IntroductionTo the order of two million people every year in the UK alone complain of ill-health which is caused or aggravated by work. Epidemiology has an important role notably in:
To gain full benefit from this resource it is important to have an understanding of epidemiologic concepts, notably causal associations, and to be aware of issues which may influence apparent association, notably bias, confounding, and chance.
Aim of this ResourceTo enable an understanding of the role and application of occupational epidemiology.
Learning ObjectivesYou should:-
Drawing analogies between the clinical and epidemiologic approach.The logic including data gathering, data processing, interpretation, intervention and hence change has a lot in common between the clinical and the epidemiologic method, as illustrated in the following table:
|Structure/ Context:||Structure/ Context:|
Consulting with an individual patient
Contending with a group of workers or other people
|Taking the individual's symptom history*||Administering a questionnaire, (and collating the replies)|
|Taking the individual's exposure history (& getting information about the work/environment)||As above by a questionnaire, but also by an objective assessment of work/environmental exposures|
|Carrying out a physical examination and tests||Gathering data from health surveillance tests etc|
|Exercising 'clinical judgement'*||Analysis of the data|
|Diagnosis||Description of profile of health and sisease|
|Prognosis||Prediction of risk|
|Change in patient's condition||Change in profile of health and disease|
|Note*: The above analogies can be extended
to the history taking process.
Types of Studies in Occupational Epidemiology
Case Control Studies
|The accompanying image shows an occupational cancer
- of the nasal sinuses - which is fortunately rarer now than it used to
In a case-control approach, 'cases' of this disease are compared to a carefully matched reference population of 'controls' (or 'referents') to determine retrospectively what differences there may have been in their occupational exposure histories. For example in one study it was shown that patients with one type of this tumour (an adenocarcinoma) had a disproportionately greater likelihood of having worked in the hardwood industry, when compared to referent patients with other pathology.
(Subsequently cohort studies were conducted - see below which supported the hypothesis of a causal association by showing that this type of tumour was much commoner in hardwood workers than in other people.)
Cross Sectional StudiesIn a cross-sectional study, the prevalence of a particular disease or of a set of symptoms or other indication of ill-health is investigated in a single time-point (or over a relatively narrow period of time). Comparisons can then be made in the frequency of ill-health for example between workers exposed to a particular hazard, and those who are not, or - better still - between workers experiencing different degrees of exposure. (See exposure-response relationships in relation to causal association).
A cross sectional study can determine the prevalence rate, which is defined as the number of EXISTING cases of disease divided by the population at a specified time point.
For example if a chest X-ray survey of quarry workers is conducted it might show that workers in quarries with high exposure to quartz (a crystalline form of silica) might have a higher prevalence of pneumoconiosis than those in quarries with little or no such exposure.
Cohort StudiesA 'cohort' was strictly speaking a fighting unit of the Roman army (roughly comparable to a 'battalion' nowadays). The term is now applied epidemiologically to a clearly defined population who prospectively share a common experience -say an occupational exposure. Comparisons in the incidence of ill-health, or in mortality can be made between exposed and non-exposed cohorts or between subsets of the same cohort but with different degrees of exposure.
In the context of a cohort study the term 'control' has a different meaning from the meaning in a case-control study (see above). In a cohort study, the 'controls' are those people who are not (or have not been) exposed to the agent under investigation.
Some cohort studies measure mortality, others measure incidence of disease. The incidence rate is defined as the number of NEW cases of a disease divided by the population at risk over a given period of time. In occupational and environmental epidemiology it is important to be able to characterise the exposure of the 'population at risk'.
For example a study was conducted in which workers were followed up during their employment in a factorybin which they were exposed to benzene. It showed that those categories of workers with a high cumulative exposure to benzene had a higher mortality from certain types of leukaemia than the control population.
Problems in Occupational Epidemiology
The Healthy Worker EffectWorkers differ from the general population from which they are drawn, and especially from unemployed people in many ways. This differences can result in serious bias in occupational epidemiology.
Can you consider some of these differences, and how they can contribute to bias?
(For example are there socio-economic differences between the employed and the unemployed, can these differences influence health, and if so - in what way?)
Other ProblemsVarious difficulties can affect the design and interpretation of studies in occupational epidemiology.
Can you consider some of these?
(For example how comparable is an occupational population to the general public in terms of age, and sex?)
Bias, confounding and chance are considered briefly elsewhere.
The Application of Occupational Epidemiology
Benefit for the workersIt should be obvious, that epidemiology has a great deal to contribute to the reduction of risks to health from work, through reducing exposure, and in other ways.
Benefit for the Community at LargeVarious direct and indirect benefits can accrue to the population at large, for example through the derivation and application of exposure limits. For example the recommendations of the Expert Panel on Air Quality Standards in relation to benzene were largely based on occupational epidemiology.
ConclusionOccupational epidemiology is an important aspect of clinical epidemiology and of occupational hygiene since it provides powerful and practical information to understand the causes and determinants of work related ill-health, to help establish what steps should be taken to reduce those risks, and to evaluate interventions for the benefits of workers, and of the community at large.
Further Reading:‘Practical Occupational Medicine'.
Ch3: ‘Epidemiological Investigation of Occupational Disease.’ pages
61-74. (1st edition)
Though not specific to occupational epidemiology, an excellent resource
on Epidemiology for Decision-Making (from the WHO) consists of Problem-Based
Training Exercises for Students.