A summary of two meta-analyses on neurobehavioural effects due to occupational lead exposure

Vulnerability of the Hippocampus to Insults: Links to Blood-Brain Barrier Dysfunction

The hippocampus is a critical brain substrate for learning and memory; events that harm the hippocampus can seriously impair mental and behavioral functioning. Hippocampal pathophysiologies have been identified as potential causes and effects of a remarkably diverse array of medical diseases, psychological disorders, and environmental sources of damage. It may be that the hippocampus is more vulnerable than other brain areas to insults that are related to these conditions. One purpose of this review is to assess the vulnerability of the hippocampus to the most prevalent types of insults in multiple biomedical domains (i.e., neuroactive pathogens, neurotoxins, neurological conditions, trauma, aging, neurodegenerative disease, acquired brain injury, mental health conditions, endocrine disorders, developmental disabilities, nutrition) and to evaluate whether these insults affect the hippocampus first and more prominently compared to other brain loci. A second purpose is to consider the role of hippocampal blood-brain barrier (BBB) breakdown in either causing or worsening the harmful effects of each insult. Recent research suggests that the hippocampal BBB is more fragile compared to other brain areas and may also be more prone to the disruption of the transport mechanisms that act to maintain the internal milieu. Moreover, a compromised BBB could be a factor that is common to many different types of insults. Our analysis indicates that the hippocampus is more vulnerable to insults compared to other parts of the brain, and that developing interventions that protect the hippocampal BBB may help to prevent or ameliorate the harmful effects of many insults on memory and cognition.

Chronic lead exposure disrupts neurometabolic activity in mouse brain: An ex vivo1H-[13C]-NMR study

Lead poisoning has been identified as a problem in adults as well as in children. Chronic exposure to lead has been implicated in neurological disorders such as amyotrophic lateral sclerosis, Parkinson's disease, and Alzheimer's disease. In the present study, we evaluated the impact of chronic lead exposure on cerebral glutamatergic and GABAergic metabolic activity in mice. C57BL6 mice were provided lead acetate in drinking water for two months. The regional cerebral metabolic activity was measured using ¹H-[¹³C]-NMR spectroscopy in conjunction with infusion of [1,6-¹³C₂]glucose. The blood Pb²⁺ increased significantly in lead acetate treated mice. Concomitantly, there was a significant reduction in the forelimb strength. The level of myo-inositol was elevated in the cerebral cortex of mice chronically exposed to lead. The glutamatergic neurometabolic activity was found to be reduced following chronic lead exposure in the cerebral cortex, hippocampus, and striatum. In contrast, the GABAergic fluxes were impaired in the hippocampus and thalamus only. The metabolic fluxes in the cerebellum were unperturbed to Pb²⁺ toxicity. In conclusion, we report that chronic lead exposure in mice leads to an impairment in forelimb strength, and a perturbation in neurometabolism in brain regions involving cognition and movement.

Blood lead levels and cognitive functioning: A meta-analysis

OBJECTIVE

The neurotoxicity of lead is well-known and even low exposure levels potentially impact neurocognitive abilities. This metaanalysis aimed to investigate associations between neurocognitive performance and lead exposure in adults and further assess potential effect thresholds.

MATERIAL AND METHODS

Articles indexed in Medline published until January 2017 reporting associations between lead exposure, blood lead, cognitive abilities and sensomotoric performance were included. The unbiased, standardized mean differences g between lead exposure and control groups extracted from the primary studies were pooled using a three-level, random-effects meta-analytic model with a restricted maximum likelihood estimator. Moderator analyses were conducted using weighted, mixedeffects regression analyses.

RESULTS

We identified 22 articles (n=3,849 participants, mean age 39.94±7.87years) published between 1976 and 2014 reporting lead exposure effects on cognitive and sensomotoric parameters (verbal abilities, visuospatial abilities, memory, attention, psychomotor function). On average, blood lead concentrations were 21.09±6.44 μg/dl higher in exposed than in control subjects. After exclusion of outliers, the random-effects three-level meta-analysis identified a significant (p<.001) pooled mean difference between exposure and control groups. Except for a smaller effect in the digit symbol test (p<.05), lead exposure did not result in different outcomes across the examined cognitive measures. Based on a marginally significant (p=.06) effect of difference in exposure levels, a blood lead increase of 10 μg/dl translated into a decline in cognitive abilities of Hedges g=.09.

CONCLUSIONS

Neurocognitive performance in adults with occupational or environmental lead exposure was significant impaired with regard to the examined parameters. However, further studies are needed for the determination of effect thresholds and reversibility.

Evidence-based toxicology: a comprehensive framework for causation

This paper identifies deficiencies in some current practices of causation and risk evaluation by toxicologists and formulates an evidence-based solution. The practice of toxicology focuses on adverse health events caused by physical or chemical agents. Some relations between agents and events are identified risks, meaning unwanted events known to occur at some frequency. However, other relations that are only possibilities – not known to occur (and may never be realized) – also are sometimes called risks and are even expressed quantitatively. The seemingly slight differences in connotation among various uses of the word ‘risk’ conceal deeply philosophic differences in the epistemology of harm. We label as ‘nomological possibilities’ (not as risks) all predictions of harm that are known not to be physically or logically impossible. Some of these nomological possibilities are known to be causal. We term them ‘epistemic’. Epistemic possibilities are risks. The remaining nomological possibilities are called ‘uncertainties’. Distinguishing risks (epistemic relationships) from among all nomological possibilities requires knowledge of causation. Causality becomes knowable when scientific experiments demonstrate, in a strong, consistent (repeatable), specific, dose-dependent, coherent, temporal and predictive manner that a change in a stimulus determines an asymmetric, directional change in the effect. Many believe that a similar set of characteristics, popularly called the ‘Hill Criteria’, make it possible, if knowledge is robust, to infer causation from only observational (nonexperimental) studies, where allocation of test subjects or items is not under the control of the investigator.

Until the 1980s, medical decisions about diagnosis, prevention, treatment or harm were often made authoritatively. Rather than employing a rigorous evaluation of causal relationships and applying these criteria to the published knowledge, the field of medicine was dominated by authority-based opinions, expressed by experts (or consensus groups of experts) relying on their education, training, experience, wisdom, prestige, intuition, skill and improvisation. In response, evidence-based medicine (EBM) was developed, to make a conscientious, explicit and judicious use of current best evidence in deciding about the care of individual patients. Now globally embraced, EBM employs a structured, ‘transparent’ protocol for carrying out a deliberate, objective, unbiased and systematic review of the evidence about a formally framed question. Not only in medicine, but now in dentistry, engineering and other fields that have adapted the methods of EBM, it is the quality of the evidence and the rigor of the analysis through evidence-based logic (EBL), rather than the professional standing of the reviewer, that leads to evidence-based conclusions about what is known. Recent studies have disclosed that toxicologists (individually or in expert groups), not unlike their medical counterparts prior to EBM, show distressing variations in their biases with regard to data selection, data interpretation and data evaluation when performing reviews for causation analyses. Moreover, toxicologists often fail to acknowledge explicitly (particularly in regulatory and policymaking arenas) when shortcomings in the evidence necessitate reliance upon authority-based opinions, rather than evidence-based conclusions (Guzelian PS, Guzelian CP. Authority-based explanation. Science 2004; 303: 1468-69). Accordingly, for answering questions about general and specific causation, we have constructed a framework for evidence-based toxicology (EBT), derived from the accepted principles of EBM and expressed succinctly as three stages, comprising 12 total steps. These are: 1) collecting and evaluating the relevant data (Source, Exposure, Dose, Diagnosis); 2) collecting and evaluating the relevant knowledge (Frame the question, Assemble the relevant (delimited) literature, Assess and critique the literature); and 3) Joining data with knowledge to arrive at a conclusion (General causation – answer to the framed question, Dose-response, Timing, Alternative cause, Coherence). The second of these stages (which amounts to an analysis of general causation), is addressed by an EBM-styled approach (adapted for the infrequent availability of human experimental studies in environmental toxicology). This involves assembling literature (through documented algorithms for database queries), excluding irrelevancies by use of delimiters as filters, and ranking and rating the remaining articles for strength of study design and for quality of execution gauged by application of either a ready-made quality assessment instrument or a custom designed checklist or scale. The results of this systematic review (including a structured review of relevant animal and in vitro studies) are then themselves systematically used to determine which causation criteria are fulfilled.

Toxicology is maturing from a derivative science largely devoted to routinized performance and interpretation of safety tests, to a discipline deeply enmeshed in the remarkable advances in biochemistry and molecular biology to better understanding the nature and mechanism of adverse effects caused by chemicals. It is time for toxicologists, like scientists in other fields, to formalize a method for differentiating settled toxicological knowledge of risk from mere nomological possibility, and for communicating their conclusions to other scientists and the public. It is time for EBT.

Rethinking Meta-Analysis: Applications for Air Pollution Data and Beyond

Meta-analyses offer a rigorous and transparent systematic framework for synthesizing data that can be used for a wide range of research areas, study designs, and data types. Both the outcome of meta-analyses and the meta-analysis process itself can yield useful insights for answering scientific questions and making policy decisions. Development of the National Ambient Air Quality Standards illustrates many potential applications of meta-analysis. These applications demonstrate the strengths and limitations of meta-analysis, issues that arise in various data realms, how meta-analysis design choices can influence interpretation of results, and how meta-analysis can be used to address bias and heterogeneity. Reviewing available data from a meta-analysis perspective can provide a useful framework and impetus for identifying and refining strategies for future research. Moreover, increased pervasiveness of a meta-analysis mindset-focusing on how the pieces of the research puzzle fit together-would benefit scientific research and data syntheses regardless of whether or not a quantitative meta-analysis is undertaken. While an individual meta-analysis can only synthesize studies addressing the same research question, the results of separate meta-analyses can be combined to address a question encompassing multiple data types. This observation applies to any scientific or policy area where information from a variety of disciplines must be considered to address a broader research question.

Study for Promotion of Health in Recycling Lead - Rationale and design

BACKGROUND

The level at which low-level lead exposure produces subclinical adverse health effects in adults remains to be established.

METHODS

The Study for Promotion of Health in Recycling Lead (SPHERL) will enroll 500 newly hired workers, whose blood lead during 2 years of follow-up is expected to increase from levels less than 2 μg/dl, as currently observed in the US population, to 20-30 μg/dl. The main outcome variables to be studied are (i) blood pressure (BP) analyzed as a continuous or categorical variable, both cross-sectionally and longitudinally, and using conventional and ambulatory BP measurement; (ii) indexes of glomerular and tubular renal function, (iii) heart rate variability analyzed in the frequency domain as measure of autonomous sympathetic modulation, (iv) peripheral nerve conductivity velocity, (v) neurocognitive performance, and (vi) quality of life. Expected outcomes. Assuming a 10-fold increase in blood lead, SPHERL will have sufficient statistical power to detect over 2 years a steepening of the age-related rise in systolic BP from 1 to 5 mmHg and a doubling of the age-related decline in the estimated glomerular filtration rate from 3.5 to 7.0 ml/min/1.73 m(2). The longitudinal design of our study complies with the temporality principle of the Bradford-Hill criteria for assessing possible causality between outcomes and exposure. SPHERL will attempt to resolve the apparent contradiction between general population studies showing associations between adverse health effects and low lead exposure with blood lead levels below 5 μg/dl and studies conducted in occupational cohorts indicating that adverse effects of lead exposure occur at much higher blood lead levels.

Cumulative lead exposure in community-dwelling adults and fine motor function: comparing standard and novel tasks in the VA normative aging study

BACKGROUND AND AIMS

Lead exposure in children and occupationally exposed adults has been associated with reduced visuomotor and fine motor function. However, associations in environmentally exposed adults remain relatively unexplored. To address this, we examined the association between cumulative lead exposure-as measured by lead in bone-and performance on the grooved pegboard (GP) manual dexterity task, as well as on handwriting tasks using a novel assessment approach, among men in the VA Normative Aging Study (NAS).

METHODS

GP testing was done with 362 NAS participants, and handwriting assessment with 328, who also had tibia and patella lead measurements made with K-X-Ray Fluorescence (KXRF). GP scores were time (s) to complete the task with the dominant hand. The handwriting assessment approach assessed the production of signature and cursive lowercase l and m letter samples. Signature and lm task scores reflect consistency in repeated trials. We used linear regression to estimate associations and 95% confidence intervals (CI) with adjustment for age, smoking, education, income and computer experience. A backward elimination algorithm was used in the subset with both GP and handwriting assessment to identify variables predictive of each outcome.

RESULTS

The mean (SD) participant age was 69.1 (7.2) years; mean patella and tibia concentrations were 25.0 (20.7)μg/g and 19.2 (14.6)μg/g, respectively. In multivariable-adjusted analyses, GP performance was associated with tibia (β per 15μg/g bone=4.66, 95% CI: 1.73, 7.58, p=0.002) and patella (β per 20μg/g=3.93, 95% CI: 1.11, 6.76, p=0.006). In multivariable adjusted models of handwriting production, only the lm-pattern task showed a significant association with tibia (β per 15μg/g bone=1.27, 95% CI: 0.24, 2.29, p=0.015), such that lm pattern production was more stable with increasing lead exposure. GP and handwriting scores were differentially sensitive to education, smoking, computer experience, financial stability, income and alcohol consumption.

CONCLUSIONS

Long-term cumulative environmental lead exposure was associated with deficits in GP performance, but not handwriting production. Higher lead appeared to be associated with greater consistency on the lm task. Lead sensitivity differences could suggest that lead affects neural processing speed rather than motor function per se, or could result from distinct brain areas involved in the execution of different motor tasks.

The protean toxicities of lead: new chapters in a familiar story

Many times in the history of lead toxicology the view that "the problem" has been solved and is no longer a major health concern has prevailed, only to have further research demonstrate the prematurity of this judgment. In the last decade, an extraordinary amount of new research on lead has illustrated, all too clearly, that "the problem" has not disappeared, and that, in fact, it has dimensions never before considered. Recent risk assessments have concluded that research has yet to identify a threshold level below which lead can be considered "safe." Although children's intelligence has traditionally been considered to be the most sensitive endpoint, and used as the basis for risk assessment and standard setting, increased lead exposure has been associated with a wide variety of other morbidities both in children and adults, in some cases at biomarker levels comparable to those associated with IQ deficits in children. In adults, these endpoints include all-cause mortality and dysfunctions in the renal, cardiovascular, reproductive, central nervous systems. In children, IQ deficits are observed at blood lead levels well below 10 μg/dL, and the dose-effect relationship appears to be supra-linear. Other health endpoints associated with greater early-life lead exposure in children include ADHD, conduct disorder, aggression and delinquency, impaired dental health, and delayed sexual maturation. Studies employing neuroimaging modalities such as volumetric, diffusion tensor, and functional MRI are providing insights into the neural bases of the cognitive impairments associated with greater lead exposure.

A proton magnetic resonance spectroscopy study of the chronic lead effect on the Basal ganglion and frontal and occipital lobes in middle-age adults

BACKGROUND

Lead is known to be a health hazard to the human brain and nervous system based on data from epidemiologic studies. However, few studies have examined the mechanism or biochemical changes caused by lead in the human brain, although recently some have used magnetic resonance spectroscopy (MRS) to test brain metabolism in vivo.

OBJECTIVES

In this study, we used 3-T MRS to investigate brain metabolism in workers chronically exposed to lead and matched nonexposed controls.

METHODS

Twenty-two workers at a lead paint factory served as chronically exposed subjects of this study. These workers did not have any clinical syndromes. Eighteen age- and sex-matched nonexposed healthy volunteers served as controls. We measured blood and bone lead and used a 3-T MRS to measure their levels of brain N-acetyl aspartate (NAA), choline (Cho), and total creatine (tCr). A structural questionnaire was used to collect demographic, work, and health histories and information about their life habits.

RESULTS

All the MRS measures were lower in the lead-exposed group. Increased blood and bone lead levels correlated with declines in Cho:tCr ratios, especially in the occipital lobe, where changes in all gray, subcortical, and white matter were significant. Increases in blood and patella lead in every layer of the frontal lobe correlated with significant decreases in NAA:tCr ratios. One of the strongest regression coefficients was -0.023 (SE = 0.005, p < 0.001), which was found in the NAA:tCr ratio of frontal gray matter.

DISCUSSION

We conclude that chronic exposure to lead might upset brain metabolism, especially NAA:tCr and Cho:tCr ratios. Brain NAA and Cho are negatively correlated to blood and bone lead levels, suggesting that lead induces neuronal and axonal damage or loss. The most significant changes occurred in frontal and occipital lobes, areas in which previous neurobehavioral studies have shown memory and visual performance to be adversely affected by lead toxicity.

Neuromotor function in a cohort of Danish steel workers

OBJECTIVE

With a longitudinal design to evaluate possible neuromotor impairment in a cohort of steel workers exposed to metal dust.

MATERIAL

Ninety-two employees from a steel works were examined in 1989 and 1995. Sixty were re-examined in 2003. A non-matched control group was examined in 1996 (n=19) and in 2003 (n=14). Median blood manganese in 1989, 1995 and 2003 was 149, 171 and 155 nmol/l. Median blood lead in 1989 and 2003 was 0.76 and 0.22 micromol/l. Median air concentration of manganese at the steel works was estimated to be 0.11 mg/m3 in 1970s and was 0.03 mg/m3 in 1990s. Median air concentration of lead was estimated to be 0.13 mg/m3 in 1970s and was 0.01 mg/m3 in 1990s.

METHOD

The Catsys 2000 system developed by Danish Product Development is computer-based device for measuring hand tremor, hand coordination and reaction time.

RESULTS

Over all there were no statistically significant differences in neuromotor function between the participating steel workers, non-participating steel workers and controls in 1995/1996. Only reaction time for the right hand was slower for the participating steel workers. Compared with the control group the steel workers showed a decline in the ability to perform fast precise hand pronation/supination and finger tapping from 1995 to 2005. Correlation analysis showed no associations between test results for fast hand coordination and blood manganese and lead. Only seniority was associated with deterioration of beat regulation of fast pronation/supination of the hands.

DISCUSSION

On a group basis the changes were subclinical, but they should none the less be taken seriously.

CONCLUSION

Changes of neuromotor function measured as the ability to perform fast precise pronation/supination of the hands and fast precise finger tapping was shown in this cohort of steel workers. No causal relationships could be shown.

Cumulative lead dose and cognitive function in adults: a review of studies that measured both blood lead and bone lead

OBJECTIVE

We review empirical evidence for the relations of recent and cumulative lead dose with cognitive function in adults.

DATA SOURCES

A systematic search of electronic databases resulted in 21 environmental and occupational studies from 1996 to 2006 that examined and compared associations of recent (in blood) and cumulative (in bone) lead doses with neurobehavioral outcomes.

DATA EXTRACTION

Data were abstracted after consideration of exclusion criteria and quality assessment, and then compiled into summary tables.

CONCLUSIONS

At exposure levels encountered after environmental exposure, associations with bio-markers of cumulative dose (mainly lead in tibia) were stronger and more consistent than associations with blood lead levels. Similarly, in studies of former workers with past occupational lead exposure, associations were also stronger and more consistent with cumulative dose than with recent dose (in blood). In contrast, studies of currently exposed workers generally found associations that were more apparent with blood lead levels; we speculate that the acute effects of high, recent dose may mask the chronic effects of cumulative dose. There is moderate evidence for an association between psychiatric symptoms and lead dose but only at high levels of current occupational lead exposure or with cumulative dose in environmentally exposed adults.

Cognitive function in a cohort of Danish steel workers

OBJECTIVE

The aim of this study was to evaluate the level of possible cognitive impairment in a cohort of steel workers occupationally exposed to manganese and lead.

MATERIAL

Ninety-two employees from an electro-steel works were examined in 1989 and 1995. Fifty-three were re-examined in 2003. Median age of the participants was 53 years, median duration of employment was 24 years, median blood manganese in 1989 and 1995 was 148 and 171 nmol/l, respectively, and median blood lead in 1989 was 0.79 micromol/l. Non-participants were comparable with participants, although they had a higher level of blood manganese in 1989 (186 nmol/l) and 1995 (186 nmol/l). Manganese level in the air was estimated below 1.9 mg/m3 in the 1970s. In the 1990s, manganese level in the air was below 0.28 mg/m3 in the majority of measurements.

METHOD

Cognitive function was examined with the Cognitive Function Scanner, a computer-based neuropsychological test battery. From a published set of norms a subgroup (n=106) matched for gender, age and social status was extracted and used for comparison.

RESULTS

Learning and memory, visuomotor and visuospatial function, concentration, attention, perception and vigilance were examined. Despite many statistically significant differences between the groups, it was not possible to interpret the results for the steel workers as being better or worse. In a visuomotor subtest, the pen-to-point test, the steel workers were much less accurate than the comparison group. This could be the result of an impaired ability to make fast accurate movements. There were no associations between pen-to-point test results and duration of employment or blood levels of manganese and lead.

CONCLUSION

Intellectual impairment could not be shown with the Cognitive Function Scanner in this cohort of low to moderate manganese and lead exposed steel workers. A slight subclinical impairment of the visuomotor function was possibly found.

Neurobehavioral science in hazard identification and risk assessment of neurotoxic agents--what are the requirements for further development?

OBJECTIVES

Modern neurobehavioral methods find useful application in research into the early effects of exposure to neurotoxic agents in the environment. This paper briefly describes the history and evolution of neurobehavioral toxicology, reviews some current trends in research in this specific discipline and identifies the most important needs and challenges to be addressed in future studies.

METHODS

All published literature was considered, including ad hoc meeting reports. Further information was obtained directly from experts in the field.

RESULTS

The number of studies (including those in occupational, environmental and pediatric exposure) using neurobehavioral evaluation is constantly increasing. Regulatory agencies are using scientific data obtained through neurobehavioral assessment, which includes other areas such as neurosensory toxicology. However, further development of this discipline is facing a number of problems and issues. Three major areas that deserve further attention have been identified: (1) specific technical issues regarding testing development, (2) epidemiological issues regarding the study design, including the need for meta-analysis/multi-center studies and for longitudinal observation, and statistical issues regarding the most adequate models for the analysis and treatment of complex neurobehavioral datasets, and (3) the need for scientific consensus on the significance of adverse effects identified with neurobehavioral methods.

CONCLUSIONS

The importance of neurobehavioral toxicology in the evaluation of mechanisms of action and for preventive purposes is progressively growing. Further development is needed for the advancement of this discipline through collaboration between experts from different fields.

Changes of neurobehavioral and sensory functions due to toluene exposure below 50ppm?

Data of a follow up study with four examinations were summarized by odds ratio statistics in order to scrutinize the results of the earlier studies with cross sectional approaches. For a "high"- (n=106) and a "low"-exposed group (references, n=86) current toluene exposures of 26ppm versus 3ppm and lifetime weighted average exposures of 45ppm versus 9ppm were ascertained. As measures of sensory functions vibration thresholds, colour discrimination, and auditory thresholds were used. Measures of psychological performances were attention (symbol-digit substitution, switching attention, simple reaction), memory (digit span forward, delayed reproduction of pictures), and psychomotor functions (steadiness, line tracing, aiming, tapping, peg board). Additionally, the frequency of diseases and symptoms were ascertained. By odds ratio statistics including relevant cofactors no significant increase of "cases with impaired functions" among the high-exposed workers was found. Evidence for neurobehavioral effects due to long-term toluene exposure below 50ppm was not established.

Reversible neurobehavioral performance with reductions in blood lead levels--a prospective study on lead workers

Lead poisoning remains an occupational hazard in Taiwan. Many studies, based on crossed-section design, have focused on changes in lead-associated neurobehavioral dysfunctions that occur at increased blood lead levels. This study evaluates the changes in neurobehavioral performance of lead workers as blood levels are reduced. We tested 27 lead workers in a lead glaze factory using the computer-based and automated Chinese edition of Neurobehavioral Evaluation System 2 (C-NES II) in 1994, 1996, and 1997. The association of blood lead levels and C-NES II results were analyzed by longitudinal data analysis methods, repeated ANOVA and mixed model analyses after adjustment for potential confounders. Over these 4 years, the mean lead blood levels of workers were reduced from 26.3(SD=12.0) to 8.3(SD=6.9) microg/dL. Based on a mixed model analysis, we found that the negative effects of exposure to lead on neurobehavioral performance can be reversed to some extent with lowering levels of blood lead. During this period, we found significant improvements in 3 subtests: finger tapping, pattern comparison reaction time, and memory. This study tentatively concluded that reversibility of the neurobehavioral performance after reduction of the lead exposure, which encourages industrial hygiene and personal health promotion to reduce their body lead burden. However, though use of NES is more sensitive to detect the changes, the chronic symptoms (using standardized questionnaire) were found to decline slowly when blood lead level is reduced, with no statistically significant difference. The result means that to avoid the lead exposure primarily is essential to prevent chronic symptoms. We conclude that the most important way to prevent and possibly reverse chronic symptoms of lead poisoning remains to be the avoidance of exposure to lead.

Delta-aminolevulinic acid dehydratase (ALAD) polymorphism and susceptibility of workers exposed to inorganic lead and its effects on neurobehavioral functions

We carried out a cross-sectional study on a group of male workers to determine the frequency of delta-aminolevulinic acid dehydratase (ALAD) polymorphisms among Chinese, Malays and Indians workers who were exposed to low to medium levels of inorganic lead. Also, the association between ALAD1 and ALAD2 genotypes and neurobehavioral functions among these workers were investigated. A total of 120 male workers were studied. Blood and urine were collected for each worker to determine the ALAD genotypes, blood lead levels, ALAD, and urinary delta-aminolevulinic acid (ALAU). ALAD1-1 was the predominant genotype for all three ethnic groups while ALAD2-2 was the rarest. The distribution of ALAD1-2 was higher among Malays (16.7%) and Indians (14.3%), compared to Chinese (3.6%). Selected tests from the World Health Organization Neurobehavioral Core Test Battery (WHO-NCTB) were used. Although workers in the ALAD1-1 and ALAD1-2/2-2 groups had comparable blood lead levels, the 106 workers with ALAD1-1 genotypes have significantly higher urinary ALA and significantly poorer neurobehavioral scores involving motor dexterity compared with those who have ALAD1-2/2-2 genotypes (13 workers). It is postulated that the ALAD2 allele may exert protective measures against the neurotoxic effects of lead. Further study involving a larger cohort of workers with the ALAD2 allele would be needed to confirm this hypothesis.