Occupation and health

Healthy worker effect

A phenomenon observed initially in studies of occupational diseases.  Workers usually exhibit lower overall death rates than the general population because the severely ill and disabled are ordinarily excluded from employment.  Death rates in the general population may be inappropriate for comparison if this effect is not taken into account.

Job exposure matrix

A method employed to assign cumulative exposure to workers in epidemiological studies based on the assignment of workers to job categories with quantified exposure levels.  All workers with the same job title and duration are usually assigned similar cumulative exposures.

Occupational disease

Morbid condition resulting from exposure to an agent during the usual performance of one's occupation.

Pneumoconiosis

A lung disease, characterised by fibrosis and scarring of the lungs, resulting from the repeated inhalation of occupationally associated dust, such as silica, asbestos and coal dust.

PMR (proportional mortality ratio)

Proportion of observed deaths from a specified condition in a defined population divided by the proportion of deaths expected from this condition in a standard population, expressed either.

SMR (standardised mortality ratio)

The ratio of the number of events observed in the study group or population to the number that would be expected if the study population had the same specific rates as the standard population.

Global burden of occupational injuries and illness

Occupation is a major contributor to injuries and disease as well as economic loss amounting to 4–5% of GDP.  The World Health Organisation (WHO) and the International Labour Organisation (ILO) estimate that there are around 2.2 million work-related deaths per year[1]. Globally only 10-15% of workers have access to a basic standard of occupational health services.

Occupational diseases

Using risk measures applied to the proportions of the population exposed to selected occupational hazards the WHO has estimated attributable fractions, deaths, and disability-adjusted life years (DALYs) for selected occupational risk factors.  Worldwide, these selected risk factors were estimated to be responsible for:

• 37% of back pain;
• 16% of hearing loss;
• 13% of chronic obstructive pulmonary disease (COPD);
• 11% of asthma;
• 8% of injuries;
• 9% of lung cancer;
• 2% of leukaemia[2].

These risks at work caused the loss of about 24 million years of healthy life – a significant and preventable proportion of the global burden of disease and injury.

Occupation factors may give rise to a very wide range of injuries and diseases.  Occupational injuries may be classified according to the type of injury, the affected body part, and the type of enterprise where the injury was sustained.

The many diseases that may be linked to occupation are too many to list, but examples from each of the main disease groupings is given in the table below.  They include infectious disease, chronic disease, malignancy, and mental disorders.
 

External agents

External agents causing occupational disease and injury include:

• noise;
• vibration;
• ionising radiation;
• man-made visible and ultraviolet light;
• non-ionising radiation;
• unspecified type of radiation;
• poisoning by and exposure to alcohol;
• accidental poisoning by and exposure to organic solvents and halogenated hydrocarbons and their vapours; to other gases and vapours; to pesticides; and to other and unspecified chemicals and noxious substances;
• overexertion and strenuous or repetitive movements.

Health promotion

Not all occupational influences are negative.  The workplace can also carry many benefits – in socialisation, in providing opportunities for social participation and recreation.  It is also a useful setting for initiatives aimed at health promotion, e.g. smoking cessation initiatives, cycling to work, and for screening for illness.  Legislation on smoking in public places and the workplace (outlawed in many European countries including, from July 2007 in England) is also likely to have had an influence on smoking prevalence.

Occupational epidemiology

Occupational settings provide opportunities for studying disease, especially through use of occupational cohorts and similar designs in which the study population is defined on the basis of employment.    The main opportunity is that the population has some shared experience or exposures, which are potentially more intense than in the general population, more specific and more readily quantified.  Indeed, many health hazards of wide public health significance in contexts beyond occupational (e.g. lead, asbestos, vinyl chloride to name some) were first identified as health hazards in occupational groups exposed to particularly high levels of the substance.

Exposure assessment: job exposure-matrix

Exposure assessment in occupational studies of chronic disease is frequently based on estimates of past exposures.  These estimates often entail constructing job-exposure matrices[3], in which exposure is categorised for certain job titles/categories which are then used for grouping individuals of the study population.

Standardised mortality ratio

Comparisons can be made of disease occurrence within an occupational population according to level of exposure (the calculation of risk or rate ratios), or between the occupational group and the wider population using standard references tables of disease rates, stratified by age-group and sex.  This is the basis of the (age/sex) Standardised Mortality Ratio (SMR), an indirect method of standardisation.

Expected deaths are the number of deaths that would be expected if age-, sex- and calendar period-specific rates for the general population applied in the occupational cohort.

Proportional mortality ratio

The Proportional Mortality (Morbidity) Ratio is another measure of relative disease risk similar in principle to the SMR. The PMR, however, uses data only on the proportion of cases classifiable to a particular cause in relation to the overall number of disease cases.  Its use is based on the question of whether a particular cause of death or disease is proportionately greater than expected among all deaths or disease cases in the occupational group as a whole.  Its advantage is that it doesn’t require the calculation of absolute disease rates.  It is usually calculated from age- and sex-stratified numbers.

Healthy worker effect

One problem with such comparisons, and with occupational epidemiology more generally, is that the working population is selected in ways that may make it hard to compare (more) exposed with less exposed without the exposure being confounded with other characteristics related to disease.  A particular issue is that of the healthy worker effect[4], a phenomenon that reflects the fact that workers usually exhibit lower overall death rates than the general population because the severely ill and disabled are ordinarily excluded from employment.