Hair Analysis Panel Discussion: Section 4.2
4.2 Temporal Considerations and Exposure Conditions
Panelists agreed that, in determining whether to use hair analysis and in interpreting analytical results, one must carefully consider exposure chronology and conditions. Because hair growth is a factor in evaluating when a contaminant might become incorporated in the hair, one must consider it when deciding whether sampling hair will identify exposures over the period of interest. With regard to this, the panelists discussed these topics:
Window of exposure that hair levels may represent. Growth
rate is a key consideration. Assuming growth at approximately 1
centimeter a month, the hair on the average person's head generally
represents a year or less of time. Therefore, hair analysis is not
the best biological medium to serve as an indicator of very recent
exposure or past exposures (greater than 1 year) (RB).
Using segmental analysis to study exposures over time.
If hair is looked at in a micro or segmental way, temporal patterns
of exposure may be identified. Understanding when exposure
might have occurred may be useful in documenting some historic exposures.
As mentioned in Section 3, neutron activation
analysis has been used to identify isolated elevations along small
segments of a hair (e.g, millimeter[s] in length) (MK).
Segmental analysis has been shown to find isolated arsenic peaks at distal points along the hair shaft. For example, studies of past acute suicidal exposures to arsenic show distinct peaks migrating away from the scalp (Leslie and Smith 1978). Such analysis can reveal past exposures even when current urinary levels are normal. Curry and Pounds (1977) demonstrated peak concentrations of arsenic in hair migrating away from the scalp following the administration of medicinal arsenic (1 hour to 72 days after ingestion).
Segmental analysis may help ATSDR scientists identify past
elevated exposures (e.g., acute high exposures from a spill event).
Segmental analysis may also rule out exposures. Houtman et al. (1978),
for example, studied hair in a population exposed to an accidental
release of arsenic dust. Segmental hair analysis revealed that concentrations
on the distal parts of hairs were elevated. However, it was determined
that the higher levels were detected on portions of hair that would
have been fully formed before the accident, thus establishing that
the arsenic in hair was the result of external contamination. In
some settings, a relatively uniform distribution of a metal such
as arsenic along the length of sampled hair can reflect relatively
stable, chronic ingestion, but even in those settings the contribution
of external contamination cannot always be readily determined (MK).
The challenge of using segmental analysis to demonstrate exposure patterns is that it requires techniques that will enable the analysis of small quantities of hair (e.g., subcentimeter sections). It also requires collection of hair in a careful way, to preserve the orientation of the hair. Further, it has been shown that uptake of arsenic—even on deliberate external contamination—was not uniform. It has been hypothesized that the use of shampoos might account for the uneven distribution. This observation might limit the interpretation of segmental analysis for measuring patterns of endogenous levels (MK).
It was also noted that concentration increases towards the tip
of the hair because it is exposed longer. This pattern is typical
with external lead exposures. Increased concentration toward the
tip is a useful clue regarding the extent of external contamination
Understanding exposure conditions/histories. Panelists
suggested obtaining complete exposure histories as part of any hair
analysis evaluation. A clinician or health assessor needs to understand
the exposure situation and work within a framework of knowing when
data may have a valid use. Using an exposure questionnaire
as part of any hair analysis exercise will help the clinician/assessor
identify sources of exposure, both site- and non-site related. Such
information will ultimately help the assessor put available data
into perspective (DP, TC, SS).
Age. The age of the individual or population tested can affect the results and interpretation of hair analysis. Studies suggest, for example, that alkaline earths and zinc are not excreted as much in early years of life. The opposite is true with aluminum, of which children excrete higher levels than adults (Paschal 1989). When skeletal growth stops, the excretion of these substances into hair is relatively constant. As part of its National Health and Nutrition Examination Survey 99+ (NHANES), CDC studied mercury levels in the hair of children and women of childbearing age. Data suggest that children had lower mercury levels than adults (CDC 2001a; CDC 2001b) (DP).