Hair Analysis Panel Discussion: Section 6.2
6.2 When Is It Appropriate To Consider Hair Analysis in Assessing Human Exposures to Environmental Contaminants?
The panelists recognized that hair analysis can serve two distinct
purposes: (1) as a tool in identifying exposures (Is the substance
reaching people? Does a competed pathway exist?) and (2) as a clinical
tool (What is the threshold for adverse health effects?) The latter
is where the largest data gaps exist. The panelists agreed that
a body of literature describes specific conditions and uses of hair
analysis for methyl mercury and arsenic. There may be a unique forensic
setting for other metals. Segmental analysis with ultra-sensitive
techniques may have a role in special cases—that is, subject-, substance-,
and situation-specific cases (e.g., identification of high-dose
The group agreed on the general criteria that need to be fulfilled in order to consider hair analysis a valid assessment tool. Panelists encourage assessors to ask: What is the predictive value of a positive or negative test? Are data available to determine whether the measured level is of sufficient magnitude to be of pathological or public health importance? The following factors are key to that determination:
- Defining the type of exposure that may have occurred and
over what time period. (What do exposure histories tell us about
the likelihood that a particular substance will be in hair at
the time of testing?)
- Understanding the type of substance and its behavior in
the body. (Are data available that relate exposure to proportional
uptake in hair? Is uptake in hair biologically plausible? Is
it a marker of external exposure?)
- Identifying the clinical relevance of a positive or negative
finding. (Are any dose-response data available that will make
useful interpretations possible?)
The panel provided this specific input on when hair analysis
can be useful:
- From an exposure perspective, hair analysis can be useful
for simply identifying or confirming exposures. Issues raised
or reiterated included (1) the difficulty in distinguishing
between internal and external contamination, (2) the qualitative
nature of any such finding, (3) the inability to confirm the
source of the substance under study, (4) the dilemma of not
being able to "take it to the next step" (i.e., to use the results
as a clinical tool). To overcome issues 1 through 3, it was
noted, it may be more feasible for some substances to confirm
the contamination source (e.g., based on the specific signature
of the substance[s] of interest). Also, more sophisticated studies
(e.g., looking at stable isotopes of certain metals) may now
be possible (TC, MK, SS, LW).
- According to the current science, the primary utility of
hair analysis is as a measure of historical exposure. The research
focus needs to be on seeking data that establish dose-response
- From a clinical perspective, the following conditions must
be satisfied before hair analysis can be viewed as a reliable
means to measure a particular substance: (1) hair contains a
substance concentration that correlates with body organs, tissues,
or fluids; (2) correlates exist and are predictive from a clinical
and/or forensic perspective; and (3) hair can be used reliably
to sample individuals, groups, and/or populations to measure
the substance (RB).
- Theoretically, potential substances for which hair analysis
may be useful include those for which the route of exposure
would limit external contamination and those for which a metabolite
might be measurable (MK).
- Because of general hair growth and cutting patterns, for
exposures longer ago than a year or quite recent, hair analysis
is not useful (RB, LW).
- Depending on the test or element under study, a negative
test can help to rule out an exposure and any potential problem.
Again, "negative" needs to be defined. That is, what is elevated
(RB, MK, TC, LW)?
- Before considering hair analysis, a practical consideration is questioning whether there are any laboratories available that provide cost-effective services and reliable results (DP).