@Copyright 1987 by The Humana Press Inc. All rights of any nature, whatsoever, reserved.
0163-4984/87/1200-0363502.20
Incorporation of Trace Elements from Environment into the Hair Structure N. LIMIt:AND V. VALKOVI( * Ruder Bo~kovid Institute, POB 1016, 41001 Zagreb, Yugoslavia
ABSTRACT A model describing the incorporation of trace elements from environment into the hair structure is presented. Model predictions for radial and longitudinal concentration profiles are given. Comparison with elemental data is satisfactory. Index Entries: Trace elements, in hair; hair; model of incorporation, of trace elements into hair; concentration profiles, radial and longitudinal.
INTRODUCTION Trace-element composition of hair is a subject that has recently received a lot of attention. There is a large number of research papers and compilations published in this field. References to early works can be found in the book by one of us (1); additional references can be found in review papers and proceedings of topical conferences (2-8). Examining blood and urine provides immense insight into h u m a n diseases. It is natural to hope that the examination of hair would be a d d e d to these examinations routinely. Hair analysis appears to offer a unique approach to the investigation of h u m a n trace-element nutrition and metabolism. There is a strong possibility that the trace-element content of hair correlates with body stores. Analyses of feces and urine are of limited value as indicators of stores, and blood has restricted use because the hemostatic mechanism operates to keep many of the components of blood constant. H u m a n head hair is a recording filament that can reflect metabolic changes of many elements over a long period of time, and thus reflects past nutritional events. The idea of hair analysis is very inviting *Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research
3,63
VoL 12, 1987
364
Limi4 and Valkovi4
because hair is easily sampled, shipped, and analyzed, and concentrations of most of the trace elements in scalp hair are an order of magnitude higher than those in body fluids or other easily accessible tissues. Correlations between trace-element levels in blood and hair are severely confounded if the hair sample is not collected close to the scalp so as to reflect the current metabolic status. Most of the reported studies are characterized by sampling errors, improper sample preparation, and faulty statistical treatment. Without controlling for sample distance from the scalp, these studies showed great variance in hair trace-element levels and very poor correlation with circulating trace-element levels. Samples taken some length from the scalp do not reflect the current metabolic status. Several investigators have shown that trace-element contents vary with geographical location, indicating the importance of environmental effects. We refer to the results of Hammer et al. (9), who studied the concentrations of As, Cd, Cu, Pb, and Zn in the hair of school children in five cities. Their results show that the concentrations of As, Cd, and Pb accurately reflect the exposure to these metals. It seems that Cu and Zn concentrations in hair do not depend on their concentrations in the air, perhaps because of their large concentrations in human nutrition. We have shown (10) in an earlier study that the elements whose concentrations increase along the hair can be identified as pollutants in the area. The concentrations of most trace elements should increase with increasing distance from the scalp if the exposure of hair to the elements of the environment is constant. For widespread pollutants, even the median value of a group of subjects can be used as a measure of the exposure. It was found that elements form two groups: One group comprises elements (Fe, Cu, Sr, Br, Se) whose concentrations do not show marked increase along the hair; and the other group comprises elements (Ni, Pb, As) that have a marked increase in concentrations along a hair. It was noted that dependences of concentration on the distance from the scalp were similar in all three cases. This indicated that elements As, Pb, and Ni are contaminants in the environment of the subject whose hair was analyzed.
TRACE ELEMENT DISTRIBUTIONS IN SINGLE HUMAN HAIRS The factors influencing trace-element levels in human hair can be grouped into four terms: (1) body stores; (2) genetic effects; (3) body fluids; and (4) environment. If the coordinate frame is attached to a single hair (see Fig. 1), with the z-axis along the hair length, then the concentration of any element within the single hair can be expressed as: C(x,y,z) = CG + CBF + CBS + CENV Biological Trace Element Research
(1) Vol. 12, 1987
365
Trace Elements in Hair
Z=V.t
Y
L v
X EPIDERMIS EXTERNAL SHEATH INTER GLAND ECCRINE GLAND HARDENING REGION
MATRIX
Fig. 1. Schematic presentation of a single hair and the coordinate frame attached to it. where Co; represents genetic contribution: CBF represents body fluids contribution; CBs represents body stores contribution; and C~NV represents environmental contribution. The d e p e n d e n c e of trace-element levels on sex, age, race, and the like is incorporated into the genetic term Cc as: Cc = Co(sex, age-hormones, race, body region, color, . . .)
(2)
In a time period represented by change in hair length, this term is constant for an individual. Furthermore, one might define: CG(X,y,z) ---- CG(X,y) = individual normal base value
(3)
The term representing the contributions of body fluids to traceelement levels in h u m a n hairs can be presented as: Biological Trace Element Research
Vol. 12, 1987
366
Limi4 and Valkovi~ CBF =
CBF[KI;tCON; D~(x,y,z); . . - ]
(4)
where, K~ represents concentrations of elements in secretions: sweat (forearm), sebaceous gland (scalp), and sudoriferous (auxilliary) gland; tCON = contact time; and D r. = diffusion coefficients. The term reflecting body stores of a particular trace element can be presented as: CBS =
CBs(KN;KBLOOD; TN)
(5)
where, KN represents body store concentration in compartment N; KBLOODis the concentration of elements in the blood supply; and ~N is the biological half-life of an element in compartment N. This term is not well defined because of the interdependency of the variable involved. The environmental term can be presented as: CENV = CENV (KENv; DENv, particle size; compounds; . . .)
(6)
where, KENV = concentrations of elements in ambient air, shampoo, water, and the like. D ENV = diffusion coefficient. It has already been shown (10) that Eq. (6) is of the form: (7)
CENv(Z ) = az + b
w h e n only z-dependence is observed. The following observables can be measured: (1) Longitudinal profile for element: C(z) =
fC(x,y,z)&@
(8)
(2) Radial profile for element: In most cases, radial symmetry for single hair can be assumed, then:
(9)
C(x,y) = x,z = const) = C(r)
(3) Total element in hair segment for element: (10)
CT = fC(x,y,z)dxdydz
where, CT is the total element for atomic absorption and neutron activation measurement comparison. (4) Concentration ratios: c(zl)
-
C Nv(Z,)
C(z2) or C(z2) -
c(zl)
CENv(Z2)
(11)
where C(z2) is taken at position z2 and considered to be an individual normal value. Biological Trace Element Research
VoL 12, 1987
Trace Elements in Hair
367
Measurements of longitudinal and cross-sectional profiles can be performed using proton-induced X-ray emission spectroscopy (PIXE) as an analytical tool. Proton beams can be focussed down to 2 x 2 ~rn 2 spot, and up to 50 points can be measured for the cross-sectional profile. Longitudinal profiles can also be measured easily using PIXE or XRF as an analytical tool.
SOME EXPERIMENTAL RESULTS The model described in the previous paragraphs contains contributions of trace elements from the body stores (the most interesting term, because, once isolated, hair can be used as a personal trace-element monitor), body fluids, and environment, togefher with a genetic term that contains biological variations. In order to test the validity of the model, PIXE measurements with the proton microbeam were performed to check contributions of the different terms (by, i.e., excessive washing procedures and the like) (11-13). In the study of washing procedure effects, hair segments of individual hairs were treated with shampoo mix. The elements present in the shampoo mix were: S, C1, K, Ca, Fe, Zn, and Se; their concentrations and pH values were determined prior to the experiments. Two concentration values were used: C1 and C2. For C7, concentrations were: S, 15%; C1, 1%; K, 350 ppm; Ca, 230 ppm; Fe, 220 ppm; Zn, 320 ppm; and Se, 1400 ppm. For C2, double concentrations of all elements were used. As an example of the obtained experimental results, the positional distributions for Zn and Se of hair segments treated with shampoo mix are shown in Fig. 2. The nontreated hair segment does not show any Se, whereas the one with the longest contact time shows elevated Se levels even in the center of the hair. Clearly, there is an Se. uptake from the shampoo that contains 1400 ppm of Se. Contrary to this, the Zn distributions are not changed; it is present in the nontreated segment in a homogeneous distribution. However, close examination shows that after a 48-h contact time, a slight flow away from the edges of the hair has taken place, despite the fact that Zn concentration in the mix is higher (320 ppm).
INCORPORATION OF ELEMENTS INTO HAIR STRUCTURE In order to explain the process of the incorporation of trace elements from the environment into hair structure, we have constructed a model based on the following assumptions: (1) There is no transport of elements along the length of hair (along z-axis); and (2) the transport of elements is symmetrical around the z-axis. Under these assumptions, the flux of an element for a given radius is proportional to the gradient of saturation. The proportionality constant Biological Trace Element Research
VoL 12. 1987
Limid and Valkovid
368
Zn
Se
aoc
e: 20(; Z Z 0
10C
0
C
-
v
POSITION/pm Fig. 2. Positional distributions of Zn and Se across the diameter of hair segments brought in contact with shampoo mix with contact time as parameter [refs. (I1-I.3)]. will d e p e n d on the radius. Because of hair structure, the saturation values can also d e p e n d on the radius. This can be expressed as: 8S T= -•(r,S) -~7
K(r,S) > 0
where, T is the flux of an element, S is the saturation value, and hair permeability. Using the continuity law, one obtains: !',:: o(r)S
=
~ 71 8-r
IrK(S) -87 6S]
(12)
K(r,S) is
(13)
and the axial symmetry implies 8S (r = 0) = 0 ~r
(14)
where p represents the maximum value for the concentration of a given element in hair and p(r)S = C; C is the concentration of an element resulting from the environmental contribution, i.e., C = CENVfrom Eq. (1). Equations (12)-(14) describe the dynamics of element transport within the hair structure. Biological Trace Element Research
Vol. 12, 1987
Trace Elements in Hair
369
It is more difficult to describe the transfer of elements between the e n v i r o n m e n t and hair structure; this is influenced mainly by b o u n d a r y conditions. In the case of element incorporation into hair structure, one deals with chemical diffusion, therefore, one has to take into consideration the surface potentials. As a result, the flux of an element from the e n v i r o n m e n t into hair does not necessarily need to be proportional to concentration differences. It can be zero even for significant concentration differences. A general law can be formulated as: T ~- ( I ) ( C , S , K E N v , S F ~ N V )
(15)
w h e r e T is the flux of element on the hair surface; C is the element concentration in hair; and KENVis the element concentration in the environment. This is a very general consideration since neither function K or a/) is known. N o w we shall discuss the incorporation into hair structure of elements like lead and selenium. The same approach can be used for a number of other elements entering hair structure from the environment, i.e., Ca, Fe, As . . . . . For the case of small saturation level functions, K and @ are constants, i.e., the Taylor series expansion gives: K(r,S) = /)(r) + S/),(r) + S2 /)(r) + . . . T(S) = ~ + s~r, + s 2 L
+ ...
(16)
and for small s:
~(r,s) ~ D(r) (17) Then:
PS -
~ a s 71 ~-~D(r)r 8r
(lSa)
1 8 D(r) 8 C r ~r p ~r
(18b)
or:
d-
w h e r e /)(r)/p = D t'Nv from Eq. (6). In addition, the following conditions should be satisfied:
ac (r
O)
0
(19)
8C (r ~r
r{~)
c~
(20)
~r
Biological Trace Element Research
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Limi~ and Valkovi4
370
where, as mentioned earlier, the axial symmetry implies Eq. (19), and the boundary conditions of transfer of an element with a constant flux is represented by Eq. (20). A simple law [Eqs. (18)-(20)] of hair contamination has some implications on the distribution of element concentration in the z-direction. Since the main quantity for a description of this distribution is the total element concentration over the whole profile of hair, we integrate C over the hair profile, then: I"
C(z = Zo) = 2~f ~ rC(z = zo, r)dr
(21)
o
and obtain the following equation:
C(z = z0) = constant
(22)
from Eqs. (18b)-(20). Let us mention that the constant value of the righthand side of Eq. (22) is a consequence of the approximation of Eq. (17) being valid for small saturation levels. In general, the right-hand side of Eq. (22) is a function of the concentrations and saturations in both hair and environment, and decreases as the saturation level of elements in the hair increases. Therefore, the simple law [Eq. (22)] of the element concentration along the side of hair is a fair approximation in the case of a small saturation level.
COMPARISON WITH EXPERIMENTAL RESULTS Experimental data from refs. (11) and (12) for Pb radial concentration profiles will be composed with model predictions. In this case it is assumed that flux is constant, i.e., the concentrations of an element in hair is small and flux is defined by the concentration of a given element in the environment. Since the number of data is limited, we shall not try to determine D(r) as a function of r. It will be assumed that D(r) is constant; then: d = D---lr ~ r ~ - aC a
(23)
aC (r = 0) = 0
(24)
5 C ( r = r0) - T 8r roD
(25)
ar
In addition, the results of calculations need to be smeared for the proton beam spot size ( - 6 ixm). In the fitting procedure to parameters, the following are adjusted: flux T and diffusion constant D. The solution deBiological Trace Element Research
Vol. 12, 1987
Trace Elements in Hair
37 ]
[LEAD(Pb!! / I
i I
\ \
\
\
II I
2 c m off root
/
75-
9
Z
\ \ \ \
P
/
O r 9
\ \
I I I
50-
\
/
\
I
I
I
//
25-
I
/
/
/
/
/
/
I
/
/
/
/
9
9
I
O~
\ \
I I
\
I
\
3~ -
2'/-,
A
1'6'
'
~}
'
\
2~ "
1~'
S'2
75-
Icrn off root
if)
~" 500
/
/
\
9 \
I /
I
I
I
~2 " 2~.
/
\
25-
\
/ \
\ x
\@
\
.I 9
1~
/
/
/~\ I
~ HAIR
,
0
~
e
DIAMETER
,
k ,~
,
1~
\
2~,
32
4
pm Fig. 3. Distribution of Pb concentration values across hair diameter in different segments of the single hair (d = 1 cm and d = 2 cm of the root). The hair donor was a professional lead melter working in a small battery factory [from ref. (12)]. The calculated curves are model predictions. Biological Trace Element Research
Vol. 12, 1987
372
Limid a n d Valkovi4
pendence on T is linear, whereas its d e p e n d e n c e on D is nonlinear. Therefore, the fitting procedure for T is trivial, and for D the gradient m e t h o d was used to find the m i n i m u m deviation. Figure 3 shows the experimental data [refs. (11) and (12)] and calculated shapes for radial concentration profiles for lead at I and 2 cm from the hair root. Both theoretical curves are fitted with the same D and T values. The numerical value for T is meaningless because only the shape of distribution is experimentally determined (the intensities are given in counts). The numerical value for D is: D = 0.11
~m2/Uof
time
(26)
where unit of time corresponds to the time interval n e e d e d for hair to grow 1 cm (this can be different for different individuals).
CONCLUSION Using the approach described in this article, one could, in principle, differentiate between various contributions to trace-element concentration levels in h u m a n hair. The measurements of radial and longitudinal concentration profiles in single hairs is n e e d e d in order to estimate different contributions to hair trace-element levels and the use of h u m a n hair as a diagnostic tool in the assessment of body stores and metabolic changes. The model discussed here should be refined on the basis of future measurements of radial and longitudinal concentration profiles.
REFERENCES 1. V. ValkoviG Trace Elements in Human Hair, Garland Publishing, New York, NY, 1977. 2. M. Anke and M. Risch, Haaranalyse und Spurenelementen Status, Gustav Fischer Verlag, Jena, 1979. 3. A. C. Brown and R. G. Crounse, eds., Hair, Trace Elements and Human Illness, Proc. of the Second Human Hair Symposium, Atlanta, Georgia, Oct. 13-15, 1978 (Preager, New York, NY, 1980). 4. G. Chittleborough, Sci. Total Environ. 14, 53 (1980). 5. J. W. Copins Peereboom, Trace Elements, Health and Hair Analysis, Proc. Int. Symp., Amstedam, 1982. 6. R. S. Gibson, J. Hum. Natur. 34, 405 (1980). 7. C. A. Pankhurst and B. D. Pate, CRC Crit. Revs. Anal. Chem. IV (no. 2 and 3), 1979, pp. 111-235. 8. V. ValkoviG 1. Appl. Cosmetol. 2, 28 (1984). 9. D. I. Hammer, J. F. Finkles, R. H. Hendricks, and C. M. Shy, Am. EpidemioI. 93, 84 (1971).
Biological Trace ElementResearch
VoL 12, 1987
Trace Elements in Hair
373
10. V. Valkovid, D. Rendid, and G. C. Phillips, Environ. Sci. Technol. 9, 1150 (1975). 1t. A. J. J. Bos, R. D. Vis, W. J. M. Lenglet, H. Verheul, V. Valkovid, and J. Makjanid, Proc. Second International Workshop on Trace Element Analytical Chemistry in Medicine and Biology (P. Br/itter and P. Schramel, eds.), Walker de Gruyters, Berlin, New York, 1983, p. 787. 12. A. J. J. Bos, Ph.D. Thesis, Free University, Amsterdam, 1984. 13. A. J. J. Bos, C. C. A. H. v. d. Stap, R. D. Vis, and V. Valkovid, Spectrochem. Acta, 9, 1209 (1983).
Biological Trace Element Research
Vol. 12, 1987
0163-4984/87/1200-0363502.20
Incorporation of Trace Elements from Environment into the Hair Structure N. LIMIt:AND V. VALKOVI( * Ruder Bo~kovid Institute, POB 1016, 41001 Zagreb, Yugoslavia
ABSTRACT A model describing the incorporation of trace elements from environment into the hair structure is presented. Model predictions for radial and longitudinal concentration profiles are given. Comparison with elemental data is satisfactory. Index Entries: Trace elements, in hair; hair; model of incorporation, of trace elements into hair; concentration profiles, radial and longitudinal.
INTRODUCTION Trace-element composition of hair is a subject that has recently received a lot of attention. There is a large number of research papers and compilations published in this field. References to early works can be found in the book by one of us (1); additional references can be found in review papers and proceedings of topical conferences (2-8). Examining blood and urine provides immense insight into h u m a n diseases. It is natural to hope that the examination of hair would be a d d e d to these examinations routinely. Hair analysis appears to offer a unique approach to the investigation of h u m a n trace-element nutrition and metabolism. There is a strong possibility that the trace-element content of hair correlates with body stores. Analyses of feces and urine are of limited value as indicators of stores, and blood has restricted use because the hemostatic mechanism operates to keep many of the components of blood constant. H u m a n head hair is a recording filament that can reflect metabolic changes of many elements over a long period of time, and thus reflects past nutritional events. The idea of hair analysis is very inviting *Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research
3,63
VoL 12, 1987
364
Limi4 and Valkovi4
because hair is easily sampled, shipped, and analyzed, and concentrations of most of the trace elements in scalp hair are an order of magnitude higher than those in body fluids or other easily accessible tissues. Correlations between trace-element levels in blood and hair are severely confounded if the hair sample is not collected close to the scalp so as to reflect the current metabolic status. Most of the reported studies are characterized by sampling errors, improper sample preparation, and faulty statistical treatment. Without controlling for sample distance from the scalp, these studies showed great variance in hair trace-element levels and very poor correlation with circulating trace-element levels. Samples taken some length from the scalp do not reflect the current metabolic status. Several investigators have shown that trace-element contents vary with geographical location, indicating the importance of environmental effects. We refer to the results of Hammer et al. (9), who studied the concentrations of As, Cd, Cu, Pb, and Zn in the hair of school children in five cities. Their results show that the concentrations of As, Cd, and Pb accurately reflect the exposure to these metals. It seems that Cu and Zn concentrations in hair do not depend on their concentrations in the air, perhaps because of their large concentrations in human nutrition. We have shown (10) in an earlier study that the elements whose concentrations increase along the hair can be identified as pollutants in the area. The concentrations of most trace elements should increase with increasing distance from the scalp if the exposure of hair to the elements of the environment is constant. For widespread pollutants, even the median value of a group of subjects can be used as a measure of the exposure. It was found that elements form two groups: One group comprises elements (Fe, Cu, Sr, Br, Se) whose concentrations do not show marked increase along the hair; and the other group comprises elements (Ni, Pb, As) that have a marked increase in concentrations along a hair. It was noted that dependences of concentration on the distance from the scalp were similar in all three cases. This indicated that elements As, Pb, and Ni are contaminants in the environment of the subject whose hair was analyzed.
TRACE ELEMENT DISTRIBUTIONS IN SINGLE HUMAN HAIRS The factors influencing trace-element levels in human hair can be grouped into four terms: (1) body stores; (2) genetic effects; (3) body fluids; and (4) environment. If the coordinate frame is attached to a single hair (see Fig. 1), with the z-axis along the hair length, then the concentration of any element within the single hair can be expressed as: C(x,y,z) = CG + CBF + CBS + CENV Biological Trace Element Research
(1) Vol. 12, 1987
365
Trace Elements in Hair
Z=V.t
Y
L v
X EPIDERMIS EXTERNAL SHEATH INTER GLAND ECCRINE GLAND HARDENING REGION
MATRIX
Fig. 1. Schematic presentation of a single hair and the coordinate frame attached to it. where Co; represents genetic contribution: CBF represents body fluids contribution; CBs represents body stores contribution; and C~NV represents environmental contribution. The d e p e n d e n c e of trace-element levels on sex, age, race, and the like is incorporated into the genetic term Cc as: Cc = Co(sex, age-hormones, race, body region, color, . . .)
(2)
In a time period represented by change in hair length, this term is constant for an individual. Furthermore, one might define: CG(X,y,z) ---- CG(X,y) = individual normal base value
(3)
The term representing the contributions of body fluids to traceelement levels in h u m a n hairs can be presented as: Biological Trace Element Research
Vol. 12, 1987
366
Limi4 and Valkovi~ CBF =
CBF[KI;tCON; D~(x,y,z); . . - ]
(4)
where, K~ represents concentrations of elements in secretions: sweat (forearm), sebaceous gland (scalp), and sudoriferous (auxilliary) gland; tCON = contact time; and D r. = diffusion coefficients. The term reflecting body stores of a particular trace element can be presented as: CBS =
CBs(KN;KBLOOD; TN)
(5)
where, KN represents body store concentration in compartment N; KBLOODis the concentration of elements in the blood supply; and ~N is the biological half-life of an element in compartment N. This term is not well defined because of the interdependency of the variable involved. The environmental term can be presented as: CENV = CENV (KENv; DENv, particle size; compounds; . . .)
(6)
where, KENV = concentrations of elements in ambient air, shampoo, water, and the like. D ENV = diffusion coefficient. It has already been shown (10) that Eq. (6) is of the form: (7)
CENv(Z ) = az + b
w h e n only z-dependence is observed. The following observables can be measured: (1) Longitudinal profile for element: C(z) =
fC(x,y,z)&@
(8)
(2) Radial profile for element: In most cases, radial symmetry for single hair can be assumed, then:
(9)
C(x,y) = x,z = const) = C(r)
(3) Total element in hair segment for element: (10)
CT = fC(x,y,z)dxdydz
where, CT is the total element for atomic absorption and neutron activation measurement comparison. (4) Concentration ratios: c(zl)
-
C Nv(Z,)
C(z2) or C(z2) -
c(zl)
CENv(Z2)
(11)
where C(z2) is taken at position z2 and considered to be an individual normal value. Biological Trace Element Research
VoL 12, 1987
Trace Elements in Hair
367
Measurements of longitudinal and cross-sectional profiles can be performed using proton-induced X-ray emission spectroscopy (PIXE) as an analytical tool. Proton beams can be focussed down to 2 x 2 ~rn 2 spot, and up to 50 points can be measured for the cross-sectional profile. Longitudinal profiles can also be measured easily using PIXE or XRF as an analytical tool.
SOME EXPERIMENTAL RESULTS The model described in the previous paragraphs contains contributions of trace elements from the body stores (the most interesting term, because, once isolated, hair can be used as a personal trace-element monitor), body fluids, and environment, togefher with a genetic term that contains biological variations. In order to test the validity of the model, PIXE measurements with the proton microbeam were performed to check contributions of the different terms (by, i.e., excessive washing procedures and the like) (11-13). In the study of washing procedure effects, hair segments of individual hairs were treated with shampoo mix. The elements present in the shampoo mix were: S, C1, K, Ca, Fe, Zn, and Se; their concentrations and pH values were determined prior to the experiments. Two concentration values were used: C1 and C2. For C7, concentrations were: S, 15%; C1, 1%; K, 350 ppm; Ca, 230 ppm; Fe, 220 ppm; Zn, 320 ppm; and Se, 1400 ppm. For C2, double concentrations of all elements were used. As an example of the obtained experimental results, the positional distributions for Zn and Se of hair segments treated with shampoo mix are shown in Fig. 2. The nontreated hair segment does not show any Se, whereas the one with the longest contact time shows elevated Se levels even in the center of the hair. Clearly, there is an Se. uptake from the shampoo that contains 1400 ppm of Se. Contrary to this, the Zn distributions are not changed; it is present in the nontreated segment in a homogeneous distribution. However, close examination shows that after a 48-h contact time, a slight flow away from the edges of the hair has taken place, despite the fact that Zn concentration in the mix is higher (320 ppm).
INCORPORATION OF ELEMENTS INTO HAIR STRUCTURE In order to explain the process of the incorporation of trace elements from the environment into hair structure, we have constructed a model based on the following assumptions: (1) There is no transport of elements along the length of hair (along z-axis); and (2) the transport of elements is symmetrical around the z-axis. Under these assumptions, the flux of an element for a given radius is proportional to the gradient of saturation. The proportionality constant Biological Trace Element Research
VoL 12. 1987
Limid and Valkovid
368
Zn
Se
aoc
e: 20(; Z Z 0
10C
0
C
-
v
POSITION/pm Fig. 2. Positional distributions of Zn and Se across the diameter of hair segments brought in contact with shampoo mix with contact time as parameter [refs. (I1-I.3)]. will d e p e n d on the radius. Because of hair structure, the saturation values can also d e p e n d on the radius. This can be expressed as: 8S T= -•(r,S) -~7
K(r,S) > 0
where, T is the flux of an element, S is the saturation value, and hair permeability. Using the continuity law, one obtains: !',:: o(r)S
=
~ 71 8-r
IrK(S) -87 6S]
(12)
K(r,S) is
(13)
and the axial symmetry implies 8S (r = 0) = 0 ~r
(14)
where p represents the maximum value for the concentration of a given element in hair and p(r)S = C; C is the concentration of an element resulting from the environmental contribution, i.e., C = CENVfrom Eq. (1). Equations (12)-(14) describe the dynamics of element transport within the hair structure. Biological Trace Element Research
Vol. 12, 1987
Trace Elements in Hair
369
It is more difficult to describe the transfer of elements between the e n v i r o n m e n t and hair structure; this is influenced mainly by b o u n d a r y conditions. In the case of element incorporation into hair structure, one deals with chemical diffusion, therefore, one has to take into consideration the surface potentials. As a result, the flux of an element from the e n v i r o n m e n t into hair does not necessarily need to be proportional to concentration differences. It can be zero even for significant concentration differences. A general law can be formulated as: T ~- ( I ) ( C , S , K E N v , S F ~ N V )
(15)
w h e r e T is the flux of element on the hair surface; C is the element concentration in hair; and KENVis the element concentration in the environment. This is a very general consideration since neither function K or a/) is known. N o w we shall discuss the incorporation into hair structure of elements like lead and selenium. The same approach can be used for a number of other elements entering hair structure from the environment, i.e., Ca, Fe, As . . . . . For the case of small saturation level functions, K and @ are constants, i.e., the Taylor series expansion gives: K(r,S) = /)(r) + S/),(r) + S2 /)(r) + . . . T(S) = ~ + s~r, + s 2 L
+ ...
(16)
and for small s:
~(r,s) ~ D(r) (17) Then:
PS -
~ a s 71 ~-~D(r)r 8r
(lSa)
1 8 D(r) 8 C r ~r p ~r
(18b)
or:
d-
w h e r e /)(r)/p = D t'Nv from Eq. (6). In addition, the following conditions should be satisfied:
ac (r
O)
0
(19)
8C (r ~r
r{~)
c~
(20)
~r
Biological Trace Element Research
Vol. 12, 1987
Limi~ and Valkovi4
370
where, as mentioned earlier, the axial symmetry implies Eq. (19), and the boundary conditions of transfer of an element with a constant flux is represented by Eq. (20). A simple law [Eqs. (18)-(20)] of hair contamination has some implications on the distribution of element concentration in the z-direction. Since the main quantity for a description of this distribution is the total element concentration over the whole profile of hair, we integrate C over the hair profile, then: I"
C(z = Zo) = 2~f ~ rC(z = zo, r)dr
(21)
o
and obtain the following equation:
C(z = z0) = constant
(22)
from Eqs. (18b)-(20). Let us mention that the constant value of the righthand side of Eq. (22) is a consequence of the approximation of Eq. (17) being valid for small saturation levels. In general, the right-hand side of Eq. (22) is a function of the concentrations and saturations in both hair and environment, and decreases as the saturation level of elements in the hair increases. Therefore, the simple law [Eq. (22)] of the element concentration along the side of hair is a fair approximation in the case of a small saturation level.
COMPARISON WITH EXPERIMENTAL RESULTS Experimental data from refs. (11) and (12) for Pb radial concentration profiles will be composed with model predictions. In this case it is assumed that flux is constant, i.e., the concentrations of an element in hair is small and flux is defined by the concentration of a given element in the environment. Since the number of data is limited, we shall not try to determine D(r) as a function of r. It will be assumed that D(r) is constant; then: d = D---lr ~ r ~ - aC a
(23)
aC (r = 0) = 0
(24)
5 C ( r = r0) - T 8r roD
(25)
ar
In addition, the results of calculations need to be smeared for the proton beam spot size ( - 6 ixm). In the fitting procedure to parameters, the following are adjusted: flux T and diffusion constant D. The solution deBiological Trace Element Research
Vol. 12, 1987
Trace Elements in Hair
37 ]
[LEAD(Pb!! / I
i I
\ \
\
\
II I
2 c m off root
/
75-
9
Z
\ \ \ \
P
/
O r 9
\ \
I I I
50-
\
/
\
I
I
I
//
25-
I
/
/
/
/
/
/
I
/
/
/
/
9
9
I
O~
\ \
I I
\
I
\
3~ -
2'/-,
A
1'6'
'
~}
'
\
2~ "
1~'
S'2
75-
Icrn off root
if)
~" 500
/
/
\
9 \
I /
I
I
I
~2 " 2~.
/
\
25-
\
/ \
\ x
\@
\
.I 9
1~
/
/
/~\ I
~ HAIR
,
0
~
e
DIAMETER
,
k ,~
,
1~
\
2~,
32
4
pm Fig. 3. Distribution of Pb concentration values across hair diameter in different segments of the single hair (d = 1 cm and d = 2 cm of the root). The hair donor was a professional lead melter working in a small battery factory [from ref. (12)]. The calculated curves are model predictions. Biological Trace Element Research
Vol. 12, 1987
372
Limid a n d Valkovi4
pendence on T is linear, whereas its d e p e n d e n c e on D is nonlinear. Therefore, the fitting procedure for T is trivial, and for D the gradient m e t h o d was used to find the m i n i m u m deviation. Figure 3 shows the experimental data [refs. (11) and (12)] and calculated shapes for radial concentration profiles for lead at I and 2 cm from the hair root. Both theoretical curves are fitted with the same D and T values. The numerical value for T is meaningless because only the shape of distribution is experimentally determined (the intensities are given in counts). The numerical value for D is: D = 0.11
~m2/Uof
time
(26)
where unit of time corresponds to the time interval n e e d e d for hair to grow 1 cm (this can be different for different individuals).
CONCLUSION Using the approach described in this article, one could, in principle, differentiate between various contributions to trace-element concentration levels in h u m a n hair. The measurements of radial and longitudinal concentration profiles in single hairs is n e e d e d in order to estimate different contributions to hair trace-element levels and the use of h u m a n hair as a diagnostic tool in the assessment of body stores and metabolic changes. The model discussed here should be refined on the basis of future measurements of radial and longitudinal concentration profiles.
REFERENCES 1. V. ValkoviG Trace Elements in Human Hair, Garland Publishing, New York, NY, 1977. 2. M. Anke and M. Risch, Haaranalyse und Spurenelementen Status, Gustav Fischer Verlag, Jena, 1979. 3. A. C. Brown and R. G. Crounse, eds., Hair, Trace Elements and Human Illness, Proc. of the Second Human Hair Symposium, Atlanta, Georgia, Oct. 13-15, 1978 (Preager, New York, NY, 1980). 4. G. Chittleborough, Sci. Total Environ. 14, 53 (1980). 5. J. W. Copins Peereboom, Trace Elements, Health and Hair Analysis, Proc. Int. Symp., Amstedam, 1982. 6. R. S. Gibson, J. Hum. Natur. 34, 405 (1980). 7. C. A. Pankhurst and B. D. Pate, CRC Crit. Revs. Anal. Chem. IV (no. 2 and 3), 1979, pp. 111-235. 8. V. ValkoviG 1. Appl. Cosmetol. 2, 28 (1984). 9. D. I. Hammer, J. F. Finkles, R. H. Hendricks, and C. M. Shy, Am. EpidemioI. 93, 84 (1971).
Biological Trace ElementResearch
VoL 12, 1987
Trace Elements in Hair
373
10. V. Valkovid, D. Rendid, and G. C. Phillips, Environ. Sci. Technol. 9, 1150 (1975). 1t. A. J. J. Bos, R. D. Vis, W. J. M. Lenglet, H. Verheul, V. Valkovid, and J. Makjanid, Proc. Second International Workshop on Trace Element Analytical Chemistry in Medicine and Biology (P. Br/itter and P. Schramel, eds.), Walker de Gruyters, Berlin, New York, 1983, p. 787. 12. A. J. J. Bos, Ph.D. Thesis, Free University, Amsterdam, 1984. 13. A. J. J. Bos, C. C. A. H. v. d. Stap, R. D. Vis, and V. Valkovid, Spectrochem. Acta, 9, 1209 (1983).
Biological Trace Element Research
Vol. 12, 1987
