Hair Analysis Panel Discussion: Section 5.2
5.2 Dose-Response and Clinical Relevance
The panelists concurred that relationships between hair and any kind of measurable outcome have only been established for methyl mercury and arsenic. The relationship between maternal hair and fetal brain levels of methyl mercury is the only well-documented hair/target tissue relationship; one panelist pointed to the benchmark dose of methyl mercury of 11 ppm in hair established by EPA2 (RB, SS, TC, MK). Data for arsenic relate largely to forensic examinations; data do not exist for arsenic that offer disease-predictive value (e.g., long-term health outcomes). The group could not identify any other environmental substances for which any hard and fast clinical relationship has been established. Dose-response curves simply do not exist.
Panel discussions regarding current knowledge and the implications for using hair analysis are highlighted below:
Can hair analysis predict cancer and other common community
health concerns? Common community health concerns relate to
health outcomes such as cancer and birth defects (according to Dr.
Harkins, ATSDR). Questions relate to what harm may have been done
or what future risk may exist as a result of environmental exposures.
One panelist stated that it is not likely for hair analysis to be used to any large extent to address public health or individual concerns related to teratology or carcinogenicity (MG). This panelist did note, however, that current efforts to measure benzene in hair might in the future provide some predictive value for aplastic anemia, but only because of the known association between benzene and aplastic anemia. Another panelist re-emphasized that hair only provides an approximate 1-year time frame in terms of possible exposures, further supporting the conclusion that hair analysis has little predictive value in studies of the carcinogenic potential of environmental exposures (RB). Judging from the current understanding of underlying science (particularly for carcinogens), another panelist commented, he would rather have exposure history instead of hair analysis data (DP).
Importance of establishing a clinical basis prior to testing. A fair amount of discussion occurred regarding the criticality of establishing a clinical basis before pursuing hair analysis. Several panelists questioned the relevance of measured levels if they cannot be used to predict health endpoints. As in other discussions, the dichotomy of using hair analysis as an exposure tool versus a clinical tool was very evident.
The physicians on the panel strongly stated that a clinical basis must be established before hair analysis can be considered a useful tool. One panelist stressed that one should not collect data that one is not prepared to use (RB). In response to an acknowledgment that a community might press for hair analysis—for example, even in the absence of supportable scientific data—two panelists were emphatic that science must be the focus: politics, litigation, and any other underlying agendas must be put aside (RB, MG). In general, one must consider what doses, under what circumstances, are relevant (RB). Part of the challenge lies in communicating to the public what the current science enables us to do. No absolutes exist in toxicology and medicine. The exposure, the form, the presentation, and the distribution must be placed in the right context (RB).
Another panelist strongly stated that the predictive value of the test result must be weighed and communicated. He emphasized that should the science clearly show no plausible correlation for a particular substance or exposure situation, then hair analysis should not be considered (MK).
One panelist reiterated that in the absence of dose-response
data, hair analysis may simply give us a better sense of exposure;
it "raises some suspicion" of possible exposure and effects (TC).
Measurements of particular substances in hair may be indicative
of exposure, but not the risk of disease (LW).
Understanding the function of various elements in hair.
In order to ultimately understand dose-response relationships and
the clinical significance of exposures, scientists need a better
understanding of the role of various elements in the hair. Two panelists
briefly commented on the basic lack of understanding of the function,
if any, of metals, cations, etc., in hair. From a practical point
of view, keratinized cells "are on their way out" with the purpose
of protecting the skin and providing warmth. It is therefore difficult
to determine the biological meaning of individual components in
hair. Some elements maintain homeostasis (e.g. potassium). Other
elements are co-factors in synthesis (e.g., chromium in collagen
synthesis). Some elements, on the other hand, are ubiquitous and
have no known purpose (e.g., lead, uranium) (RB, LW).
Substances for which hair analysis might prove useful.
Panelists provided a couple of examples of other elements for which
hair analysis may hold some promise. The panelists agreed that if
strong hypotheses exist, the scientific merit of these types of
relationships may be worth pursuing (RB, DP, TC, SS).
Thallium might be useful in hair because it is an unusual toxicant. (Thallium was suggested based on a "classic picture" of thallium intoxication studied by CDC in Florida.) (DP)
The possible correlation between excessive manganese levels (as measured in hair) and violent and other antisocial behaviors has been studied in incarcerated populations. While study findings suggest some correlation and have some merit on the surface, many potentially confounding factors exist that need to be examined more closely, such as hair color, race, and social context (DP). Panelists questioned the overall scientific merit of the correlation, based on the possible lack of biological plausibility—that is, symptoms are not necessarily consistent with documented health effects associated with manganese (DP). One panelist noted that manganese exposures would more likely be expected to cause neurological effects that lead to more withdrawn or inactive behavior (e.g., Parkinson-like symptoms) (SS). Another panelist noted that, because manganese is an essential trace element, it is reasonable that it will get into hair (TC). Another study, by Bader et al. (1999), showed some correlation between axillary hair and airborne manganese (attributed to contamination by dust and water), but overall did not support the use of hair for manganese analysis (SS).
2EPA has established a methyl mercury benchmark dose (in maternal hair) of 11 ppm. This is equivalent to 46 to 49 micrograms of methyl mercury per liter of maternal blood; the critical effect is developmental neurological abnormalities in offspring (U.S. EPA 2001).