Different stages of disease, changes in heaviness of work and life cycle.

The aim of this 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 (iron foundries), medium (manufacture of metal products), and light (manufacture of electrical devices). The population comprised 15 714 men hired in 1950-1976 to work in the three branches of the metal industry. A questionnaire on occupational history, morbidity, and the causes of turnover was sent to 3 450 current and former workers. The occupational histories of current and former workers were compared for changes in heaviness throughout their complete occupational histories. Occupations during a lifetime were also classified into three levels of exposure on the basis of physical demand (heavy, medium, and light). The three levels of exposure showed different patterns of changes in heaviness of the work according to age throughout the workers' complete occupational histories. Selection into and out of jobs within and between different levels of exposure seemed to be some kind of continuous process, a chain of selection. Similarly as changes in the heaviness of the work formed a chain of selection during lifetime, the workers' health also changed to form a chain through their life cycle. Hard and soft measures formed a continuum (dissatisfaction - death), and they followed each other as explanations for termination of employment. The points of inflection in age, where the measures of different stages of disease turned from soft to hard, varied according to the level of exposure. The life-table technique was used to show the turning of the measures.