Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
characterizing the magnitude, duration and time patterns of human
contact with environmental contaminants may follow a direct approach
or an indirect approach. Direct approaches to exposure assessment
include point-of-contact measurements and measures of biological
markers of exposure. Indirect approaches include environmental
monitoring, modelling and questionnaires. These approaches may be
employed in various types of exposure studies that are typified by the
manner in which the study population is selected; for example,
comprehensive studies that include all members of the study
population, descriptive studies consisting of a non-probability
sample, or surveys based on a randomly selected, representative sample
of individuals.
4. STATISTICAL METHODS IN EXPOSURE ASSESSMENT
4.1 Introduction
Statistics is a necessary and critical tool in exposure
assessment studies. Statistics can be employed at each stage of the
exposure assessment study. At the planning stage, statistics aids in
selection of study design and determination of the amount and form of
data to collect. After the data are collected, statistical description
of the results helps understanding of the basic characteristics of
exposure and its determinants. Statistics is also essential during
final analysis of the data for hypothesis testing, characterizing
exposure through various routes and media, and exploring relationships
between ideal measurements (e.g., exact lung uptake) and feasible
measurements (e.g., ambient, indoor, or personal measures).
Furthermore, statistical inference allows one to generalize the
observations derived from a sample to a wider population from which
the sample was drawn. Finally, as noted in Chapter 11, statistics play
an important role in quality assurance (QA) programmes.
Selected applications of descriptive and inferential statistics
in exposure assessment studies are discussed in the following
sections. This chapter is not a substitute for a course in statistical
methods, but is intended to provide a brief review and useful
references. Widely available statistical software for personal
computers can be used to perform data processing and necessary
calculations. One example of such packages is the statistical
programme Epi Info developed for and distributed by WHO (Dean et al.,
1995).
Throughout the chapter, data collected as part of a lead exposure
study performed in Malta and Mexico (WHO, 1985b) (Table 8) will be
used to illustrate some key statistical concepts and methods. The
purpose of this study was to investigate the relative importance of
lead exposure via different routes of exposure. Blood lead
concentrations were considered to be an indicator of lead uptake from
all exposure routes, whereas faeces lead concentrations were
considered to represent exposure via ingestion. In the course of this
study, blood lead and faeces lead measurements were obtained from 36
and 19 individuals in Malta and Mexico, respectively.
4.2 Descriptive statistics
Descriptive statistics summarize data in a simple manner to
discern key points about the collected information. We typically
assume that the collected data are a sample from a larger population
of possible measurements and that the sample is representative of the
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