A 2010 outbreak of acute lead poisoning in rural Zamfara State, northwestern Nigeria resulted in severe neurological morbidity and mortality in several hundred children, a result of lead-contaminated gold ore processed with low technology methods.¹ Gold ore processing became common in Zamfara after gold prices rose during the 2008–2012 global economic recession.² Lack of emission controls during processing, and the co-occurrence of gold ore with bioaccessible lead in the region’s underlying geology caused extensive contamination of villages. By March 2010, >l400 children had died and >2,000 children were left permanently disabled from inhalation and ingestion of lead particulates.³ An initial investigation found two mining villages where 118 of 463 (25%) children ≤5 years old had died in the previous 12 months, 82% of whom experienced symptoms consistent with lead poisoning.¹ Of the surviving children, 97% had blood lead levels (BLLs) ≥45 µg/dL, the level recommended by the US Centers for Disease Control and Prevention (US CDC) for chelation therapy, with measured BLLs as high as 700 µg/dL.⁴ Lead concentrations in residential soil exceeded the US Environmental Protection Agency’s (USEPA) 400 µg/g limit in 85% of samples, ranging to >100,000 µg/g, the limit of detection for the handheld x-ray fluorescence analyzer used. A follow-up investigation reported that the odds of lead poisoning or contamination were 3.5 times higher in ore processing villages versus non-ore-processing villages.⁵

Lead toxicity can affect every organ system, often with irreversible effects.⁶ The World Health Organization estimates that ≥16% of children worldwide have BLLs ≥10 µg/dL, with 90% living in low-income countries.⁷ Children are especially vulnerable to lead toxicity due to their physiology, and are exposed primarily through hand-to-mouth behaviors and inhalation.⁸ Childhood lead poisoning is associated with impaired neurodevelopment and cognitive function, lower IQ, impaired hearing, and reduced stature.⁹ Symptoms of acute lead poisoning include convulsions, coma, and death.¹⁰ There is no known safe BLL in children.¹¹ The US CDC currently uses a BLL in children of 5 µg/dL for action to reduce exposures.¹² International efforts have successfully reduced lead in many parts of the world, although many sources remain.¹³ Subsistence gold mining using low-technology methods such as manual ore grinding and liquid mercury amalgamation accounts for an estimated 20–30% of global gold production and is an important potential source of environmental contamination, and Zamfara’s underlying geology makes subsistence gold mining in this region particularly hazardous.^2,14

Previous investigations in Zamfara focused on highly contaminated villages for priority environmental and medical interventions. In these investigations, background lead levels in soil in villages which did not process ore were <25 µg/g. In villages which did process ore, soil lead levels ranged from 1,560–69,700 µg/g, and soil lead was highly bioaccessible, with an estimated absorption through the gastrointestinal system of 39–66%.²

Our objectives were to estimate the statewide prevalence of elevated BLLs in children ≤5 years old in ore processing and non-ore-processing family compounds (clusters of proximate polygamous family dwellings) and villages (dispersed collections of compounds in a rural area), and to identify factors associated with elevated BLL.

The study protocol was granted approval by the US CDC Institutional Review Board-A and the National Health Research Ethics Committee of Nigeria. Field work was performed during June and July of 2012.

We collected data from 392 compounds. Blood samples were insufficient for nine children, leaving a final sample of 383 children with a mean age of 35.2 months (standard deviation 16.2), with 52% boys and 48% girls. Less than 6% of the 383 compounds reported being engaged in lead-related occupations, except for auto repair, which was reported by 12.8% of compounds. Of the 383 children, 28.9% (111) had breastfed in the previous 12 months, 94.5% (362) lived in homes cleaned daily, and 87.0% (333) had worn lead-sulphate-based eyeliner in the previous 30 days (79% of boys and 96% of girls).

The estimated prevalence of having a compound member who had processed ore in the previous 12 months was 17.2% (95% CI: 9.7, 24.7; n=55). The estimated prevalence of each processing activity was as follows: 12.8% for breaking (95% CI: 7.1, 18.6; n=43), 7.6% for grinding (95% CI: 2.9, 12.4; n=31), 16.3% for washing (95% CI: 9.3, 23.2; n=50), 13.1% for drying (95% CI: 6.7, 19.5; n=45), 7.3% for separating (95% CI: 2.3, 12.4; n=30), and 4.9% for vaporizing mercury (95% CI: 0.5, 9.2; n=18).

Elevated BLLs were widespread in all compounds, although were more prevalent in children whose compound members processed ore, as shown in Table 1.

The geometric mean (GM) BLL was 9.5 µg/dL (95% CI: 7.9, 11.4) in compounds with members who processed ore (range: 2.6–61.3 µg/dL) and 6.5 µg/dL (95% CI: 6.1, 6.8) in compounds with no members who processed ore (range: 1.6–44.7 µg/dL). The GM lead concentration in dust samples was 40.5 mg/kg (95% CI: 32.4, 50.5) in compounds with members who processed ore (range: 14.0–710.0 mg/kg) and 30.7 mg/kg (95% CI: 27.6, 34.1) in compounds with no members who processed ore (range: 6.8–20,000 mg/kg). The GM lead concentration in soil samples was 39.2 mg/kg (95% CI: 31.1, 49.3) in compounds with members who processed ore (range: 11.0–560.0 mg/kg) and 24.3 mg/kg (95% CI: 22.3, 26.4) in compounds with no members who processed ore (range: 5.3–1,100 mg/kg).

Fifty (62.5%) water samples tested below the limit of detection for lead (1 µg/L). Wells in non-ore-processing villages (n=22) had a similar mean lead concentration to wells in ore processing villages (n=58) [2.2 µg/L (95% CI: 0.0, 5.3) vs 2.4 µg/L (95% CI: 1.1, 3.3)].

There were statistically significant positive Pearson correlations between natural-log transformed lead concentrations in soil and blood (r=0.3), dust and blood (r=0.3), and soil and dust (r=0.6), as well as between the number of ore processing activities performed by the child’s compound members and BLL (r=0.3). Children in compounds with members performing all six processing activities had GM BLLs of 14.1 (95% CI: 9.0, 21.9), higher than children in compounds performing fewer activities. The GM BLL was highest among children whose compound members participated in vaporizing mercury to remove it from the ore (Figure 1).

Differences in GM BLL were statistically significant (p<0.05) for children living in villages where any village resident participated in any ore processing activity, washing, drying, or vaporizing mercury compared to children in villages where it was reported that no one processed ore, regardless of whether children’s family compound members processed ore (Table 2). Although not statistically significant, breaking and grinding showed a similar trend.

The covariates identified as risk factors for natural-log transformed elevated BLL in the final linear regression model included living in a compound in which family members participated in vaporizing mercury (standardized coefficient (β=0.45, 95% CI: 0.10, 0.81), ore grinding (β= 0.30, 95% CI: 0.06, 0.55), and separating (β=0.16, 95% CI: −0.20, 0.51) in the last 12 months, using a piped tap as the primary drinking water source (β=0.63, 95% CI: 0.33, 0.93), lead-sulphate-based eyeliner exposure in the past 30 days (β=0.15, 95% CI: −0.02, 0.31), breastfeeding in the past 12 months (β=0.14, 95% CI: 0.002, 0.28), male sex (β=0.12, 95% CI: 0.01, 0.23), and child’s age (β=−0.004, 95% CI: −0.008, −0.0004).

We found elevated BLL rates lower than those reported in previous studies of highly contaminated villages conducted before widespread education on the dangerous but then-common practice of processing ore within compounds.^1,5 Nevertheless, we found that children with family members involved in ore processing were more likely to have elevated BLLs, and that BLLs tended to increase with the number of processing activities.

Our estimates for the GM BLL in children in non-ore-processing compounds (6.5 µg/dL) approximate those reported for children in urban environments in Nigeria.^20,21 Elevated BLLs in non-processing compounds might result from exposure to mine tailings generated by other village residents, as suggested by data in Table 2, or from using contaminated soil or water to make house bricks, processing prior to the 12 months in question, exposure to lead-sulphate-based eyeliner, or drinking water from leaded pipes.

Activities most strongly associated with higher BLL were grinding, separating, drying, and vaporizing mercury. Vaporizing was the least prevalent, but most strongly associated, possibly due to highly respirable aerosolized particulates produced from heating lead. Our finding that having compound members engaged in more activities was associated with higher BLLs may be explained by generation of more lead tailings, leading to environmental contamination, and to take-home lead on workers’ clothing and skin.

The USEPA’s recommended 400 µg/g limit for play area soil was exceeded by four sampled compounds, and indoor sleep area dust samples exceeded this level in 10 sampled compounds. By comparison, 85% of compounds sampled in 2010 had soil concentrations >400 µg/g.¹ While GM lead concentrations in soil and dust were higher in compounds where ore processing workers lived in our study, the highest concentrations were found in compounds that reported no processing in the previous 12 months. These compounds may have processed ore prior to the previous 12 months, or used soil or water contaminated from nearby processing to make bricks for their homes.

Our findings that elevated BLLs were associated with being male, younger age, lead-sulphate-based eyeliner exposure, and breastfeeding are consistent with the current literature.^22–24 Our observed association between piped tap water and elevated BLL could be due to lead pipes or solder. Data on lead in pipes were not collected.

This study benefited from a large sample of compounds with and without members engaged in ore processing, survey data, and blood, soil, and dust samples. These data are likely free from effects of seasonal variation in lead exposures, as all field work was performed during the beginning of the rainy season, when tailings are less likely to be spread by dry, windy conditions.²⁵

Drinking water is a possible lead ingestion route, although we did not observe an association between processing and lead in water. Most families use multiple seasonal water sources, including rainwater, so a single sample would not reflect exposures. Additionally, village-level water samples are a more crude exposure measure than compound-level soil, dust, and blood samples.

Study limitations include a lack of data on the actual locations of ore processing, and a lack of ability to distinguish processing-related take-home lead exposures from direct exposures to tailings. Additionally, we did not collect data on children’s behaviors that could impact exposure, such as hand-to-mouth behaviors or time spent near ore processing or tailings. Our inclusion of child’s age and sex in regression models may have controlled for some potential effect of behaviors on lead exposure. Another limitation is the lack of detailed dietary data on factors that could reduce lead uptake, such as iron supplements, or that could increase exposures, such as eating animals hunted with lead ammunition.²⁶

There are a number of opportunities for further study. Little is known about the synergistic effects of multiple metal exposures, and exploring this relationship in Zamfara could be informative. We did not find alarmingly high mercury levels in children, although they were elevated by US standards. Blood manganese and cadmium levels were also elevated for some children (data not shown). It would also be informative to perform follow-up cognitive studies of the children in the Zamfara area, because little is known about the impact of such high blood lead levels in children who survived the earlier disaster.

The results of this study demonstrate that elevated BLLs are widespread in Zamfara, and are higher among children living in compounds with members who processed lead-containing gold ore. The proportion of children with BLLs ≥45 µg/dL appears lower at the statewide level than previously reported for highly contaminated villages. Although elevated BLLs <45 µg/dL are not associated with mortality, they can have life-long health impacts. These children, and those who continue to be exposed, can suffer intelligence loss, learning and behavioral disorders, anemia, reduced stature, hearing loss, peripheral neuropathy, hypertension, and reproductive and renal effects. The societal and economic effects of aggregate intelligence loss in areas with widespread lead exposures can be substantial. Continued remediation, education, BLL surveillance, emission controls, and occupational hygiene measures are needed to prevent childhood lead exposures. The lead poisoning deaths in May 2015 of 28 children related to ore processing in Zamfara’s neighbor state of Niger demonstrate that these measures are needed throughout the region.²⁸