Hair Analysis Panel Discussion: Section 4.1
4.1 Distinguishing Between Endogenous and Exogenous Sources of Metals
All of the panelists agreed that using hair analysis as an exposure or diagnostic tool for metal contamination is severely limited by difficulties in distinguishing between internal and external sources of metals. It is further complicated by the natural occurrence of many of the trace elements (several of which are essential nutrients) within the body. The group recognized, however, that this distinction is not a limitation when a metabolite or a substance with no external source is being measured (e.g., organic compounds such as methyl mercury or many drugs of abuse).
Dr. Kosnett led discussions on the difficulties that exist in
distinguishing endogenous and exogenous substances in hair. Other
panelists expanded upon these issues. Individual points are summarized
- Hair analysis data do not necessarily enable you to determine
where the measured contaminant came from and how it got there.
High hair levels may provide "markers of potential exposure,"
but that does not tell us how much is internally incorporated.
If hair analysis is used in ATSDR's evaluations of exposures
to contaminants in air (e.g., in the form of particulates),
water, or soil/dust, it must be realized that this distinction
cannot necessarily be made (MK).
- An Alaskan study of arsenic levels in tap water, urine,
and nails (Harrington et al. 1978), reveals some interesting
trends. Individuals drinking bottled water, but bathing in tap
water with arsenic averaging 345 micrograms per liter (µg/L),
had higher average levels of arsenic in hair (5.7 parts per
million, or ppm) compared to those drinking and bathing in tap
water with arsenic containing 30 µg/L (0.46 ppm arsenic in hair).
Urine levels were similar, however. This example helps illustrate
the difficulties in using hair concentrations alone to draw
inferences regarding the magnitude of the internally absorbed
dose of a metal (MK).
- Though they are not applicable to the example above (based
on arsenic toxicokinetics), another reviewer noted that the
following caveats could further confound the interpretation
of such a scenario: (1) other exposures could be occurring (e.g.,
cooking, brushing teeth), (2) dermal absorption could be occurring,
and/or (3) a pool of the contaminant could be sequestered in
and later released from the bone (e.g., this can be true with
- Effect of washing hair. Dr. Kosnett described various
studies that have looked at the role and/or effectiveness of
washing hair in order to distinguish between endogenous and
exogenous sources of arsenic. These studies suggest that no
truly good washing method exists to remove arsenic: If hair
is not washed aggressively, exogenous arsenic will remain. If
hair is washed too aggressively, endogenous arsenic may be removed.
— Smith (1964) showed that detected concentrations of arsenic in
hair will vary depending on washing method, with no method shown
to be capable of removing all arsenic. The results of applying different
washing methods (to hair purposely externally contaminated with
12.08 ppm arsenic) are highlighted in Table 4-1. The arsenic concentration
in hair before contamination was measured at 0.14 ppm.
Table 4-1. Effect of washing method and time on arsenic levels in hair
|Washing Method||Washing Time (mins)||Arsenic (ppm)|
Source: Smith 1964
— Van den Berg et al. (1968) showed similar findings. Depending
on the washing regime, this study revealed that even after 1,600
minutes of washing, externally deposited arsenic was still detected
Measuring total concentrations in hair. Depending on the purpose of your testing, it may not be critical to distinguish between internal and external contamination. For example, in an industrial hygiene exposure investigation, identifying elevated levels of an element may be enough to suggest that the potential for exposure exists and protective measures are needed. While urine data may reveal that existing protective measures have prevented internal exposures, knowledge that employees have exposure potential may be important (e.g., contamination could be carried home) (MK, TC, MG). Several panelists reiterated, however, the limitations of using such data for clinical evaluation or interpretation.