Working Hours and Ischaemic Heart Disease in Danish Men: A 4-Year Cohort Study of Hospitalization

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1 International Journal of Epidemiology International Epidemiologies! Association 1993 Vol. 22, 2 Printed in Great Britain Working Hours and Ischaemic Heart Disease in Danish Men: A 4-Year Cohort Study of Hospitalization FINN TOCHSEN Tuchsen F (National Institute of Occupational Health, Department of Occupational Medicine, Lersf* Parkalle 105, DK-2100 Copenhagen 0, Denmark). Working hours and ischaemic heart disease in Danish men: a 4-year cohort study of hospitalization. International Journal of Epidemiology 1993; 22: Four groups of men with non-daytime work were identified in two surveys. The relative risk of being admitted to hospital due to ischaemic heart disease (IHD) (ICD-8, ) was measured in a 4-year cohort study of all economically active men in Denmark, aged years. Compared with occupational groups having day-work only, men in occupations with frequent night and early morning work had an excess standardized hospitalization ratio () of 193, occupational groups with late evening work had an excess of 216, groups working in rosters covering 24-hour services had an excess of 174, and groups having other irregular working hours had an excess of 172. We conclude that night work rather than shifts is responsible for a raised risk of IHD. More research is needed to develop preventive strategies. Some older studies have failed to find associations between shift work and increased risk of ischaemic heart disease (IHD). 1 " 3 Since 1978, however, a number of studies have found such an association.*" 9 The most recent and best designed study has demonstrated an exposure-response relation between years of shift work and risk of coronary heart diseases, 10 " 12 but little is known about the relative risk of the many forms of abnormal working hours used in modern industries, public and private service trades. Recent analyses of Swedish data show that various types of night work rather than shifts are responsible for excess risks of acute myocardial infarctions. 13 The purpose of the present study is to measure the risk of IHD in male workers working abnormal hours and to identify risks associated with i) night/early morning work; ii) late evening work; iii) rosters covering 24-hour services; iv) other irregular working hours. METHODS All men aged on 1 January 1981, identified in the Central Population Register of Denmark, have been followed and all hospitalizations over the 4 years to 31 December 1984 have been recorded. This has been achieved by the construction of The Occupational Hospitalization Register that National Institute of Occupational Health, LCTM* Parkalle 105, DK-2100 Copenhagen 0, Denmark. links data from the population census of Denmark to the National Inpatient Register. Coding of occupations was by means of The Employment Classification Module at Denmark's Central Bureau of Statistics. Data on working hours were linked on an aggregated level partly from the 1976 round of the Survey of Living Conditions in Denmark (SLC) 14 and partly from a survey on bakers' occupational health carried out in The Employment Classification Module Using different administrative registers, all men in Denmark aged 16+ years are classified annually according to occupation, in the Employment Classification Module (ECM). The ECM contains information on economic activity, occupational classification and industry. The occupational code is an extended classification of the International Standard Classification of Occupations (ISCO) 1958 version, and includes the self-employed. The industrial code is a national extension of the International Standard Classification of all Economic Activity (ISIQ 1968 version. The data on occupation and industry have been used to construct groupings that could be identified in the Survey of Living Conditions in Denmark in This survey included 5166 respondents and had a response rate of 86.4%. Via the CPR register the nonrespondents have been compared with respondents for age, county of residence, and marital status. There 215

2 216 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY were more non-respondents among residents of the capital city and among divorced people, but the differences are too small to influence the selection of occupational groups to be included in this study except for the minimum number of respondents. This means that more groups could have been included if more people had responded, but no systematic error could occur. All occupational groups in the SLC with at least 20% (minimum five men) giving positive answers to a question on non-day work were included if they could be identified in The Occupational Hospitalization File. The two groups working predominantly at night and in the early mornings have been identified by means of a survey carried out in 1979 including 1782 respondents, all bakers. 12 The response rate was 64.1%. Of these respondents 45 % started work between 9 pm and 4 am while 43% started work before 6 am. Even if all nonrespondents had day work only this group would have satisfied the inclusion criteria. The National Inpatient Register The National Inpatient Register which holds data on all admissions to Danish hospitals is updated each year. Hospitalizations for all men except foreigners treated in Danish hospitals between 1981 and 1984 are included in this study. The Personal Identification Number (PIN) from the National Inpatient Register is used in the cross-linking procedure, but excluded in the final file. This study uses first admissions during with a discharge diagnosis of ICD-8 codes i.e. acute myocardial infarction, other acute and subacute forms of IHD, old myocardial infarction, angina pectoris, and other forms of chronic IHD. The use of the PIN has enabled exclusion of re-admissions in the follow-up period. The Central Population Register From the Central Population Register, we have included information on PIN, from which we extracted the information about gender, date and year of birth. The Central Population Register was used to identify men, actually living in Denmark on 1 January We have restricted the analysis to age groups with a high percentage of economically active men (20-59 years). Validity of the Data Age and gender are part of the PIN and are carefully checked by the authorities. The completeness and accuracy of these data are shown by the fact that matching from different registers on PIN was 100% complete. Data on occupation are not routinely checked by the primary data producers but information from various sources is compared and pooled in the ECM. From 1972 to 1981 everyone who was admitted to hospitals in the two largest cities in Denmark with the diagnosis acute myocardial infarction was registered in the Heart Register. The most relevant validation of the National Inpatient Register, for this study, compared registrations of admissions with acute myocardial infarctions in the National Inpatient Register with similar registrations in the Heart Register during the period , and a sample of the admissions was re-assessed with regard to the diagnosis. 16 Almost everyone registered in the Heart Register could be found again in the National Inpatient Register with the same diagnosis. Re-assessment of the diagnosis revealed that approximately 90% of the admissions with acute myocardial infarction could be verified, without modification, by reviewing the text of summaries. ANALYSIS Standardized hospitalization ratios () were calculated by dividing the observed number of hospitalizations in a given subcohort defined by occupation and/or industry code by an expected number. The expected number was based on incidence ratios for men economically active on 1 January 1981 and on age distribution in the cohort. Men are no longer at risk of being admitted to a hospital in Denmark, from date of first emigration ( men) or from date of death (24075 men). From the date of such an event these men are excluded from the denominator in the risk calculations. The period of risk is calculated for each person. This information is collected from 4 years of the migration files and the death certificate files in the Central Population Register. A referral bias may relate to hospital workers. The increased hospitalization ratio of semi-skilled, and unskilled groups working in hospitals may relate to ready access to treatment from that source. For most of the occupational groups in this study there is good agreement between the risk estimates found here and the risk estimates in The Danish Occupational Mortality Study Examples are self-employed bakers with an SMR of 133 with 142 cases of IHD (90% confidence interval [CI]: ), skilled bakers with an SMR of 120 with 122 cases (: ), and taxi operators with an SMR of 171 with 225 cases ( : O.8). 16 In contrast to this the 1861 male nurses and therapists had an excess of 131 with 11 cases ( : ) but their SMR in

3 WORKING HOURS AND HEART DISEASE IN DENMARK was only 77 ( : ). 17 Likewise the assisting male nurses and porters had an of 135 and 112 observed cases ( : ), but their standardized mortality ratio was only 89 for 61 deaths ( : ). These two occupational groups have therefore been excluded from the present analysis. Ninety per cent confidence limits (one-tailed) are calculated assuming a binomial distribution estimated by a normal distribution as recommended by Rothman." Logarithmic scale transformation was used to introduce symmetry in the intervals. RESULTS Table 1 shows raised risks, compared with all economically active men, for the two occupational groups with working hours at night and in the early mornings. In Table 2 we have listed occupational groups in which at least 20% have late evening work. A significant excess risk was found for all the groups of occupations working in late evenings. For the group of occupations in which at least 20% indicate that they have rosters covering 24-hour services, a significant excess risk was found among fishermen and for the group as a whole, compared with all economically active men (Table 3). The occupational groups indicating that they have other forms of irregular working hours have a statistically significant excess risk; none of the groups have risks below the average of all economically active men (Table 4). In Table 5 we have listed all the occupations in which all respondents answer that they have day work only. With a few exceptions they had decreased risks compared with all economically active men. None of them had statistically significant excess risks. When the day-working groups were used as the standard instead of all economically active men we found statistically excess risks in all the four groups with non-daytime work: firstly those groups doing night/early morning work, secondly groups working late evenings, thirdly occupational groups where at least 20% work in rosters covering 24-hour services, and fourthly occupational groups where at least 20% have other irregular working hours. For all four groups CI are narrow (Table 6). DISCUSSION A register-based study such as this one has advantages and disadvantages. It has the advantage of covering the whole population in a comparable way; on the other hand exposure information is usually insufficient. TABLE 1 Standardized hospitalization ratios () and 90% confidence intervals (CI) for ischaemic heart disease (ICD-8, ) in male occupational groups who work predominantly at night and in the eariy morning Self-employed bakers Skilled bakers ( ) ( ) ( ) TABLE 2 Standardized hospitalization ratios () and 90% confidence intervals (CI) for ischaemic heart disease (ICD-8, ) for male occupational groups in which at least 20% work late in the evening 90* CI Taxi operators and transport activities not coded elsewhere ( ) Self-employed in hotels, restaurants and cafes ( ) Cooks and waiters ( ) ( )

4 218 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY TABLE 3 Standardized hospitalization ratios () and 90% confidence Intervals (CI) for ischaemic heart disease (ICDS, ) for male occupational groups in which at least 20% cover 24-hour services Fishermen ( ) Senior traffic staff ( ) 43040, Shipping and railway staff ( ) Bus, coach and road transport staff ( ) Rescue services, customs and excise, police ( ) ( ) TABLE 4 Standardized hospitalization ratios () and 90% confidence intervals (CI) for ischaemic heart disease (ICDS, ) for male occupational groups m which at least 20% have other forms of irregular working hours Salaried data processing staff Doormen, ticket inspectors, marshals, security staff Drivers Production workers Waged employees (not coded elsewhere) The Occupational Hospitalization Register has the disadvantage that it holds no information on specific exposures. Therefore we have linked, at the level of occupations, additional information from the Survey of Living Conditions in Denmark Such ecological studies also have advantages and disadvantages. It is an advantage that exposure information is unrelated to, and therefore unbiased by the memory of the people involved. On the other hand, precision is reduced compared to studies with individual exposure information. The fundamental methodological idea of the present study is to divide up all occupational groups into clusters of occupations sharing working hours, in an objective way, that is by means of existing written knowledge from the period before the start of the follow-up period. Men's choice of career may be determined by the pre-cxistence of disease, particularly a chronic disease such as IHD. In a longitudinal study such as this, the ( ) ( ) ( ) ( ) ( ) ( ) healthy worker effect is reduced compared to crosssectional studies. It will also be less than in studies that rely on mortality because hospital admission is an earlier endpoint than death. By using economically active men as the reference group instead of the general population, the study at least removes that portion of the healthy worker effect that relates to removal of sick people from active employment. It does not deal with movement of sick people from more to less active jobs. Indeed this selection may account for some of the apparent excess risk in light occupations such as caretakers, but no such jobs are involved in the present study. One study showed a higher prevalence of diseases of the circulatory system in people who moved from heavy to light work than in those who had consistently pursued light work." This has been a particular problem in studies of shift work and IHD. The excess risk of IHD is not found among active shift workers but among ex-shift workers. The cohort design is appropriate for dealing with the healthy

5 WORKING HOURS AND HEART DISEASE IN DENMARK 219 TABLE 5 Standardized hospitalization ratios () and 90% confidence intervals (CI) for ischatmic heart disease (ICDS, ) for male occupational groups with day work 90ft CI Salaried architects, academic engineers Lecturers at institutes of higher education Scientific technicians Production and structural technicians Veterinary assistants Teachers Managers Office supervisors Book-keepers, caihiers Shop assistants Chief power station engineers Skilled workers in agriculture and horticulture Skilled slaughterers Skilled painters Bricklayers, carpenters or other skilled building workers Unskilled warehouse, stock or wholesale workers Unskilled workers in agriculture and horticulture Unskilled building workers J ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) TABLE 6 Standardized hospitalization ratios () and 90% confidence intervals (CI) for ischaemk heart disease (ICDS, ) for male occupational groups with various forms of abnormal working hours compared with day work Working hours Night/early morning work Later evening work Rosters covering 24-hour services ( ) ( ) ( ) Other irregular working hours ( ) worker effect and the problem is also reduced if people remain in day jobs in the same occupation. Using hospital admission as a measure of disease introduces referral bias. Some IHD cases are diagnosed in hospitals, some by GPs or private specialists. In Denmark acute myocardial infarctions are rarely treated outside hospital. 13 They account for 58 7o of male IHD admissions. For other IHD diagnoses, there may be minor differences in the treatment pattern in rural areas compared to urban areas. Since all Danish hospitals were public in and had the same admission system, only minor differences in the admission pattern between occupational groups are expected.

6 220 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY As in ad hoc studies, there may be misclassifications of occupations. This study only records the major occupation during 1 year. Migration from index to reference groups or vice versa, before or during the follow-up period, will lead to underestimation of the risk. Non-daytime work may influence occurrence of IHD via three pathways: (i) mismatch of circadian rhythms -» (A) internal desynchronization or (B) increased susceptibility, or (C) sleep/wake disturbances - IHD, or -», (ii) behavioural changes in diet, smoking and exercise etc. -» IHD, and (iii) disturbed sociotemporal patterns -» social insufficiency -» stress - IHD. One implication of this model might be that some behavioural changes are steps in the causal chain and therefore not confounders. For instance some of the groups with high risks like waiters are known to have a higher smoking frequency than other occupational groups. Since it has been shown that shift workers in general smoke more than others, 920 smoking could be seen as an element in the causal chain rather than as a confounder. Smoking should therefore not be controlled. Some confounding may, however, still arise if men who choose, or accept, night work are more likely to be smokers than men who are recruited to day work. The same arguments are valid for alcohol consumption. We have, therefore, standardized for employment grade. Since no groups of self-employed or 'other salaried employees' have stated that they have day work only, these groups have been excluded in this standardization. The calculation which includes control of intervening variables, reduces the SMR considerably but significant excess risks remain (night/early morning work: SMR = 157; : ; late evening work: SMR = 140; 90*% CI : ); rosters covering 24-hour services: SMR = 149; : ; other irregular working hours: SMR = 122; : ). The magnitude and the consistent pattern of the risk differences thus exceed what can be explained away by confounders. An individual coping strategy against gastrointestinal complaints, caused by shift work, may often include change in diet. The impact of age has been controlled through standardization. In a former study we have shown the high risk of IHD in bus drivers, bakers, fishermen, employees in hotels and restaurants, senior police and salvage corps employees, and other occupational groups. 21 The present study may offer some explanation of the mechanisms behind these findings. This project has revealed a very consistent pattern. All groups working in late evenings, at night and in the early morning have raised risks of IHD compared with groups who only work during the day. ACKNOWLEDGEMENTS Thanks to Professor Thorbjorn Akerstedt, The Swedish National Institute of Psychosocial Factors and Health and Assistant Professor Lars Alfredsson, Institute of Environmental Medicine, Stockholm, who suggested the present method. We are indebted to Mr Erik Jtfrgen Hansen, Danish National Institute of Social Research, and to Danish Data Archives who kindly provided us with a complete data set and documentation of the Danish Welfare Study Thanks also to Mr Otto Andersen, Danish Bureau of Statistics who did the basic linkage of the Occupational Hospitalization File. The set up of the basic, linked register as well as the present study, were supported by the Danish Work Environment Fund, Grant REFERENCES 1 Aanonsen A. Shift Work and Health. Oslo: Universitetsforlaget, Thiis-Evensen E. Skiftarbeid og Heist. Pongrunn: Andreas Jakobsens Boktrykkeri, Thiis-Evensen E. Skifttrbeid. Mediclnsk Xrbog (Copenhagen) 1963; 7: Koller M, Kundi M, Cervinlui R. 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