Concept 5: occupational epidemiology: sentinel surveillance, healthy worker effect, retrospective cohort study

Occupational epidemiology as the discipline of study that deals with the distribution and determinants of workplace related illnesses and use of that knowledge to prevent harm and promote wellness of the workers has two distinctions:
As workers within a workplace setting are relatively readily identifiable, therefore many epidemiological studies can rely on all members of the population. A common study design to assess the distribution of diseases in workplace is surveillance or workplace surveillance. This is a variation in the case series study design where an epidemiologist:
The cases are then recorded and reported to the disease surveillance units or occupational surveillance units for further analysis to identify causes of the diseases and risk factor. For assessment of occupational risk factors, epidemiologists rely on cross-sectional surveys, case control studies, and retrospective or prospective cohort studies. Cross-sectional surveys are where employees are assessed for a specific disease condition or health condition. For example, if 100 employees work in a an enterprise and they are surveyed for high blood pressure, this type of study is a cross-sectional survey. A limitation of cross-sectional survey in workplaces to identify true rates of disease is healthy worker effect. If some employees are absent on the day the survey is conducted due to an illness, say for example for the blood pressure, then the epidemiologists would not be able to measure the true prevalence of hypertension in this workforce. Instead, the epidemiologists would only be able to measure the prevalence of hypertension among those workers who were healthy to attend work on that particular day. This phenomenon is referred to as "healthy worker effect" and therefore a remedy to that would be to repeatedly survey the workplace for the particular disease or health state that investigators are interested to study.
Because workplace settings provide opportunities for pre-employment examination and thereafter periodic examinations, therefore, workplaces are ideal settings for conducting retrospective cohort studies. Retrospective cohort studies are those where the investigators would start with a group of exposed and non-exposed individuals, all of whom are free of the health outcomes they are interested to study. Some workers are exposed to specific toxins due to their natural exposure as a result of their work, others would not be exposed, or exposed to less extent, again, due to the nature of their work. For example, if you were to study nurses and orderlies in a hospital where they were required to lift heavy weights and patients such as operation theatres, and compare them with nurses and workers in other parts of the hospital such as chronic disease wards where they were required to tend to only occcasionally patients who would require heavy lifting of equipments and then wanted to study the association between certain shifts and the risk of low back pain, you could use a retrospective cohort study. In a retrospective cohort study like this, you would start with the health records of all healthy workers in the hospitals who were free of low back pain complaints in the beginning of their work and then you would follow different sets of workers in different work contexts to find out their relative risks of developing low back pain.
Retrospective cohort studies would also enable the epidemiologist to set up a nested case control study design within this setting and this too is used in occupational health setting. Imagine you are interested to study whether exposure to certain chemical (let's call it "Ch") is associated with increased risk of high blood pressure among a group of employees. The investigator might take blood samples of all employees during their pre-employment physical examination to measure the dose of Ch in blood and then follow up employees over the years. After a few years, when sufficient numbers of employees have developed high blood pressure and others have not developed blood pressures, then, within this cohort that are being followed up over the years, the researchers would set up a case control study; in this case control study for instance, where they would test the association between presence of Ch in the blood sample many years earlier and the risk of blood pressure, they would be able to use an odds ratio of the the presence or absence and indeed various levels of Ch in the body. 
So these are the opportunities and costs of conducting epidemiological studies within the workplace.

Concept 6: history of occupational health, from Hippocrates to Percival Pott

The history of occupational health is as old as that of modern medical science. Hippocrates in 5th century BC in his work, airs, waters, and places wrote about the importance of places and occupations in health states and advices to the doctors.