Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
A specific example for an approach used in external QC is the
UNEP/WHO regression method. The regression method is based on the QC
programmes developed in the UNEP/WHO pilot project on assessment of
human exposure to pollutants through biological monitoring for the
determination of lead in blood and cadmium in kidney cortex (Vahter,
1982). This external QC method was also applied in the UNEP/WHO Human
Exposure Assessment Location (HEAL) study. For the studies on lead and
cadmium, reference external QC samples of blood, air filter material,
dust and diet were sent to participating laboratories (Vahter &
Slorach, 1990). The same procedure was also adopted for the HEAL
nitrogen dioxide study, using standard sodium nitrite solutions and
nitrogen dioxide-exposed sample badges as QC samples (Matsushita &
Tanabe, 1991).
Table 37. External QC schemes
Scheme Country Reference
Centre de Toxicologie du Quebec Canada Weber, 1988
(CTQ)
Danish External Quality Assessment Denmark Anglov et al.,
Scheme (DEQAS) 1993
External quality control in the Germany Schaller, 1991
toxicological analyses of biological
materials, German Society of
Occupational Medicine
Japanese External Quality Japan Sugita et al.,
Assessment Scheme 1990
NIOSH Proficiency Analytical USA NIOSH, 1979
Testing Programme
United Kingdom National External United UKNEQAS, 1993;
Quality Assessment Schemes Kingdom Bullock, 1986
(UKQAS)
The accuracy and precision of the analytical results were
evaluated by using a statistical technique known as the maximum
allowable deviations (MAD) method (Vahter, 1982; WHO, 1986; UNEP/WHO,
1991). This method is based on the linear regression line of reported
versus "true" values and established acceptance criterion on how much
the line may deviate from the ideal line where the measured equals the
expected value. Acceptance criteria or the MADs have to be decided on
separately for each pollutant or for each medium. The stringency of
the MAD is primarily based on the required quality of the actual
monitoring data and the sensitivity, accuracy and precision of the
analytical methods. As in the use of control charts, decision
concerning acceptance and rejection of results is based on statistical
criteria, i.e., on the probability of making right or wrong decisions.
A laboratory may be erroneously rejected when in fact its methodology
is satisfactory, or it may be erroneously accepted when its
methodology is not satisfactory. Power analysis is done to determine
the probability of including an unsatisfactory laboratory. The concept
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