Salud Pública de México

 Determining lead sources in Mexico using the lead isotope ratio

Determining lead sources in Mexico using the lead isotope ratio


Madhu Chaudhary-Webb, MS,(1) Daniel C. Paschal, Ph D, (1)(5) Bill Ting, Ph D,(1) Crawford Elliot, Ph D,(2) Harry Hopkins, Ph D,(3) Luz Helena Sanín, MD, MPH,(4) Mahamad A. Ghazi, Ph D.(2)

(1) Centers for Diease Control and Prevention, División of Environmetal Healt Laboratory Sciences, Atlanta (GA). 30341, United States of america (USA).
(2)Departament of Geology, Georgia State University, University Plaza, Atlanta (GA). 30309, USA.
(3) Departament of Chemistry, Georgia State University, University Plaza, Atlanta (GA). 30309 USA.
(4) Instituto Nacional de Salud Pública, Cuernavaca, Morelos, and Universidad Autónoma de Chihuahua, Chihuahua, México.
(5) Pan American Healt Organization, México, DF, México. Isabelle Romieu, MD, Ph D, Mahamad A. Ghazi, Ph D.


Objetivo. Identificar la fuente principal de exposición a plomo entre mujeres residentes en la Ciudad de México, por medio de la comparación de las razones de isótopos de plomo en sangre, cerámica y gasolina. Material y metodos. La población de estudio fue seleccionada aleatoriamente de participantes de una muestra obtenida de enero a diciembre de 1996 y consistió en 16 mujeres con niveles de plomo mayores a 10 µg/dl, que usaban cerámica vidriada con plomo. Las razones de isótopos de plomo se obtuvieron con un espectrómetro de masa de plasma inductivamente acoplado (ICP-MS) Perkin Elmer 5000, en interfase con un sistema de vaporización electrotérmica (ETV) Perkin Elmer HGA-6OOMS. Resultados. Las razones de isótopos (206Pb/204Pb, 207Pb/204Pb, y 208Pb/204Pb), tanto de los especímenes de sangre, como de los de cerámica se correIacionaron fuertemente, con valores de r2 de 0.9958,0.99 15, y 0.9890, para las tres razones de isótopos, respectivamente, sugiriendo que la exposición a plomo probablemente se debió al uso de la cerámica. Las mediciones de las razones de isótopos de plomo de la gasolina con plomo, que todavía se usaba al momento de la toma de muestras de sangre, fueron distintas de las encontradas en sangre y en cerámica. Conclusiones. La determinación de las razones de isótopos de plomo puede ser una herramienta eficiente para la identificación de las fuentes principales de exposición a plomo y para apoyar la implantación de medidas de salud pública para prevención y control.


Objective. Lead poisoning can, in some cases. be traced to a specific route or source of exposure on the basis of the individual's blood lead isotope ratio. To assess the major source of lead exposure among women residing in Mexico City, we compared blood, ceramic, and gasoline lead isotope ratios. Material and Methods. The study population, randomly selected from participants of a large trial, (1/1996-12/1996) comprised of 16 women whose lead levels exceeded 10 ug/dl and who reported using lead-glazed ceramics. Lead isotope ratios were performed on a Perkin Elmer 5000 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) interfaced with a Perkin Elmer HGA-6OOMS Electrothermal Vaporization System (ETV). Results. The isotope ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) of both the blood specimens and their correspondin ceramic specimens were highly correlated. with r=0.9979, r2=0.9958, r= 0.9957, r2=0.9915 and r=0.9945, r2=0.9890 values for the three isotope ratios, respectively, suggesting that the lead exposure most likely resulted from the use of these ceramic, Measurements of lead isotope ratios from leaded gasoline in use at the time of blood sampling, differed from those in blood and ceramics. Conclusions. Determining lead isotope ratios can be an efficient tool to identify a major source of lead exposure and to support the implementation of public health prevention and control measures.


Environmental lead exposure in Mexico City results from two main sources –leaded gasoline and traditional lead-glazed ceramics1 The use of lead as an additive in gasoline has decreased since 1986, and unleaded gasoline was introduced in Mexico in 19911 A survey conducted to determine blood lead levels (BLLs) among the population of Mexico City showed that umbilical cord BLLs decreased from 13.5 u/dl in 1980 to 6.9 ug/dl in 1996.2,3 However, BLLs in 33% of women at delivery exceed 10 ug/d1,4 Therefore, deter­mining the major source of lead exposure in this po­pulation is important from a public health standpoint, because of the impact of elevated BLLs on neurobe­havioral development in both fetuses and children.5,6

Lead poisoning can in some cases be traced to a specific route or source of exposure by measuring an individual's blood lead isotope ratio. Elemental lead comprises four naturally occurring stable isotopes: 201Pb, 206Pó, 207Pb, and 208Ph. Except for 204Pb, these isotopes are products of radioactive decay of either uranium or thorium.7,-9 Lead isotope ratio measurement –correlating body lead to possible external sources of lead exposure– is sometimes referred to as "lead fin­gerprinting." The isotopic content of lead introduced into a human body remains constant over time and changes only if another source of lead with differing lead isotopic content is introduced. In such a case, the body's new lead isotopic content will be a combina­tion or mixture of the two lead sources. Since the body treats lead as one unit and does not metabolically alter it, all measurements of lead isotope ratio from a blood specimen can potentially be correlated to an outside source, if the number of lead sources are limited and little lead mixing has occurred.8,9

The goal of this study was to determine the major source of lead exposure among women residing in Mexico City, whose BLL's exceeded 10µg/dl, by comparing blood, ceramic, and gasoline lead isotope ratios. The investigation began in 1996 and was com­pleted in 1998.

Material y Métodos

Participating women were part of a double-blinded ran­domized trial to evaluate the impact of calcium supple­mentation in lactating women on their venous BLLs. The expected outcome of calcium supplementation was a decreased remobilization of lead stored in bone. This decrease would minimize lead exposure to the fetus. From January 1994 through June 1995, potential study participants were identified from maternity wards in three hospitals in Mexico City and interviewed. Methods have been described elsewhere4 The study population initially participating in the trial comprised 529 women (270 placebo, 259 intervention). Twenty women were randomly selected from lactating women and in puer­perium, who had been assigned to receive placebo and who had reported using lead-glazed ceramics. Blood samples were collected for analysis, and each woman provided one intact earthenware ceramic utensil that she routinely used for cooking and serving food. Addi­tionally, we obtained samples of leaded gasoline and determined the isotope ratio.

The research protocol was approved by the Hu­man Subjects Committee of the National Instihrte of Public Health of Mexico. All participants received a detailed explanation of the study and procedures used, as well as counseling on reduction of lead exposure.

We measured the total lead content of the blood specimens on a Perkin Elmer Model 5100 Zeeman Gra­phite Furnace Atomic Absorption Spectrometer (ZGFAA). The analytical method employed was a well-established method used in many clinical labs in the United States.10,11

We determined the total lead content of the cera­mics by measuring the leachates from each ceramic using a 4% nitric acid solution as the leaching solution on an Instruments SA JY70+ Inductively Coupled Plas­ma Optical Emission Spectrometer (ICP-OES).9,10 Sam­ples of the nitric acid solution were collected at 1 hour, 6 hours, and at 24 hours, from each container.10

The isotope ratio measurements were performed on a Perkin Elmer ELAN 5000 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) interfaced with a Perkin Elmer HGA-600MS Electrothermal Vaporiza­tion System (ETV)10,12-14 described in a previous publi­cation. We assessed the accuracy of isotope ratio measurements comparing the levels which were de­termined to the target value of know composition. The percent age error varied between –0.2% to 3.1%)10

For statistical analysis, we compared the isotope ratios 206pb/204pb, 207pb/204pb and 208pb/204pb from each blood analysis, with isotope ratios from the earthenware ceramic specimen obtained from the same household, to determine whether lead exposure was likely to have been caused from using the earthenware. Similarly, the blood isotope ratios were compared with the gasoline isotope ratios10

Lead ratios were measured for blood specimens with lead values greater than or equal to 10 µg/dl (N=16). Lead isotope ratios for the blood, aqueous pottery lea­chates, and gasoline extracts, were tabulated and com­pared with each other to determine possible statistically significant correlations. We obtained the association bet­ween each blood specimen and its corresponding cera­mic by calculating slopes for each blood/ceramic pair.

The group correlation of all the blood samples to the ceramics within each isotope ratio was also calcuated10. In addition we calculated the Mahalanobis distance between lead isotope ratios present in blood, and gasoline and blood and ceramc pots.15


Table I presents the total BLLs of the 16 women whose levels exceeded 10 µg/dl and the lead isotope ratios of the blood samples and their corresponding ceramics for each woman. The blood and ceramic isotope ratios from each woman/ceramic pair match with each other. The highest errors were observed in the 208Pb/204Pb ratio measurements because of the difference in the relative abundance of the two isotopes. The 208Pb isotope signal is 37 times more intense than the 204Pb isotope, leading to higher measurements and precision errors.

Table II reports the isotope ratio association bet­ween each blood specimen to its corresponding cera­mic specimen from that household, as determined by the calculated slopes of each pair. Additionally, a to­tal group correlation was calculated for all the blood/ceramic pairs for each isotope ratio. The slopes were very close to 1 and correlation coefficients (r2=0.99 or better) strongly suggesting that use of ceramics was a significant contributor to the women's blood lead level, given the accuracy of the method (see methods).

We compared the average isotope ratios of the ceramics with the average isotope ratios obtained for the blood specimens. The lead isotope ratios of both the blood and the ceramic resemble the lead isotope ratios found in the local mines of Mexico or California, making them the most likely source of lead in these specimens.5,12,13 Additionally, the average isotope ratios for the blood samples and their corresponding ceramics from the same household showed a high degree of correlation, suggesting that lead exposure for these women resulted from their use of these ceramics.

The blood isotope ratios were tightly clustered. By comparing each blood lead isotope ratio with the corresponding 1 hour leachate lead isotope ratio of the pottery obtained from the same household, we observed that the isotopic content of the blood specimens and the ceramic leachates were virtually identical. This high correlation for all three isotope ratios indicates that the pots are the most likely source of lead in blood (Figure 1); however, exposure to an additional source in conjunction with the pottery cannot be totally dis­counted.

We subsequently measured lead isotope ratios from leaded gasoline in typical use at the time of sampling. The isotope ratios measured for gasoline differed from those in blood or ceramics. Figure 2 shows the cluste­ring of the blood and ceramic isotope ratios in compari­son to the distinctly different gasoline lead isotope ratios.

In addition, we calculated the Mahalanobis distan­ce between blood and gasoline, including the 3 lead isotope ratios, which gave a result of 4.6 (p<0.001) sug­gesting that isotopes ratios were significantly different. The Mahalanobis distance between the lead isotope ra­tios of blood and ceramic pots was 2.3, suggesting that lead ceramic was the main source of blood lead.


The lead isotope ratios in blood and in ceramic were highly correlated, strongly suggesting that use of lead-glazed ceramic was the major source of lead exposure in our population. In contrast, the gasoline lead isoto­pe differed, indicating that gasoline was not of major concern for our population.

These results agree with reports from Mexico City that observed a strong correlation of elevated blood lead with use of ceramic cookware14,16,17 and elevated BLLs among ceramic workers in Mexico.18

Determining lead isotope ratios has been used to identify sources of exposure. Delves reported high cor­relations between paint and water lead ratios and blood lead ratios5,7,12,19,20
Where the source of lead exposure is not clearly established, determining lead isotope ratios in blood and corresponding environmental samples may help determine the major source of exposure and allow for primary prevention, permitting the persons at risk to eliminate or minimize their exposure. There are, however, limitations to this technique. For isotope measurement ("lead fingerprinting") to be useful, poten­tial sources of lead exposure must be limited in number and scope, and the lead sources must be isotopically dis­tinct. If two or more sources of lead exist internally (within the body) or externally (in the environment), mixed isoto­pe ratios result, reflecting a combined isotope ratio of all the sources of exposure. Such mixed isotope ratios gene­rally provide no useful information in identifying lead sources because they then represent the average lead isotope ratio, and separation into the original isoto­pe ratios is difficult7,19,21,22 Another important factor is the accumulated lead in bone. Long after lead exposure has ceased and BLLs have dropped, a new stress on the body (such as a bone break or pregnancy) could shift the equilibrium and consequently release accumulated stores of lead from the bones, elevating the BLLs inde­pendent of a current external source of exposure. Lead exposure would then be difficult to attribute to any ex­ternal source if demographics such as employment, housing, or the living conditions of the exposed person had changed since the initial exposure6,7 Although wo­men in our sample were lactating, which increases the lead mobilization from their bones, their living conditio­ns had not changed over time; therefore, the source of their lead exposure would not have changed dramatica­lly over time. Any blood lead isotope ratios measured for this population sample would probably represent current exposure. The lead isotope ratios measured for the women in this study, compared with the lead isotope ratios measured for the ceramics, strongly suggest the ceramics are the major contributor of lead exposure in these women. The gasoline lead isotope ratios are suffi­ciently distinct from the blood lead isotope ratios to eli­minate them as a possible lead exposure source in these women.7,8

The number of women in our study was small, but given the large difference in lead isotope ratios between lead-glazed ceramics and gasoline, the tech­niques we used were likely to discriminate between the different lead sources. The large difference in lead isotope ratios between gasoline and lead glaze suggests that the lead used in these two media originated from different sources. In Mexico, the lead tetraethyl added to gasoline by the government-owned petroleum com­pany (Pemex) comes from providers outside of Mexico. In contrast, the lead used to prepare ceramic glazes comes from ores different to those ores used in lead tetraethyl and which is likely to have been ex­tracted from Mexican mines.

These results are of major public health relevance because of the widespread use of lead- glazed ceramic in Mexico –approximately 40%.22,23 Alternative tech­nology is available based on a borate glaze that is inert and provides ceramics with similar aesthetic to the tra­ditional. Public programs are needed to promote and implement alternative technology to decrease the lead burden of the Mexican population.


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Salud Pública de México es una publicación periódica electrónica, bimestral, publicada por el Instituto Nacional de Salud Pública (con domicilio en Avenida Universidad núm. 655, col. Santa María Ahuacatitlán, Cuernavaca, Morelos, C.P. 62100, teléfono 329-3000, página web,, con ISSN: 1606-7916 y Reserva de Derechos al Uso Exclusivo con número: 04-2012-071614550600-203, ambos otorgados por el Instituto Nacional del Derecho de Autor. Editor responsable: Carlos Oropeza Abúndez. Responsable de la versión electrónica: Subdirección de Comunicación Científica y Publicaciones, Avenida Universidad núm. 655, planta baja, col. Santa María Ahuacatitlán, Cuernavaca, Morelos, C.P. 62100, teléfono 329 3000. Fecha de última modificación: 7 de junio de 2018. D.R. © por el sitio: Instituto Nacional de Salud Pública.

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