Maternal exposure to arsenic and mercury and associated risk of adverse birth outcomes in small-scale gold mining communities in Northern Tanzania.

BACKGROUND

Exposure to arsenic and mercury in artisanal and small-scale gold mining (ASGM) communities is an issue that predominantly affects low and middle-income countries. Large epidemiology studies in these communities are rare, and the impact of such exposures on reproductive outcomes are not well understood.

OBJECTIVE

To examine associations between prenatal maternal arsenic and mercury exposure and birth outcomes in both ASGM and non-ASGM communities in Northern Tanzania.

METHODS

This longitudinal prospective study included 961 women (ASGM = 788, non-ASGM = 173) of the original cohort of 1056 who were followed until a pregnancy outcome was registered. Maternal spot urine samples and dried blood spots were used to measure total arsenic (T-As) and total mercury (T-Hg) in the second trimester of pregnancy. Data on adverse birth outcomes were collected in 5 categories: spontaneous abortion, stillbirth, preterm birth, low birth weight, and visible congenital anomalies. Mann-Whitney U-tests were used to test for differences between median T-As and T-Hg by area of residence. Logistic regression models were used to estimate the odds of stillbirth and visible congenital anomalies given maternal T-As and T-Hg levels. Modified Poisson regressions were used to estimate relative risk ratios between maternal T-As and T-Hg levels and composite adverse birth outcome, spontaneous abortion, low birth weight, and preterm birth.

RESULTS

Statistically significant differences were found in median T-As (9.6 vs. 6.3 µg/L, Mann-Whitney U-tests, Z = -3.50, p < 0.001) and median T-Hg blood concentrations (1.2 vs. 0.70 µg/L, Z = -9.88, p-value < 0.001) between women living in ASGM and non-ASGM areas respectively. In ASGM areas, the adjusted relative risk (aRR) of a composite adverse birth outcome increased with increasing T-As (aRR 1.23, 95%CI: 1.14-1.33, p < 0.0001) and T-Hg (aRR 1.17, 95%CI: 1.1-1.25, p < 0.0001) exposure. Spontaneous abortion (aRR 1.53, 95%CI: 1.28-1.83), stillbirth (adjusted odds ratio (aOR) 1.97, 95%CI: 1.45-2.66) and preterm birth (1.17, 95%CI: 1.01-1.36) were significantly associated with elevated T-As, whereas elevated T-Hg was significantly associated with stillbirth (aOR 2.49, 95%CI: 1.88-3.29) and visible congenital anomalies (aOR 2.24, 95%CI: 1.3-3.87).

CONCLUSION

Over half (54.7%) of women in ASGM areas of Northern Tanzania had adverse birth outcomes and the risk of adverse birth outcomes was significantly associated with increased prenatal exposure to arsenic and mercury.