signs and spirometric evidence of severe airway obstruction also underwent an exercise test. The test comprised 6 min of treadmill running at a speed of 5 km/h arriving at an incline of 10° while breathing room air (22°C, 50% relative humidity). Lung function was measured 5 and 10 min after the exercise to determine the percentage of fall in forced expiratory volume in 1 s (FEV1).

As a result of these examinations, the subjects were divided into the following categories: Category O is subjects who had no evidence of past and/or present asthma. This category was created for this study only and is not used in routine practice in the IDF. Category A is subjects who had asthma in total clinical remission for at least 3 years with normal spirometry (FEV1 > 70% of predicted) and a normal response to exercise (decrease in FEV1 after exercise of <10%). Category B is subjects who had rare and/or mild attacks of dyspnoea with normal spirometry and normal response to exercise. Category C is subjects with mild asthma who did not require regular daily medications and either mild impairment in spirometry or evidence of moderate bronchial hyperactivity. Category D is subjects with active asthma who required regular daily treatment or with moderate impairment in spirometry or evidence of severe bronchial hyperactivity.

The recruits in asthma Categories O, A and B were posted to all types of duty. Those in asthma Category C were not posted in CU and those in Category D were not posted in either CU or MU. They were either assigned to CT or totally dismissed from military service. The major differences in stress imposed by the three categories of duty are indicated by the following descriptions: CU is characterized by very intensive physical activities, living in field conditions; MU is characterized by moderate physical activities and exposure to non-specific environmental hazards (mostly gases and fumes, grease, etc.) and CT is characterized by sedentary office work.

All the recruits in Category B were followed for 6 weeks after their induction into the IDF and their status verified at that time to eliminate cases of asthma and/or bronchial hyperactivity overlooked at the time of the initial examination. The group remaining in Category B was then followed for 30 months. Their subsequent assignment to duty categories was based on a general evaluation of their skills and not on a physical basis alone. If any symptoms suggestive of asthma developed during the different periods of follow-up, the subjects were re-evaluated by a trained respiratory physician who performed clinical examination and spirometry. If indicated, exercise challenge and, in some cases, methacholine challenge were carried out in order to confirm or rule out the presence of bronchial hyperreactivity or asthma.

In this historical prospective study, we noted the occurrence of newly diagnosed asthma that caused a change in category (see above) or that led to a change in jobs. In addition, the records of each case were examined and the character of the newly diagnosed asthma was determined: onset of clinical asthma (with functional evidence) that required regular treatment and onset of (EIA) or evidence of a positive methacholine challenge test (MCT) only. The records of subjects diagnosed as having asthmatic symptoms during the first 6 weeks of their service (see above) served as a comparison group. None of them were assigned to CU.

All data were recorded in real time on a computer and analysed later. Statistical analysis was carried out on a software package for PC (SAS V.6.03). Percentages were compared by means of {chi}2 test and proportion test. P values of <0.05 in two-tailed tests were considered to be significant.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflicts of interest
 References
 
During the period of this study (1 January 1992–31 December 1994), 799 new recruits to the IDF were identified to fit the definition of Category B after their first 6 weeks of observation: 370 were assigned to CU, 236 to MU and 193 to CT. Their status after 30 months follow-up is shown in Table 1.


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  		Table 1. Change in symptom status and RR for developing asthma

 
The pulmonary status of 125 subjects (16% of the total) worsened during the observation period and they were found to have newly clinical and/or pulmonary function test evidence of asthma. The fraction of those assigned to CU that developed asthma was 21% as compared to 15% for MU and 5% for CT (Table 1). Analysis of these different prevalences revealed a significantly greater relative risk (RR) of developing asthma in CU as compared with CT (RR = 3.7, P < 0.001) and MU (RR = 1.4, P < 0.05). The RR of serving in MU was significantly greater as compared with CT (RR = 2.7, P < 0.001).

The clinical records of 116 of the 125 subjects with deterioration of their status were analysed for details of the severity of the asthma (Table 2). Three general categories of asthma were determined as follows: overt clinical asthma requiring continuous treatment, EIA only and airway hyperreactivity as identified by a MCT. EIA was observed as the only manifestation of asthma in 32% of the soldiers serving in CU compared to 13 and 11% in maintenance and CT, respectively. The RR for developing EIA in the CU compared to MU and CT were 2.6 and 2.9, respectively (P < 0.05).


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  		Table 2. Causes of change to a lower functional category in soldiers with newly diagnosed asthma

 
There were 58 subjects rejected from the above study because they developed overt asthma during the 6-week period after their induction into the IDF (findings shown in Table 3).


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  		Table 3. Type of asthma developed during the first 6 weeks of service related to type of duty

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflicts of interest
 References
 
EIA is one manifestation of asthma and may sometimes be the only expression of the disease. In the current study, asthma was seen more frequently among those subjects serving in CU (RR = 3.7) than in either MU or CT (RR = 2.7 and RR = 1.4, respectively). The rate of EIA difference was statistically significant only between CU and MU (32/13%, P < 0.05). When we evaluated (Table 3) the rate of EIA in new cases of asthma in MU and CT during the first 6 weeks of military training, we found no difference in the rates of EIA in both groups (41 versus 42%), suggesting that there was no selection bias when original duty assignments were made. In our study, the rate of new onset asthma in the MU was higher than the rates reported in civilian athletic groups in different studies (6–14%) [8–12]. A higher rate of EIA was found only in cross-country skiers [12,14], which might be explained by the strenuous training at low temperature and inhalation of large amounts of cold air, conditions which do not exist in the Middle East. The differences in the rates of asthma between our study group and groups of athletes can be explained by several other factors. The athletes are a highly motivated population that sees sports achievements as a major target. They probably under-report their diseases. The athletes who quit competitive sports activity are not found in cross-sectional studies and therefore those studies are biased. This bias is known as the ‘healthy worker effect’. In our study, we reduced the bias by using the complete database of this study group. The advantage of this study is the follow-up of three groups with different activities but with similar preliminary medical status. In other studies [8,9,10,12,14], no matched control population was included. We did not find any prospective studies following similar population in the military service. We found only a few cross-sectional, retrospective studies based on different methods evaluating soldiers discharged from the army [16] or tested for underdiagnosed asthma [17,18] without any follow-up.

It is generally accepted that the pathogenesis of EIA is closely associated with the fluxes of heat and water that develop within the tracheobronchial tree during the conditioning (warming and humidification) of large volumes of air [5]. The physical activity of MU soldiers is similar to that of a long distance runner. Helenius et al. [19] have found that the prevalence of asthma is high in elite athletes who are associated with long distance runs. This may be due to prolonged hyperventilation and increased exposure to inhalant allergens and irritants during endurance training and competition, and this pathogenesis is typical to the known trigger of EIA.

The military lifestyle in particular may be especially unforgiving of a disease that is both so unpredictable and so potentially life threatening as asthma. The major differences among the three categories were discussed in the methods: CU is characterized by very intensive physical activities, living in field conditions; MU is characterized by moderate physical activities and exposure to non-specific environmental hazards (mostly gases and fumes, grease, etc.) and CT is characterized by sedentary office work. However, we do not have data about environmental allergens such as pollen which are unavoidable in many military jobs. Since we had no smoking records of the soldiers, we could not ascertain whether this particular item, as well as the preceding ones, biased our findings or not.

Our study seemed to demonstrate that the CU setting is where more asthma, especially EIA, develops and the clerical worker is at least risk among the three groups screened. The increased risk of EIA in CU compared to MU and CT may indicate that any one or all the factors associated with CU service conditions could contribute to this increased risk of uncovering the mild cases of asthma, especially EIA, that had been overlooked up to the time of induction into the army. Further research is warranted to verify whether other factors mentioned, but not considered, might be significant.


Key points
  • The prevalence of exercise-induced symptoms in patients with asthma has been reported to range from 40 to 90%.
  • The reasons for this wide range are related to exogenous factors, such as differences in the intensity of the exercise, lack of uniformity in the methods used to detect the response and failure to standardize the environmental variables which influence the magnitude of the obstruction.
  • Our study demonstrated that the CU setting is where more asthma, especially EIA, develops and the clerical worker is at least risk among the three groups screened.

 


   Conflicts of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
References
 
None declared.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 

  1. Anderson HR. Is the prevalence of asthma changing? Arch Dis Child (1989) 64:172–175.[Free Full Text]

  2. Burney PGJ, Chinn SR, Rona J. Has the prevalence of asthma increased? Evidence from a national study of health and growth in 1973–1986. Br Med J (1990) 300:1306–1310.[Abstract/Free Full Text]

  3. Burr ML, Butland BK, King S, et al. Changes in asthma prevalence: two surveys 15 years apart. Arch Dis Child (1989) 64:1452–1456.[Abstract/Free Full Text]

  4. Gregen PG, Mullaly DI, Evance R. National survey of prevalence of asthma among children in the United States: 1976–1980. Pediatrics (1988) 81:1–7.[Abstract/Free Full Text]

  5. McFadden ER, Gilbert IA. Exercise-induced asthma. N Engl J Med (1994) 330:1362–1367.[Free Full Text]

  6. Jones RS, Buston MH, Wharton MJ. The effect of exercise on ventilatory function in the child with asthma. Br J Dis Chest (1962) 56:78–86.[CrossRef][Web of Science][Medline]

  7. Poppius H, Muittari A, Kreus K-E, Korhonen O, Viljanen A. Exercise asthma and disodium cromoglycate. BMJ (1970) 4:337–339.[Abstract/Free Full Text]

  8. Fitch KD. Management of allergic Olympic athletes. J Allergy Clin Immunol (1984) 73:722–727.[CrossRef][Web of Science][Medline]

  9. Rupp NT, Guill MF, Brundo DS. Unrecognized exercise-induced bronchospasm in adolescent athletes. Am J Dis Child (1992) 146:941–944.[Abstract/Free Full Text]

  10. Voy RO. The U.S. Olympic committee experience with exercise-induced bronchospasm, 1984. Med Sci Sports Exerc (1986) 18:328–330.[CrossRef][Web of Science][Medline]

  11. Weiler JM, Metzger WJ, Donnelly AL, et al. Prevalence of bronchial hyperresponsiveness in highly trained athletes. Chest (1986) 90:23–28.[CrossRef][Web of Science][Medline]

  12. Larsson L, Hemmingsson P, Boethius G. Self-reported obstructive airway symptoms are common in young cross-country skiers. Scand J Med Sci Sports (1994) 4:124–127.

  13. Heir T, Oseid S. Self-reported asthma and exercise-induced asthma symptoms in high-level competitive cross-country skiers. Scand J Med Sci Sports (1994) 4:128–133.

  14. Larsson K, Ohlsen P, Larsson L, et al. High prevalence of asthma in cross-country skiers. Br Med J (1993) 307:1326–1329.[Abstract/Free Full Text]

  15. Katz I, Moshe S, Sosna J, et al. The occurrence, recrudescence, and worsening of asthma in a population of young adults and the impact of varying types of occupations. Chest (1999) 116:614–618.[CrossRef][Web of Science][Medline]

  16. Dickinson JG. Asthma in the army: a retrospective study and review of the natural history of asthma and its implications for recruitment. J R Army Med Corps (1988) 134:65–73.[Medline]

  17. Morris MJ, Grbach VX, Deal LE, et al. Evaluation of exertional dyspnoea in the active duty patient: the diagnostic approach and the utility of clinical testing. Mil Med (2002) 167:281–288.[Web of Science][Medline]

  18. Nish WA, Schwietz LA. Underdiagnosis of asthma in young adults presenting for USAF basic training. Ann Allergy (1992) 69:239–242.[Web of Science][Medline]

  19. Helenius IJ, Tikkanen HO, Haahtela T. Association between type of training and risk of asthma in elite athletes. Thorax (1997) 52:157–160.[Abstract]


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This Article
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