Hazard Assessment and Contaminated Sites - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 543

minimised. Case control studies are subject to several forms of bias and weaknesses
related to time-sequence but, if well designed, may still provide useful evidence
for the causal nature of an association. Cross-sectional studies are weaker as they
provide no direct evidence on the time sequence of events.
Ecological studies are the least satisfactory because of the dangers of incor-
rect extrapolation to individuals from data derived from regions or countries.
However, where certain exposures cannot normally be measured individually (e.g.,
air pollution, pesticides residues in food, fluoride in drinking water) evidence
from ecological studies may be important in environmental health decision mak-
ing (Beaglehole et al. 1993 ). Time-series studies demonstrating health outcomes
associated with fluctuating air contaminant levels may be one particularly useful
example.
The ranking in this Table 12.3 assumes that studies are well designed and
well conducted in each case. Even the presence of “a strong ability to 'prove'
causation” should be supplemented by mechanistic knowledge to be confident of
causation.

Ta ble 12.3 Relative ability of different types of study to “prove” causatio n

Type of study Ability to “prove” causation

Randomised controlled trials Strong

Cohort studies Moderate

Case-control studies Weak/Moderate

Cross-sectional studies Weak

Ecological studies Weak

Adapted from Beaglehole et al. ( 1993 )

12.4.5 The Strengths and Limitations of Observational
Epidemiology Versus Experimental Toxicology

Epidemiological studies are crucial for assessing effects directly in humans and esti-
mating population attributable risks. However, their power of resolution is limited,
mainly because of the difficulties in estimating exposure precisely and in controlling
bias. Toxicological studies are necessary for elucidating causal mechanisms, which
may be important for determining dose-response relations and extrapolating to low
doses in Risk Assessment, but direct generalisations to humans based on animal
data are often uncertain (Pershagen 1999 ).
Epidemiological studies are often given increased weighting because they come
from humans but, compared to toxicological studies of animals, may be more
costly and time consuming and more likely to result in ambiguous findings (Samet
et al. 1998 ). However, substantive findings have been obtained at times through
opportunistic studies of highly exposed groups - such as occupational cohorts or
communities that have been inadvertently exposed to contaminants e.g., via food or