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The Relationship Between Lead Exposure and Homicide
Paul B. Stretesky, PhD;
Michael J. Lynch, PhD
Arch Pediatr Adolesc Med. 2001;155:579-582.
ABSTRACT
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Context Previous studies have suggested that excessive lead exposure is related
to aggressive and violent behavior.
Objective To evaluate the association between estimated air lead concentrations
and homicide rates.
Design Cross-sectional ecological study.
Setting All counties in the contiguous 48 states of the United States.
Exposure Measure Estimated air lead concentrations and blood lead levels.
Main Outcome Measure The homicide rate in each county.
Results Negative binomial regression was used to examine the relationship between
air lead concentrations and the incidence of homicide across counties in the
United States (N = 3111). After adjusting for sociologic confounding factors
and 9 measures of air pollution, the only indictor of air pollution found
to be associated with homicide rates was air lead concentration. Across all
counties, estimated air lead concentrations ranged from 0 to 0.17 µg/m3. The adjusted results suggest that the difference between the highest
and lowest level of estimated air lead is associated with a homicide incidence
rate ratio of 4.12 (95% confidence interval, 1.02-16.61).
Conclusion The results of this study support recent findings that there is an association
between lead exposure and violent behavior.
INTRODUCTION
NUMEROUS behavioral, neuropsychological, and biological studies suggest
that sufficient exposure to lead, a metallic neurotoxin, can promote brain
dysfunction.1 Called the neurotoxicity hypothesis, this position states that lead exposure alters
neurotransmitter and hormonal systems and may induce aggressive and violent
behavior.2, 3, 4, 5
Two studies have drawn specifically on the neurotoxicity hypothesis to examine
the relationship between lead exposure and criminal behavior.2, 5
Denno2 traced the behavioral patterns of 987
African American youth from birth to age 22 years. She found that among the
dozens of sociologic and biologic correlates of delinquency, lead poisoning
was among the strongest for male subjects.
Needleman et al5 studied bone lead levels
and self-reported antisocial behavior among 301 grade school students as part
of the Pittsburgh Youth Study. Using the Child Behavior Checklist, they found
that parents and teachers were more likely to rate the behavior of children
with high bone lead levels as delinquent and aggressive, and that these children
reported engaging in more delinquent acts than children with low bone lead
levels. The authors concluded that lead exposure "should be included when
considering the many factors contributing to delinquent behavior."5(p369)
Additional research examining a variety of settings and behaviors is
needed if the relationship between lead exposure and aggression is to be considered
more than speculative. One alternative is to examine the association between
air lead levels, blood lead levels, and homicide through the use of ecological
data. We focus on homicide as the most extreme outcome associated with expressions
of aggression that lead exposure might generate.6
Lead exposure occurs through a variety of sources, but has most often
been associated with the ingestion of lead-based paint by children.7, 8 Empirical research, however, suggests
that airborne lead is an important exposure pathway.9, 10
Seventy percent of inhaled lead less than 1 µm in size is absorbed directly
into the bloodstream, while larger lead particles may be trapped in mucous
and swallowed. Numerous studies have documented a strong positive association
between air lead levels and blood lead levels.9, 11, 12, 13
Following the ban on leaded gasoline, the Environmental Protection Agency
determined that most air lead comes from smelters, battery plants, and industrial
facilities that process lead. Given the importance of airborne lead as one
potential source of lead exposure, this study tested the hypothesis that a
positive, statistically significant association exists between estimated air
lead concentrations and the incidence of homicide across US counties in 1990.
METHODS
Data on potential lead exposure were drawn from the Environmental Protection
Agency's Cumulative Exposure Project (CEP).14
The CEP provides estimated concentrations of various air pollutants covered
under the Clean Air Act.15 Estimated air lead
concentrations, reflecting the average amount of air lead in micrograms per
cubic meter, for 1990 were available for all 3111 contiguous counties in the
United States. Long-term, large land tract air lead estimates were modeled
using the Assessment System for Population Exposure Nationwide (ASPEN). These
estimates were based on the amount of lead generated at fixed sources (eg,
large waste incinerators and factories), small, dispersed emitters (eg, gas
stations and machine repair shops), and mobile sources (eg, cars and off-road
equipment). The ASPEN model accounts for various meteorological conditions
(eg, wind speed and direction, atmospheric stability, decay, and secondary
formation) to adjust lead estimates to more closely mirror actual environmental
levels. The pollution estimates of ASPEN are highly correlated with actual
levels of environmental pollution.14
Research on the association between air lead concentrations and blood
lead levels indicates that air lead is an important source of levels of lead
in the body.9, 10, 11, 12, 13
To determine if CEP estimates of air lead concentrations were correlated with
blood lead levels at the county level, we obtained data from the Ohio Department
of Health on children younger than 6 years who had blood lead levels greater
than 10 µg/dL during 1998. The Centers for Disease Control and Prevention
(CDC) (Atlanta, Ga) identify the data from Ohio as more valid than data from
most states because a large proportion of children across all Ohio counties
are tested for lead poisoning. We found that across Ohio's counties, CEP-estimated
air lead concentrations were positively and significantly correlated with
the percentage of children (of those children screened) who had elevated blood
lead levels (Pearson correlation coefficient, 0.44; P<.001;
n = 88). That relationship persisted (Pearson correlation coefficient, 0.48; P<.001; n = 88) despite adjustments in the percentage
of houses built before the 1950s (1990 census estimate), reinforcing previous
findings that suggest that lead exposure may result from a variety of contamination
sources, including lead-contaminated air.
The National Center for Health Statistics mortality files at the CDC
contain homicide counts for each of the 3111 counties in the contiguous United
States for 1990. A death was counted as a homicide if it was coded in the
International Cause of Death classification as E960 to E969. We followed the
CDC procedures and averaged the number of homicides during a 3-year period
(1989-1991) to minimize random year-to-year fluctuations.16
We included 15 additional variables that may have potentially confounded
the relationship between air lead concentrations and the incidence of homicides.
Because homicide rates and air pollution are high in urban areas, we were
concerned that an association between atmospheric lead and homicide incidence
might reflect an association with air pollution in general. Therefore, we
included 9 additional air pollutants drawn from CEP data in our cross-sectional
homicide models. All air pollutants were measured in micrograms per cubic
meter. Four air pollutants included in the analysis (toluene, benzene, ethylene
dibromide, and ethylene dichloride) are or have been constituents of gasoline
(as was lead prior to its discontinued use in that capacity); 1 pollutant
(ethylene glycol) is used in antifreeze and brake fluids; and 4 pollutants
(cyanide, trifluralin, naphthalene, and ethylbenzene) were estimated by the
CEP to be most highly concentrated in urban areas.
We included 6 sociological confounding factors that have been hypothesized
to be related to potential lead exposure and homicide rates.5, 17, 18
These were derived from the Bureau of Census Summary Tape File 3C for 1990
and include area of county (square kilometers); number of persons aged 16
to 29 years; number of persons living beneath the Social Security Administration's
determined poverty level; number of African-Americans; a census-defined urban
indicator; number of individuals aged 18 years and older who have not completed
high school; and whether or not the county is located in a southern state.
To investigate the association between estimated air lead concentrations
and the incidence of homicides we used negative binomial regression.19 All statistical computations were performed using
statistical software (SAS Institute, Cary, NC). The natural logarithm of the
1990 population (eg, the midpoint between 1989 and 1991) was entered as an
offset term. All of the pollutants entered into the model were scaled so that
a 1-unit change represented the difference between the highest and lowest
levels across all counties.
RESULTS
Descriptive statistics and bivariate correlations for all of the variables
in this analysis are presented in Table
1. The mortality data indicate that many counties (992 [34%]) did
not report a homicide between 1989 and 1990. A few counties (eg, Los Angeles
County [Los Angeles, Calif] and Cook County [Chicago, Ill]), however, reported
a large number of homicides. Fewer than 1% of all counties reported more than
100 homicides annually. Estimated air lead concentrations and other measures
of pollution were also skewed (Table 1),
indicating that most counties faced relatively low levels of air pollution
in 1990 and were well within the benchmark concentrations suggested in the
toxicological literature.20 Table 2 presents the negative binomial regression estimates. Incidence
rate ratios are based on the range (eg, a change between the maximum and minimum
value) of each pollutant. All sociologic covariates and air lead levels were
entered into the unadjusted and adjusted air pollutant models. In the unadjusted
model, the difference between the highest and lowest levels of air lead is
associated with an incidence rate ratio of 14.09 (95% confidence interval
[CI], 3.68-53.99) The difference between the highest and lowest levels of
air lead in the adjusted model are much more conservative, with an incidence
rate ratio of 4.12 (95% CI, 1.02-16.61). This means that even while adjusting
for sociologic covariates and 9 other air pollutants, the incidence of homicide
was estimated to be nearly 4 times higher in a county that had an air lead
level equivalent to 0.17 µg/m3, the maximum air lead concentration
observed among all counties, than in a county with an air lead level equivalent
to 0 µg/m3. The other 9 air pollution indicators have CIs
for rate ratios that encompassed both negative and positive values, which
indicate that the relationship between estimated air lead concentrations and
homicides is not likely to be the result of a relationship between homicide
and air pollution in general.
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Table 1. Characteristics of 3111 Counties Analyzed*
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Table 2. The Relationship Between Estimated Air Lead Concentrations
and Homicide Rates Across US Counties, 1990*
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COMMENT
The major finding of this study is that there is an association between
air lead concentrations and homicide rates in the contiguous United States
in 1990. At this time, however, it is not possible to say that the observed
relationship is causal. Nevertheless, the finding of an association between
lead exposure and homicide is consistent with the few individual-level studies
examining the role of lead exposure in delinquency and aggression. Moreover,
the results of this study contribute to the emerging and controversial issue
concerning the role of lead exposure in predisposing some individuals to committing
crime and displaying violent behaviors.
The following weaknesses limit the generalizability of these findings
and suggest directions for future research. First, we studied ecologic aggregates
rather than individuals. The implications of our findings indicate that persons
who commit homicide tend to be exposed to higher levels of lead in the environment
than other persons. However, without further conformation of these results
at the individual level of analysis, we cannot be certain that this is the
case.
Second, our analysis is based on estimated air lead concentrations rather
than actual measures of air lead. While these estimates are generally deemed
valid, there is a possibility that some systematic bias in the estimation
procedure, of which we are unaware, artificially produced the observed relationship
between air lead concentrations and homicide rates.
Third, our analysis fails to consider sources of lead in the environment
other than air lead concentrations. Data for water and soil concentrations
of lead were not available for the entire United States during the period
under investigation. Because recent research indicates that soil and water
are important pathways to lead exposure,21, 22
such an omission might be substantial if air lead, soil lead, and water lead
concentrations are determined to be unrelated. Currently, however, we are
not aware of any empirical evidence that would indicate such a possibility.
In short, it is unlikely that counties with high air lead concentrations also
have low soil lead and water lead concentrations.
Fourth, the homicide data used in this study represent the homicide
victim's rather than homicide offender's county of residence. To the extent
that offenders and victims may not have resided in the same county, measurement
error may be present in our analysis and is likely to weaken the observed
relationship between lead and homicide.
For much of the 20th century, lead's deleterious effects on health were
widely accepted. Often, however, these affects were only associated with extreme
cases of lead exposure that occurred through occupational exposure or lead
paint ingestion by children. In recent years, research has pointed toward
the neurotoxicity and behavioral effects of lead at subclinical levels and
through a variety of environmental mediums. The results of our ecological
study indicate that these additional environmental pathways may be more ubiquitous
than imagined, affecting patterns of serious forms of violence such as homicide.
If the association uncovered in this analysis is truly reflective of a causal
relationship, these findings may have important policy implications that link
the need for continued efforts toward lead abatement, human health and behavior,
and crime control.
AUTHOR INFORMATION
Accepted for publication January 18, 2001.
We thank John Cochran, PhD, Thomas Miezckowski, PhD, Mike Lacy, PhD,
Thomas Cole, MD, and the several anonymous JAMA reviewers
for their valuable comments and suggestions on an earlier version of this
article.
From the Department of Sociology, Colorado State University, Fort Collins,
(Dr Stretesky); and the Department of Criminology, University of South Florida,
Tampa (Dr Lynch).
Corresponding author and reprints: Paul B. Stretesky, PhD, Department
of Sociology, B258 Clark Bldg, Colorado State University, Fort Collins, CO
80523 (e-mail: pstretes{at}lamar.colostate.edu).
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