Hazard Assessment and Contaminated Sites - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 540

Epidemiological studies, depending on their design, may serve two purposes;
hypothesis-generation or assessment of a causal relationship. Their ability to
evaluate a causal relationship may be limited by a lack of control of potential
confounders or a lack of power (which is usually the result of limited sample sizes)
(Samet et al. 1998 ).

12.4.3.1 Observational Studies

Different observational study designs have different applications, advantages and
disadvantages (see Table 12.2 ). These comparisons assume the different types
of studies are equally well designed. Even so, design variations may affect their
performance and provide exceptions. See Beaglehole et al. ( 1993 ) for a more
detailed description.

Table 12.2 Guidelines for the assessment of causation

Temporal relation Does the cause precede the effect (essential)

Plausibility Is the association consistent with other knowledge? (mechanism of

action; evidence from experimental animals)

Consistency Have similar results been shown in other studies?

Strength What is the strength of the association between the cause and the

effect? (In general, relative risks greater than 2 can be considered
strong)

Dose-response
relationship

Is increased exposure to the possible cause associated with increased
effect?

Reversibility Does the removal of a possible cause lead to reduction of disease risk?

Study design Is the evidence based on a strong study design?

Judging the
evidence

How many lines of evidence lead to the conclusion?

Beaglehole et al. ( 1993 , p. 76)

12.4.4 Assessing the Relationship Between a Possible Cause and
an Outcome

A cause is “an event, condition, characteristic or a combination of these factors
which plays an important role in producing the disease” (Beaglehole et al. 1993 ,
p. 76).
Causation of adverse health effects is affected by four types of factor:

• predisposing factors such as immune deficiencies, gender and previous illness;

• enabling factors such as poor nutrition and bad housing may favour the develop-

ment of disease;

• precipitating factors such as the exposure to a specific disease contaminant; and

• reinforcing factors such as repeated exposure which may aggravate an established

disease or state (Beaglehole et al. 1993 ).

The term “risk factor” is commonly used to describe factors that are positively
associated with the risk of development of a disease, but that are not sufficient in