Health selection among metal workers

Health selection among metal workers. Scand j work environ health 9 (1983) 155-161. The objective of the present study was to determine which age and exposure categories are the most prone to health selection. Mortality and morbidity were studied on three different exposure levels, defined primarily according to the physical demands of the work: heavy level (iron foundries), medium level (manufacture of metal products), and light level (manufacture of electrical devices). The population comprised about 15,700 men employed in 1950- 1976 in the three branches of the metal industry. The number of person-years of follow-up was about 215,800. During the period 1950-1978, 1,407 deaths occurred. Occupational hi~story, morbidity, and turnover causes were studied by means of a questionnaire sent to 3,500 current and former workers. The survival curves showed no great differences between the three exposure levels. However the heavy level had the highest degree of mortality, and the medium level the lowest. Foundry workers had the highest overall rates of disability. But, in the older age groups, the disability rates of the metal product workers were the highest. Less social selection was suggested for foundry workers than for the two other cohorts. Metal product workers seemed to be selected by both social and health factors. Young electrical workers were occupation- ally trained. On the other hand older workers with poor health were selected to this light level (negative health selection).

Workers are selected into and out of physically demanding jobs with regard to their health. The study of occupational mortality and morbidity is hampered by this selection. Furthermore, social selection and rapid turnover are involved in health selection (2,3,4). Because different stages of disease form only one dimension (dissatisfactiondeath) '(5, 7), the correct interpretation of the results also requires softer measures than mortality. Earlier studies have concluded that soft and hard measures of health follow one another as explanations of the termination-of-employment rate (5,6).
The aim of the present study was to determine which age and exposure cate-Reprint requests to: Ms R-S Koskela, Institute of Occupational Health, Haartmaninkatu 1, SF-00290 Helsinki 29, Finland. gories are the most prone to health selection.
It was initially assumed that low mortality in an occupation may be caused by health selection, whereby workers with health problems change to other occupations; thus health selection causes excess mortality in these occupations (fig 1).
The following hypotheses were formed: (i) if the levels of exposure are defined as heavy, medium, and light, according to the physical demands and exposures connected with the work, the heavy level has the strongest health selection and, consequently, low mortality rates; ~(ii) the medium level has the highest degree of mortality ,due to three reasonsfirst it includes persons who are primarily unfit for heavy work; second it includes persons who could not continue doing heavy work; third the work itself involves considerable exposure; Ciii) persons a t the light level have a strong occupational and "negative" health preselectionselection during the period of employment is low; (iv) selection mechanisms result in different turnover rates at the three levels, (v) high mortality and morbidity rates are not necessarily concentrated at the same level(s). The levels of exposure in this study were defined primarily according to the physical demands and exposures connected with the work: heavy level (iron foundries), medium level (manufacture of metal products), and light level (manufacture of electrical devices). The population comprised about 15,700 men employed between 1950 and 1976 in the three branches of the metal industry representing the different levels of exposure (4,000-6,000 workers from each branch). The number of person-years of follow-up was about 215,800. During the period of 1950 to 1978, there were 1,407 deaths. In addition another methodological (crosssectional) cohort was comprised of employees who had been hired earlier and were still working in 1950. The period of follow-up for disability began in 1969 and ended in 1978. Data for the analyses of mortality and disability were obtained from national death and disability registers. A sample was used to study the occupational histories, morbidity, and the reasons for turnover. A postal questionnaire was sent to 400 current and 600 former workers from each branch. In addition a questionnaire about occupational history was sent to the next of kin of 450 deceased workers.
The results were compared between the levels for the analyses of mortality, disability, and turnover, and within the levels for the analyses of turnover and morbidity.
When the mortality of the three exposure-level cohorts were compared with the mortality of the general male population of Finland (8), the standardized mortality ratios (SMRs) were exceptionally high: 124 for the foundry workers, 92 for the metal product workers, and 107 for the electrical workers.
The standardized mortality ratios of occupational cohorts usually remain below 100. The most probable reason for the high ratios was that, for methodological reasons, there were no criteria for the duration of exposure.
No great differences were found for the survival curves of the three exposurelevel cohorts. The order of the survival curves implied that the heavy-level cohort had the highest degree of mortality, and the medium level the lowest. The curve of the light level was similar to the medium one until the workers aged, when the curve attained that of the heavyexposure cohort (fig 2).
Social and health selection are intermingled. Therefore variations in the numbers of unskilled workers may have caused different mortality patterns among the three exposure-level cohorts. Laborers and other unskilled workers had a tendency towards higher mortality than skilled workers. But, when the unskilled workers were excluded from each level, the order of the survival curves remained unchanged between the industries. Some cause-specific findings supported social and health selection (fig 3). The probability of violent death was highest for the foundry workers (about 1.5 times that of the metal product workers). The foundry workers also had the highest incidence of deaths for which alcohol was a contributing cause (ICD codes 291, 303, 571, E 979 & E 980). The metal product workers had the lowest incidence. The highest incidences of deaths with alcohol as a contributing cause were found for the older age groups of electrical workers. This finding may reflect negative social and health selection. Negative health selection among electrical workers was also compatible with the rather high rate of mortality from tumors within the group of aged workers.
Mortality from coronary heart disease seemed the lowest for the electrical workers, a result which may indicate that health selection is low during employment a t the light level. This assumption was confirmed when mortality from cardiovascular diseases was compared with that of the cross-sectional cohorts which included workers who were occupationally active in 1950 but had been hired earlier and, therefore, were highly selected groups. The cross-sectional cohort of electrical workers had higher rates of coronary heart disease than the main cohort. On the contrary, a comparison of these cohorts revealed health selection during employment in the heavy and medium levels of exposure. The main cohort of foundry workers had a higher rate of mortality from cardiovascular diseases than the cross-sectional one. The difference was mainly due to the mortality from coronary heart disease. A similar difference between the cross-sectional and main cohorts was found for the metal product workers, but it was not explained by coronary heart disease.
When compared to the active male population of Finland (I), foundry workers, metal product workers, and electrical workers had standardized disability ratios of 121, 108, and 97, respectively. The effect of selection can be summarized with a disability index defined in such a way that the observed number equals 100 for each age category (fig 4).
The high disability rates of the young foundry workers were mainly due to mental disorders (p < 0.001, Poisson    (fig 4). The primary causes of the excess were respiratory diseases and mental disorders. Nevertheless, no specific cause reached statistical significance.
The electrical workers had disability rates below the expected values. The youngest age class, for which mental of the foundry workers had entered disorders caused an excess of disability (p < 0.001), was an exception.
A comparison of the turnover of employees in the three industries showed that the foundry workers had the highest termination-of-employment rate until the age of 4 0 4 4 a, whereafter the rate seemed to be the highest among the electrical workers. In all age categories the metal product workers had considerably lower rates than the two other groups. The termination-of-employment rates were the highest for young workers regardless of the level of heaviness ( fig 5).
In addition to the individual factors (workers' health and social factors) turnover also depends on job market, ie, on the opportunity of securing other employment. The termination-of-employment rates were influenced by the fact that these three industries had different. amounts of new employees. Thus economic factors introduced additional selective features.
Selection into and out of industries was analyzed on the basis of the complete occupational histories collected by a questionnaire. The occupations of each worker throughout his lifetime were classified according to three levels of heaviness: heavy, medium and light. Most either from hea"y-level occupations (such as agriculture, forestry, and construction work) or from medium-level occupations (chiefly the manufacture of metal products). Although 26 O/o of the foundrymen transferred to heavy work after leaving the foundries, most of the workers sought lighter work (mostly in the metal industry). One of every three foundry workers whose new job involved heavy work transferred to another foundry ( fig 6). The metal product workers either began their work lives within the metal product industry or they transferred to it from work of the same level of heaviness. When the metal product workers left a job, they generally transferred to medium-level occupations. They either continued doing the same task, or they chose a lighter form of work in the metal industry.
The electrical workers had seldom transferred from the same level. They generally switched from medium-level work, mainly within the metal industry; several electrical workers had begun their work lives within that industrial branch. After leaving the work, they sought either medium-level or light-level work in the metal industry. The metal ~r o d u c t workers and the electrical workers were occupationally preselected because more of the workers had begun their work lives in those branches. Health selection during employment was revealed in the foundry workers' tendency to move to lighter work. To some extent a similar tendency was also found among metal product workers.
The foundry workers can be taken as an example of health selection within industries. The occupational histories of current and former foundry workers (as classified when the original cohorts were formed in 1976) were compared for changes in the heaviness throughout their complet,e occupational histories (fig 7). The occupations were again classified into three levels of exDosure on the basis of physical demand (heavy, medium, and light). Transitions from one level to another were grouped into transitions to heavier levels of exposure and to lighter levels. The changes are expressed as percentage differences (percentages of transitions to heavier levels minus those to lighter levels of exposure) in relation to the total number of workers in an age group. Current workers had transferred mainly to heavier work until the age of 35. Former workers with a long duration of exposure had moved to heavier work until the age of 25, whereafter they switched to lighter work. Former workers with a short duration of exposure had sought lighter work from the very beginning.
The timing of the foundry work within the workers' complete occupational histories was examined , (fig 8) in the same way as the changes in job level (light, medium, heavy). Changes in the heaviness of the work were clearly associated with occupational histories within foundries ' (fig 7).
The workers' health was also associated with changes in the heaviness of the work. Both the current and the former workers' own assessments of their present health showed the same pattern of change, from a good to a poor health state, according to age and changes in the heaviness of their work. Health selection between current and former workers was also revealed by the higher prevalences of angina pectoris, chronic bronchitis, and musculoskeletal diseases found among the groups of former workers.
In summary, contrary to the initial hypotheses, mortality was the highest at the heavy level of exposure and the lowest at the medium one. The preliminary results indicate that the two most probable explanations are the high level of exposure and the low social preselection. Although turnover is rapid and workers transfer to the medium level of exposure, the medium-level workers with health problems correspondingly transfer to the light level of exposure. Thus mortality in the older age groups of workers a t the light level of exposure was rather high (ie, negative health selection). On the other hand, low mortality at the medium level may be caused by high health preselection and high social preselection. Future analyses of the softer measures of health should provide more-detailed information regard-ing the differences between and within the levels of exposure.