1110 VOLUME 112 | NUMBER 10 | July 2004 • Environmental Health Perspectives

Children’s Health Article

Population lead exposure has been one of the

main environmental health problems in

Mexico (Albert and Badillo 1991; Friberg and

Vahter 1983; Olaiz et al. 1996; Romieu et al.

1994; Rothenberg et al. 1993). Several standards have been published in the last decade to

restrict emissions and reduce the exposure to

this metal. Lead in gasoline, one of the main

sources of environmental exposure eliminated

in the past decade, was reduced by > 98.5%

between 1986 and 1997. In September 1997 it

was eliminated from all gasoline sold in the

metropolitan zone of the Valley of Mexico.

Thanks to the elimination of this source, from

1988 to 2002 the mean lead air level was

reduced from 2.80 μg/m3 to 0.07 μg/m3

(Departamento del Distrito Federal 1998;

Driscoll et al. 1992; Romieu et al. 1994).

Notwithstanding the success in reducing air

lead, there are still other sources of population

exposure, such as fixed industrial sources that

contaminate restricted zones in their surroundings and lead-glazed ceramics that affect most

of the population (Hernandez-Avila et al.

1991, 1996; Navarrete-Espinosa et al. 2000;

Olaiz et al. 1996).

The results of several cross-sectional studies from the population in Mexico City performed during the last decade reveal a

reduction in blood lead levels, an indicator of

environmental lead exposure.

However, these data do not allow evaluation of the effect of the reduction of environmental exposure across time. For this reason,

our objective in this work was to determine the

secular trend in blood lead concentrations of a

cohort of children evaluated every 6 months

during their first 10 years of life. We also

quantified the relative contribution of several

other sources of exposure to this metal to

their blood lead levels. Nevertheless, because

of the difficulty in specifying and measuring

the different lead sources, the data of this

study can only provide the lower limit of the

contribution of each source.

Materials and Methods

Subjects. We recruited 502 pregnant women

attending the prenatal clinic at 12 weeks of

pregnancy at the National Institute of

Perinatology in Mexico City. They all signed

an informed consent form approved by the

ethics committee of the institute. Only 383 of

the children born to these women fulfilled the

inclusion criteria: 5-min Apgar score ≥ 7, birth

weight > 2,000 g, gestation age ≥ 36 weeks,

without major or minor congenital anomalies

or being the product of multiple birth. Of

these children, 62 were lost to follow-up

between 1 and 6 months after birth.

We followed 321 children, born between

1987 and 1992, through structured interviews

of the parents and with psychometric tests and

blood lead determinations every 6 months

from delivery. Table 1 presents the characteristics of the sample. Parents were informed of

potential sources of lead, routes of exposure,

toxicity of lead, and some ways to avoid it.

They also received nutritional and hygienic

orientation. None of the children received

chelation therapy.

Blood lead measurements. We drew

venous blood into purple-top Vacutainers that

contained ethylenediamine tetraacetic acid.

Blood samples were analyzed in duplicate

by anodic stripping voltammetry, at the

Environmental Associates Laboratories (ESA

Labs; Chelmsford, MA, USA). Samples with

mean duplicate values < 5 μg/dL were reanalyzed via atomic absorption spectrometry.

Quality control information is provided elsewhere (Rothenberg et al. 1994). ESA Labs is

a reference laboratory for the Centers for

Disease Control and Prevention’s Blood Lead

Laboratory Reference System (Atlanta, GA,

USA) and participates in the Commonwealth

of Pennsylvania Department of Health Blood

Lead Proficiency Testing Program (Exton,

PA, USA).

For the statistical analysis, we calculated

the arithmetic mean of the biannual concentrations of lead determined during each year

of the child’s life and natural log transformed

the means for use in modeling.

Air lead measurements. We extracted air

lead information from the Statistical Compendium of the System of Atmospheric

Monitoring of the Metropolitan Zone of the

Valley of Mexico (MZVM), 1986–2002

(Departamento del Distrito Federal 2003).

Air lead concentrations were obtained through

the Automatic Network of Atmospheric

Monitoring (ANAM) of the MZVM that

started operations in 1986 (Departamento del

Distrito Federal 2003).

At present, the ANAM includes 19 stations with manual equipment. The network

records in particle filters the concentrations of

total suspended particles, particles < 10 μm,

and metals such as lead. The ANAM generates

a 24-hr sample every 6 days. Air lead concentrations were analyzed by atomic absorption

spectrometry.

We used air lead data collected at three air

monitoring stations that reported the most

complete data during the study period. In the

Address correspondence to S.J. Rothenberg, Instituto

Nacional de Salud Pública, Avenida Universidad 655,

Sta. Ma. Ahuacatitlán, CP 62508, Cuernavaca,

Morelos, México. Telephone and fax: +52-739-395-

0662. E-mail: drlead@prodigy.net.mx

We gratefully acknowledge the extended participation of the parents and the children who contributed

to the study.

This study was supported in part by the Consejo

Nacional de Ciencia y Tecnología grant SALUD2002-C01-7159, by the Secretariat of Health, Mexico

(Secretaria de Salud), and the U.S. Environmental

Protection Agency.

The authors declare they have no competing

financial interests.

Received 5 August 2003; accepted 13 April 2004.

Blood Lead Secular Trend in a Cohort of Children in Mexico City (1987–2002)

Lourdes Schnaas,1 Stephen J. Rothenberg,2,3 María-Fernanda Flores,1 Sandra Martínez,1 Carmen Hernández,1

Erica Osorio,1 and Estela Perroni1

1National Institute of Perinatology, Mexico City, Mexico; 2National Institute of Public Health, Cuernavaca, Morelos, Mexico; 3Charles R.

Drew University of Medicine and Science, Los Angeles, California, USA

We determined the secular trend in blood lead levels in a cohort of 321 children born in Mexico

City between 1987 and 1992. Blood lead level was measured every 6 months during a 10-year

period. We modeled the effect of yearly air lead concentration nested within the calendar year in

which the child was born, family use of lead-glazed pottery, socioeconomic status, year in which the

child was born, age of the child at the time of blood lead measurement, place of residence, and an

indicator variable for subjects with complete or incomplete blood lead values. The yearly mean of air

lead of the Valley of Mexico decreased from its highest level of 2.80 μg/m3 in 1987 to 0.07 μg/m3 in

2002. The contribution of air lead to blood lead according to year of birth was strongest for subjects

born in 1987 and fell to nearly zero for children born in 1992. The geometric mean of the entire

cohort rose from 8.4 μg/dL in the first year of life to 10.1 μg/dL in the second and decreased thereafter until it reached 6.4 μg/dL at 10 years of age. Children of families who used lead-glazed ceramics

had blood lead levels 18.5% higher than did children of nonusing families. Children who belonged

to the lowest socioeconomic levels had blood lead levels 32.2% higher than did those of highest

socioeconomic levels. Children who lived in the northeast part of the city had blood lead levels

10.9% higher compared with those who lived in the southwest. Key words: air lead, blood lead, children, gasoline, lead-glazed ceramic, Mexico City. Environ Health Perspect 112:1110–1115 (2004).

doi:10.1289/ehp.6636 available via http://dx.doi.org/ [Online 15 April 2004]
selection of these stations, we also looked for

a geographic area with high contamination

levels (Xalostoc, an industrial zone placed

behind a smelter in the northeast sector of the

city), a zone with intermediate levels (Merced,

a mixed high-density commercial–residential

zone with heavy vehicular traffic in the center

of the city), and a zone with low atmospheric

lead levels (Pedregal, a low-density, upscale

residential zone in the southwest of the city).

We calculated each year’s average of

3-month means for each station’s air lead

data. We used natural-log–transformed yearly

means for statistical modeling.

Other variables. Children were placed

into separate subcohorts according to the year

in which they were born. To avoid collinearity of air lead over time with age of child, air

lead was nested within each of the resulting

six subcohorts in subsequent analyses, instead

of being used as a separate independent variable. The resulting coefficients for the six

nested air lead variables thus reflected the

effect of lead on secular trend of air lead on

subcohort blood lead across all ages instead of

on blood lead according to age of child.

Socioeconomic status (SES) was computed

by equally weighting the sum of three separate

nine-point scales measuring education and

occupation of the head of the family and the

total family income based on integral multiples of the minimum wage. SES of the families

ranged from low-low to lower-upper. For this

analysis, we collapsed the resulting sum of the

three SES scales into only three categories: low

SES (the low-low and middle-low categories),

upper-lower and lower-middle SES, and middle-middle and higher SES. The information

about family use of lead-glazed ceramic was

obtained during the pregnancy and was coded

as a “yes” or “no” response on the questionnaire. Place of residence was fixed into one of

the three nonoverlapping zones in which the

air monitoring stations were located based on

family residential address. The northeast sector, represented by the Xalostoc air monitoring station (with the highest air lead values),

was used as the omitted dummy variable in

the residential analysis. We recorded the family residential address at recruitment and at

varying times after. We created a dummy variable indicating whether the address changed

during the participation of the child in the

study. We coded as “unchanged address” the

53 subjects (16.5% of the sample) whose postnatal address was unverifiable but who contributed postnatal lead data. Because not all

subjects had blood lead data for every age, we

created a dummy variable indicating complete

or incomplete (missing data at one or more

ages) blood lead for each subject.

Statistical analyses. We used Fisher’s exact

tests (StatXact 5; Cytel Software Corp.,

Cambridge, MA, USA) to compare frequencies

of sex, SES, and leaded ceramic ware use

between subjects included in the analysis and

subjects whose mother was recruited into the

study but who failed to present to the study

after 1 month of age. We compared the same

variables with Fisher’s exact tests to determine if

there were significant differences between subjects with complete lead data from 1 to 10 years

of age and those with missing data points for

1 or more years. We used t-tests for independent groups to determine differences in blood

lead concentration at each age between subjects

with complete lead data and those with missing data at one or more ages. We applied a

Kruskal-Wallis chi-square test with exact probabilities (StatXact) to determine if there was a

significant association between SES and use of

lead-glazed ceramic ware.

We used the SPSS version 11.5 (SPSS,

Inc., Chicago, IL, USA) linear mixed-effects

models procedure to analyze the data set as a

repeated-measures, unbalanced analysis of

variance design using heterogeneous firstorder autoregressive covariance structure of

the repeated measure, natural log blood lead

at each age (10 levels), and a fixed-effects air

lead (mean annual air lead concentration for

each calendar year of the study) nested within

the subcohort according to the calendar year

in which the child was born (six levels). The

other fixed variables were year in which the

child was born (six levels), family SES (three

categories), use of lead-glazed ceramic ware

(two levels), indicator variables for place of

residence (three variables), and an indicator

variable for subjects with complete or one or

more missing blood lead values.

We selected the covariance structure of the

repeated-measures variable by inspection of the

covariance matrix and by the use of information criteria of the model as a whole. The

restricted log likelihood measures the fit of the

data to the model, and Bozdogan’s criterion

(Bozdogan 2000) provides an alternate measure

of fit, penalizing the fit measure according to

the number of parameters in the model. We

used the heterogeneous first-order autoregressive structure because it was in accordance with

the observed unrestricted covariance matrix

and maximized the fit measures.

We used the Sidak procedure to adjust

two-tailed p-values in all post hoc multiple

comparisons. We used the Jonckheere-Terpstra

test in post hoc analyses of the trend in blood

lead across year of birth at each age. We used

exact p-values as calculated by StatXact.

Results

Table 1 shows the differences between included

and excluded data. There were no differences

between the groups in sex, SES, or use of leadglazed ceramics. Only 31.2% (n = 100) of subjects provided lead data at all 10 ages; 48.3%

(n = 155) of subjects had three or fewer missing lead values. The proportion of children

who used lead-glazed ceramic ware was lower

in the group of subjects with incomplete data

(Table 2). The proportion of children living in

the southwest sector was lower and in the

northeast sector was higher in children with

incomplete data. The geometric mean of blood

lead (GMPb) was also significantly higher in

the incomplete group at almost all ages except

at 1, 2, and 4 years of age (Table 3).

Figure 1 shows the time course of lead in

air and lead in blood of the entire cohort

according to the year of the study. In this

graph, blood lead is confounded by age and

year because in the early years of the study only

younger children were available and in the

later years of the study only older children contributed data. Yearly average air lead concentration decreased from its highest level of

2.8 μg/m3 in 1987 to 0.07 μg/m3 in 2002.

Dates of important changes in gasoline lead

content are indicated by the arrows in Figure 1.

Children’s Health | Blood lead trend in children in Mexico City

Environmental Health Perspectives • VOLUME 112 | NUMBER 10 | July 2004 1111

Table 1. Characteristics of tested versus not-tested subjects.

Subjects included (n = 321) Subjects not tested (n = 62)

Characteristics No. (%) No. (%) p-Valuea

Sex

Male 174 (54.2) 32 (51.6) 0.781

Female 147 (45.8) 30 (48.4)

SES

Lowest 52 (16.2) 8 (12.9) 0.714

Medium 189 (58.9) 36 (58.1)

Highest 80 (24.9) 18 (29.0)

Clay pot useb

Yes 128 (39.9) 24 (46.2) 0.447

No 193 (60.1) 28 (53.8)

Cohort size (year)c

1987 38 (11.8) 13 (21.0)

1988 68 (21.2) 16 (25.8)

1989 34 (10.6) 2 (3.2)

1990 60 (18.7) 14 (22.6)

1991 49 (15.3) 5 (8.1)

1992 72 (22.4) 12 (19.3)

aFisher’s exact test or Pearson chi-square exact probability, subjects tested versus subjects not tested. bIn some nottested subjects, there was no information about clay pot use. cYear in which child was born.
SES, the age of the child at the time of

blood lead measurement, the year in which

the child was born, family use of lead-glazed

ceramic, and the yearly mean of air lead of the

Valley of Mexico significantly predicted the

children’s blood lead levels (p ≤ 0.001) as well

as place of residence and having missing data

(p < 0.05; Table 4).

The contribution of air lead to lifetime

blood lead according to year of birth was estimated in the mixed model (Table 4). It was

strongest for subjects born in 1987 (each natural

log decrease in air lead was accompanied by a

23.7% decrease in blood lead concentration

across the first 10 years of life) and fell to nearly

zero in the children born in 1992. Post hoc

contrasts showed that the most important difference in lifetime blood lead was between the

part of the cohort born in 1987 and 1988

compared with the part of the cohort born in

1991 and 1992 (Figure 2). The difference in

lifetime blood lead concentration between the

two subcohorts was 43.0% [t = 5.919, degrees

of freedom (df) = 725.9, p < 0.0005]. It is

notable that completely unleaded gasoline was

introduced into Mexico City in the last quarter of 1990. Leaded gasoline completely disappeared from the Mexico City market in

1997 (Figure 1). There was a significant

decrease (p < 0.01) in the percentage of children with blood lead ≥ 10 μg/dL (the current

Mexican action limit) across year of birth

(subcohort) during each of the first 7 years of

life (Table 5).

The estimated GMPb of the entire cohort

rose from 8.4 μg/dL [95% confidence interval

(CI), 7.7 to 9.2] in the first year of life to

10.1 μg/dL (95% CI, 9.3 to 11.0) in the second

and decreased thereafter linearly to 6.4 μg/dL

(95% CI, 5.8 to 7.1) when children reached

their tenth birthday (Figure 3). The GMPb of

the cohort at 2 years of age was > 10 μg/dL, a

level considered unacceptable in children by the

national Mexican and international standards

for lead exposure. Blood lead concentration at

2 years and 3 years of age was significantly

higher than at 1 year of age (Sidak p ≤ 0.001).

Blood lead concentration of the cohort at 9 and

10 years of age fell significantly below the 1-year

concentration (Sidak p < 0.01).

Children belonging to the lowest and

middle SES had 32.3% (95% CI, 17.0 to

49.5) and 9.3% (95% CI, 0.2 to 19.1) higher

GMPb than children belonging to the highest

stratum (Table 4). In preliminary modeling

we failed to find a significant interaction

between SES and either age or year of birth

on blood lead. Children from families who

used ceramic pottery had GMPb levels 18.5%

(95% CI, 10.0 to 27.6) higher than did children from families who did not use such

pottery (Table 4). Although there was a significant association between ceramic pottery

use and poverty (Kruskal–Wallis chi-square =

5.921, df = 1, p = 0.015; 53.8, 38.9, and

31.6% used clay pottery as SES increased

from the lowest to the highest stratum), the

interaction between poverty and use of

ceramic pottery in the prediction of blood

lead level was not significant in the multivariate analyses (p = 0.88), and it was not

included in the final model. The group of

children with complete data had a GMPb

8.6% (95% CI, 1.3 to 15.4) lower than the

group with incomplete data (Table 4).

The results showed that the children living

in the southwest (residential) and central

(mixed commercial–residential) sectors of the

city had a GMPb 10.9% (95% CI, 2.4 to

18.6 and 7.2% (95% CI, –2.1 to 15.6) lower

than did those who lived in the northeast sector (Table 5). The effect of moving residence

during the study was not significant (p =

0.192) in the mixed model and was not

included in the above estimations.

Discussion

Design issues. Our results have at least three

limitations. We used only three air monitoring stations to estimate the annual means of

atmospheric lead, which were not necessarily

representative of the place of residence of all

subjects. A better estimate would have

Children’s Health | Schnaas et al.

1112 VOLUME 112 | NUMBER 10 | July 2004 • Environmental Health Perspectives

Table 2. Characteristics of subjects with complete versus incomplete data.

With complete data (n = 100) With incomplete data (n = 221)

Characteristics No. (%) No. (%) p-Valuea

Sex

Male 51 (51.0) 123 (55.6) 0.469

Female 49 (49.0) 98 (44.4)

SES

Lowest 10 (10.0) 42 (19.0) 0.126

Medium 63 (63.0) 127 (57.5)

Highest 27 (27.0) 52 (23.5)

Clay pot use

Yes 48 (48.0) 79 (35.7) 0.048

No 52 (52.0) 142 (64.3)

Cohort size (year born)b

1987 10 (10.0) 28 (12.7)

1988 17 (17.0) 51 (23.1)

1989 13 (13.0) 21 (9.5)

1990 16 (16.0) 44 (19.9)

1991 16 (16.0) 33 (14.9)

1992 28 (28.0) 44 (19.9)

Place of residence

Southwest 48 (48.0) 71 (32.1) 0.008

Center 33 (33.0) 77 (34.8)

Northeast 19 (19.0) 73 (33.1)

aFisher’s exact test or Pearson chi-square exact probability, complete data versus incomplete data. bYear in which child

was born.

Table 3. Differences in GMPb between subjects with complete and incomplete data.

With complete data With incomplete data

Age (years) Mean ± SD No. Mean ± SD No. p-Valuea

1 9.2 ± 1.7 100 9.3 ± 1.7 202 0.762

2 10.5 ± 1.8 100 11.0 ± 1.7 164 0.480

3 9.3 ± 1.7 100 10.9 ± 1.6 118 0.014

4 8.4 ± 1.6 100 9.4 ± 1.7 107 0.101

5 7.4 ± 1.6 100 9.5 ± 1.6 81 0.001

6 6.6 ± 1.6 100 8.1 ± 1.6 65 0.008

7 6.2 ± 1.6 100 7.8 ± 1.5 64 0.002

8 5.5 ± 1.6 100 6.6 ± 1.6 54 0.022

9 5.2 ± 1.6 100 6.8 ± 1.5 45 0.001

10 4.9 ± 1.6 100 6.2 ± 1.5 34 0.014

at-Tests for independent groups.

14

12

10

8

6

4

2

GMPb (μg/dL)

3

2

1

0

Mean air Pb (μg/m3)

1989 1991 1993 1995 1997 1999 2001

Calendar year

▲

●

●

● ●

●

●

● ● ● ● ● ● ● ● ●

▲ ▲

▲

▲

▲ ▲

▲

▲ ▲

▲ ▲

▲ ▲ ▲

A

B

C

D E

Figure 1. Yearly GMPb concentration with geometric

95% CIs (blue triangles, dashed line) of entire cohort

and yearly mean air lead concentration with natural

log air lead regression line (black circles, solid line)

as a function of calendar year. Regression adjusted

R2 = 0.96, indicating adequate specification of air

lead by natural log transformation in statistical modeling. Arrows indicate important reductions in permitted lead content of gasoline in Mexico: A, 0.5–1.0

mL/gal; B, 0.3–0.54 mL/gal; C, 0.2–0.3 mL/gal;

D, 0.1–0.2 mL/gal; E, 0.0 mL/gal.
resulted from adjusting the children’s air lead

exposure according to their place of residence.

Because we did not have those data, we

selected the station that had the highest

atmospheric lead levels (Xalostoc) and the

lowest levels (Pedregal) and a station with

middle levels (Merced), and we fixed the

place of residence of the children based on

family residential address into one of the three

nonoverlapping zones in which the air monitoring stations were located. Residence in the

highest air lead zone (Xalostoc) was significantly associated with higher lifetime blood

lead concentrations (p = 0.013) compared

with those living in the lowest air lead zone

(Pedregal), as had already been found in a

prior study in Mexico City (Lara-Flores et al.

1989). We found that 7.5% (n = 24) had

changed their address sometime during their

participation in the study. We included this

information in a preliminary model as a

dummy variable and found no significant

(p > 0.20) effect. However, we lost contact

with 53 subjects before verifying whether

their address had changed, and so these data

are likely incomplete. Our presented model

did not include a term for change of address.

Another limitation of our study is that the

use of lead-glazed ceramic pottery was not

quantified and was formally assessed only at

the start of the study. We asked only whether

they used this kind of pottery to serve, cook, or

keep foods. For our analysis we used the data

provided by the mother during pregnancy. We

advised all mothers after application of the

pregnancy questionnaire that use of lead-glazed

pottery was a risk factor for lead exposure, and

we repeated that advice throughout the study.

We found that similar data gathered after pregnancy were not very reliable, because mothers

admitted “limited use” in the home when

home visits were made, despite answering no

in previous and subsequent questionnaires, and

also revealed that their children would occasionally or frequently eat with family members

outside their own home, families that continued to use lead-glazed pottery.

The variable of air lead nested within subcohort showed some collinearity with age

of subject. Test models (data not shown)

without this variable calculated age coefficients and standard errors 1.5–2 times and

1–2 times, respectively, higher than the age

coefficients and standard errors calculated

with the nested air lead term in the model.

Nevertheless, the degree of collinearity was

not severe. The pattern of change in the subcohort main effect remained unaffected by

the addition of the nested air lead variable.

Furthermore, the pattern of change in the

coefficients of the air lead variable nested with

subcohort shows that the strongest relationship of air lead on lifetime blood lead was in

the earliest-born subcohort and the weakest in

the latest-born subcohort.

Public health implications. Among the

variables tested, atmospheric lead levels were

the strongest predictors of blood lead levels of

the children across their first 10 years of life.

Increased blood lead levels during the second

year of life have been reported by several investigators and explained as a consequence of,

among other factors, an increase in gastrointestinal capacity to absorb lead, as well as to the

characteristic hand-to-mouth activity of this

age in which children introduce materials into

their mouths, including their generally dirty

hands (Baghurst et al. 1992; Rothenberg et al.

1996). This age group has also a greater prevalence of iron deficiency, a factor associated

with increased lead absorption and retention

(Ruff et al. 1996).

The presence of high blood lead levels

among children born in the first 2 years of the

study is unlikely to be the result of greater lifetime accumulation of lead in the bones of children born earlier in the study when air lead

levels were high, because bone lead is highly

labile in infants and young children (O’Flaherty

1995). As shown in Table 4, the coefficients

of air lead nested within year of birth fall with

later year of birth, reaching essentially zero for

the part of the cohort born in 1992. Thus,

the higher lifetime blood lead levels of children born in 1987 and 1988 compared with

those born in 1991 and 1992 may be most

likely caused by the higher air lead concentrations to which the former children were

exposed during their lifetime. The model presented here supports the importance of air

lead concentration in determining blood lead

level throughout the first 10 years of life.

Children’s Health | Blood lead trend in children in Mexico City

Environmental Health Perspectives • VOLUME 112 | NUMBER 10 | July 2004 1113

Table 4. Estimates of fixed effects.a

Parameter Estimate [ln(μg/dL)] df p-Value 95% CI

Intercept 1.615 1073.84 0.000 1.388 to 1.842

SES

Low 0.280 368.84 0.000 0.157 to 0.402

Medium 0.088 368.84 0.046 0.002 to 0.175

High 0.000b

Age (years)

1 0.274 1140.60 0.000 0.120 to 0.428

2 0.456 1131.48 0.000 0.315 to 0.596

3 0.405 1009.02 0.000 0.279 to 0.531

4 0.339 880.97 0.000 0.229 to 0.449

5 0.309 796.55 0.000 0.210 to 0.408

6 0.199 623.72 0.000 0.116 to 0.283

7 0.188 524.82 0.000 0.114 to 0.261

8 0.084 439.96 0.011 0.019 to 0.148

9 0.034 316.94 0.167 –0.014 to 0.082

10 0.000b

Year born

1987 0.437 677.11 0.000 0.266 to 0.608

1988 0.472 620.64 0.000 0.316 to 0.627

1989 0.328 535.15 0.000 0.151 to 0.506

1990 0.205 512.58 0.014 0.042 to 0.368

1991 0.194 516.75 0.033 0.016 to 0.371

1992 0.000b

Clay pot use

Yes 0.170 364.6 0.000 0.096 to 0.244

No 0.000b

Locationc

Southwest –0.115 3670.0 0.013 –0.205 to –0.024

Center –0.074 379.11 0.125 –0.169 to 0.021

Air lead (year born)

1987 0.213 924.26 0.000 0.114 to 0.312

1988 0.166 834.78 0.000 0.086 to 0.246

1989 0.162 986.16 0.002 0.061 to 0.261

1990 0.116 998.51 0.005 0.035 to 0.196

1991 0.143 1004.30 0.001 0.056 to 0.229

1992 –0.003 1191.42 0.934 –0.083 to 0.076

Incomplete blood lead 0.089 360.95 0.023 0.012 to 0.167

aDependent variable was ln(blood lead). bThis parameter is set to zero because it is redundant. cNortheast (Xalostoc) is

the omitted dummy variable.

Figure 2. Effect of year of birth on GMPb. Shown

are geometric means with 95% CIs estimated from

the mixed model.

12

10

8

6

GMPb (μg/dL)

Birth year

1987 1988 1989 1990 1991 1992

●

●

●

● ●

●
Belonging to a family with low SES significantly contributed to higher blood lead levels

at all ages. Other studies have already reported

an association among high lead levels and

poverty (Carter-Pokras et al. 1990; Lanphear

et al. 1998; Mahaffey et al. 1986).

According to the National Nutrition

Survey of Mexico of 1999 (Rivera-Domarco

et al. 2001), the median of the percentage of

consumption of the minimum daily requirement of calcium, iron, and zinc of children

12–59 months of age in Mexico City was

110, 48.6, and 40.7%, respectively. It is possible that frequent and extended conditions of

insufficient nutrition in our cohort favored

greater digestive absorption as well as a greater

retention of lead in the poorer children.

In addition, the poorer residential zones in

Mexico City are usually the most industrialized

and their pollution is higher. Perhaps children

living in those areas have not been favored by

the reduction of air lead to the same degree as

those with medium SES, because lead emissions

from stationary industrial sources were not

affected by gasoline lead reduction. However,

our models continue to show an effect of

poverty even when we account for area of residence. It is clear that we are unable to account

for many sources of lead variance in our data

set. Some of these unaccounted sources may

be partially measured by our poverty index.

Nonetheless, the association of SES and lead

exposure appears in many investigations from

other cultures, each culture likely having its

own unique pattern of unmeasured variables

that contribute to the effect. After examining

the sources specifically addressed in our model

as not being responsible for the effect of poverty on blood lead, we found that SES differences in nutrition remain the most plausible

explanation of the association.

From a public health viewpoint, the significant effect of the use of clay pots on children’s blood lead levels is important. To this

date there have been no effective efforts by

authorities to reduce their production, distribution, and use, despite passage of controlling

regulations in 1993. Despite the personal

counseling that the mothers of our study

received about the dangers implied in the use

of lead-glazed ceramics, blood lead differences

among users and nonusers remain until the

end of our study. It is difficult to change

behaviors that are considered part of one’s

cultural identity, especially when the effects of

lead can be nonspecific, subtle, and delayed

and can easily be attributed to other causes.

If, in the short term, education by itself

has not been enough to obtain a behavioral

change in the lead-glazed ceramic users of our

cohort, it is important that federal authorities

recognize the importance of strict enforcement of existing regulations to eliminate this

source of exposure. According to the results of

our study, if no children of our cohort had

used this type of pottery, there would have

been a reduction of 18.8% in the number of

blood lead measurements that exceeded the

current standard for lead during the study

period. We consider that such lead reduction

in the Mexican population would potentially

have a significant public health impact.

Conclusions

In the last decade, the Mexican government

has achieved notable advances in the reduction

and control of different lead sources, resulting

in a corresponding reduction of blood lead levels of children and adults. Among the adopted

measures, the elimination of this metal in

gasoline is the one that has had the greatest

measurable impact on the reduction of the

blood lead levels of the inhabitants of Mexico

City. Similar effects have been reported in

other countries, where the changeover from

leaded to unleaded gasoline has been the major

antecedent of sharp declines in average blood

lead levels and in the number of children with

elevated lead level (Hayes et al. 1994; Luo et al.

2003; Neo et al. 2000; Schuhmacher et al.

1996; Stromberg et al. 1995; Wietlisbach et al.

1995). Since the gradual removal of lead in

gasoline started in several countries in the

1970s, nearly 50 nations have already banned

lead in gasoline (Landrigan 2002).

In November 1993, the Mexican government established regulations to control the lead

content of low-temperature glazed ceramic ware.

However, its use is still a very common practice

among certain sectors of the Mexican population, with serious implications from a public

health viewpoint. It is a problem that is hard to

control and eliminate because the clay pots are

produced largely by small family enterprises

without quality control. Despite the standard to

limit the amount of lead that can be released

from this kind of earthenware, no official

inspection program oversees the production,

distribution, and sale of such receptacles. Such

lead-glazed vessels can release large amounts of

lead into foods and beverages, and today leadglazed pottery is the principal source of elevated

blood lead level in Mexico for populations not

exposed to industrial sources.

Several studies have reported that Mexican

potters and their families have blood lead levels

several times higher than the already elevated

levels of the general population (Fernandez

et al. 1997; Molina-Ballesteros et al. 1980;

Serrato and Olaiz 1996). The beneficiaries of

reducing the presence of lead-glazed ceramics

would not be limited to just the users but

would also include the producers, and an

important source of occupational exposure to

lead in Mexico would be eradicated.

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Children’s Health | Schnaas et al.

1114 VOLUME 112 | NUMBER 10 | July 2004 • Environmental Health Perspectives

Table 5. Percentage of children with blood lead levels ≥ 10 μg/dL during the first 10 years of life by year

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12

10

8

6

GMPb (μg/dL)

Age (years)

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