Page 281 - Industrial Ventilation Design Guidebook

242 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

The purpose of epiderniological studies is to try to identify whether there
are causal relationships between the occurrence of diseases or other biological
effects and exposures to various agents. There are three main types of epide-
rniological studies: cross-sectional, cohort, and case-control studies. The
working population is, on the average, healthier than the general population.
Due to this "healthy worker effect," comparisons should be made with an-
other worker group instead of the general population. The reason for the
healthy worker effect is the fact that it is difficult for sick or disabled people to
stay in employment due to the limitation caused by their diseases. Poor health
may also prevent a person from getting a job in the first place.

Cross-sectional Studies
In a cross-sectional study, exposure and effect are studied simultaneously.
This approach contains an inherent problem because exposure must precede
the effect. However, it can be used to investigate acute effects and also mild
chronic effects (which do not force people to leave their jobs) if exposure has
remained rather stable for a long time. When the prevalence of the effects
studied are compared with the prevalence in other worker groups (controls or
references) which correspond otherwise with the study group but are not ex-
posed to the agent investigated, indicative evidence of possible causality may
be obtained. For example, cross-sectional studies have been applied success-
fully to reveal the associations between mild neurotoxic effects and exposure
to organic solvents. 11

Cohort Studies
In a cohort or follow-up study, a group of workers exposed to the same
agent is followed for a certain period, which can be either retrospective (starts at
some time in the past and continues to the present) or prospective (starts in the
present and continues for a certain time into the future). A cohort of controls
should be formed with the same selection criteria as used for the study groups,
except that they lack the exposure. Thus, exposure to one agent only can be
studied whereas several health outcomes can be included. A cohort study is the
only possible study method when the exposure studied is rare. The results of the
cohort study are expressed as relative risks (risk ratios, RR) for various diseases
(see Table 5.8 for results of different types of epiderniological studies on cancers
in printing workers and epiderniological terms) (IARC, 1996),
r> i • r>- i /nr»\ (exposed with disease)/(all exposed)
Relative Risk (RR = }—-—j - . j- ; .; ;.—-—r--
(controls with disease)/(all controls)
r-vjj T» • <r\n\ (exposed cases)/(non-exposed cases)
Odds Ratio OR = -—i—^ 7-477 ~ 3 VT
(exposed controls)/(non-exposed controls)
The benefit of a prospective cohort study is the possibility for accurate ex-
posure assessment. However, these are not common, because many occupa-
tional diseases (including cancers which are being intensely investigated
currently) require long exposure times to develop. It is not practical or ethical
to wait for decades before one obtains the result.
The problems often encountered in retrospective cohort studies include
poor exposure data and incomplete follow-up of all individuals. The accuracy
of health outcome data may also be low.