Since Mn dust loading is likely to be on the casual pathway for some of our predictors of exposure, such as farm worker shoes in the home and proximity to agricultural use of Mn fungicides, we used a structural equation model to evaluate casual pathways of exposure in the model that included participants with Mn dust measurements. We constructed a structural equation model to simultaneously estimate Mn tooth levels and Mn dust loading as outcome variables, with Mn dust loading also included as a predictor variable in the Mn tooth model. Maternal smoking during pregnancy was associated with a 34% and 40% decrease in MnPN levels in models with and without Mn house dust loading, respectively. We excluded from the multivariable models those variables that were not significant predictors of MnPN , including traffic density, Mn outdoor air concentration, acres of lettuce near the home, estimated total dietary Mn and iron intake, tap water consumption, estimated prenatal Mn tap water concentration and Mn tap water intake, maternal country of birth, maternal education, household income, housekeeping practices,raspberry grow in pots and maternal hematocrit to hemoglobin ratio during pregnancy. The coefficient of determination was 22% for the model with Mn tooth measurements and 29% for the model including both Mn tooth and house dust measurements.
There was no spatial auto correlation between the residuals for either model . Table 3 also provides the proportion of the variance explained for the predictor variables from multi-variable models of MnPN for all children with tooth measurements and those with Mn measured in both teeth and dust . The number of farm workers storing shoes in the home , maternal smoking during pregnancy , prenatal residence on Antioch Loam soil and agricultural use of Mn fungicides within 3 km of residence explained the greatest amount of variability of MnPN in the model without Mn house dust loading. Maternal smoking , prenatal residence on Antioch Loam soil , the number of farm workers storing shoes in the home , Mn house dust loading and maternal farm work during pregnancy explained the largest proportion of variability of MnPN in the model for children that also had Mn measured in prenatal house dust. Using structural equation models, the same predictor variables were significant and no new significant predictors of Mn levels in teeth were identified. The percentage change and significance level was nearly identical for maternal smoking, maternal farm work and residence on Antioch Loam soil which were predictors of Mn levels in teeth. However, agricultural use of Mn fungicides near the home and the number of farm worker shoes stored in the home were significant predictors of Mn dust loading. As a result, the percentage change associated with an increase in Mn dust loading corresponding to the interquartile range was 17.4% in the structural equation model compared to 3.4% in the ordinary regression model because Mn dust loading now included the effects of agricultural Mn fungicide use near the home and farm worker shoes stored in the home.
We report that Mn levels measured in prenatal dentin using laser ablation inductively coupled plasma mass spectroscopy were associated with estimates of prenatal environmental Mn exposure. Our findings suggest that deciduous teeth provide a biomarker of prenatal Mn exposure that is available retrospectively for the study of Mn related health effects, which would be especially useful in case-control studies. We observed that agricultural applications of widely used Mn-containing fungicides, maneb, and mancozeb, contribute to higher Mn tooth levels in this population of children living in an agricultural community. This is the first study to evaluate Mn measurements in deciduous teeth as an age-specific indicator of exposure from agricultural or industrial use of Mn. The only previous study that assessed Mn exposure from fungicides found that pregnant women who reported pesticide spraying less than a kilometer from their house had significantly higher blood Mn concentrations in a community where apple orchards were sprayed with mancozeb.An evaluation of Mn concentrations in house dust found higher levels in residences located within 500 m of agricultural fields than residences located farther from fields.Ethylenethiourea measured in urine has been used as an indicator of occupational exposure to maneb and mancozeb.In the CHAMACOS cohort, ethylenethiourea was detected in 24% of maternal urine samples collected near the beginning of the second trimester suggesting maternal exposure to maneb occurred during pregnancy in this cohort.38 Our results add to the existing evidence that household proximity to farmland and parental occupational take-home increases children’s exposure to other classes of pesticides.
Importantly, agricultural-related variables such as farm work by the mother, storage of farm worker’s shoes indoors and agricultural use of Mn containing fungicides within 3 km of the residence were significantly associated with increased tooth Mn levels and along with maternal smoking explained the largest proportion of the variance in this cohort. Including Mn house dust loading in the ordinary regression model reduced the amount of variability explained by the number of farm workers storing shoes in the home and agricultural use of Mn fungicides, and based on a structural equation model this was a result of Mn dust loading being on the casual exposure pathway for Mn from these sources. Nevertheless, the predictors we identified explained only 22–29% of the variability in Mn levels in prenatal dentin suggesting that other unknown factors contributed to Mn body burden. Iron status and iron metabolizing genes such as hemochromatosis and transferrin may play an important role in Mn biomarker levels. Mn levels in blood were 12% lower among women carrying any variant allele of HFE than women with no variant alleles and these results were replicated in a knockout mice model, suggesting that HFE contributes to variability in Mn exposure biomarkers.Mn levels in hair and estimated ambient Mn air concentrations near a ferromanganese refinery in Ohio were significantly correlated only when HFE or TF genotypes were included in the models.Women with low serum ferritin levels had higher blood Mn levels than the normal group in Korea.A limitation of the present study is that we did not have information on iron-metabolizing genes HFE and TF or serum ferritin levels. We did not observe a relationship between maternal hematocrit to hemoglobin ratio or estimated dietary iron intake during pregnancy and MnPN. Although we had few mothers that smoked in our population , we observed significantly lower MnPN levels in children whose mother smoked during pregnancy in multi-variable models. One previous study also observed a negative relationship between smoking and Mn blood levels in the second trimester but not at delivery,30 planter pot while a national study in Korea also found lower Mn blood concentrations among current and former smokers.Similar findings have previously been reported in relation to placental transfer of zinc; umbilical cord blood zinc levels were lower in mothers who smoked during pregnancy compared to nonsmokers.Mn is an essential nutrient that protects against oxidative stress.As a result, Mn levels in blood may be lower in smokers and less available for fetal transfer in pregnant smokers. Further studies are needed to evaluate the relationship between smoking and biomarkers of Mn and to identify the mechanisms by which smoking reduces Mn transfer to the fetus. We also found higher MnPN levels in children whose prenatal residence was located on Antioch Loam, soil which can be high in manganese content.A previous study found an association between Mn levels in soil outside the residence and Mn concentrations in house dust, showing that Mn levels in the home can be influenced by Mn soil concentrations.Previous exposure studies found that Mn levels in children’s hair decreased with residential distance from a ferromanganese alloy plant, and residential duration and proximity to the plant explained 37% of the variance.A recent study using a new method for cleaning hair prior to analysis found significantly higher Mn levels in children living in the vicinity of active, but not historic, ferroalloy plant emissions.
Mn measured in personal air for 38 children living near a ferromanganese refinery were associated with distance to the refinery but Mn in blood and hair were not.Higher nitrogen dioxide concentrations, a proxy for motor vehicle emissions, have been associated with higher Mn levels in cord blood.We did not observe an association between MnPN and traffic density, which is relatively low in our study area, but we did see a borderline significant increase in MnPN with estimated outdoor Mn air concentrations in the model that included Mn house dust loading. We observed higher Mn levels in teeth during the second trimester than the third trimester while previous studies have found higher maternal blood Mn concentrations later in pregnancy.While maternal blood Mn levels fluctuate during pregnancy, they do not necessarily reflect variations in fetal exposure. The use of dentin Mn allows us to measure fetal Mn exposure directly and we observed higher Mn levels in dentin formed during the second trimester in comparison to dentin formed later in gestation. There are no known variations in tooth mineralization over this period that would affect Mn uptake in dentin, and it is possible that the higher Mn levels in dentin formed during the second trimester reflect increased fetal uptake. Future studies should assess the within person variability in MnPN using multiple teeth per child and evaluate Mn levels in different types of teeth that develop at slightly different times. This study had a number of other limitations. We did not have information on time activity patterns for the mothers and using only residential locations to assess proximity to Mn fungicide use and other Mn sources could result in misclassification of exposure. We did not collect personal environmental or duplicate diet samples to measure Mn exposure. Drinking water quality data was collected for regulatory purposes not to determine exposure levels and sampling occurred irregularly over time. Most of our study population drank less than one glass per day of tap water and Mn was not detected frequently in public water supplies in our study area. Future studies should collect tap water samples for Mn analysis to better characterize potential exposure from drinking water. We used data from the Total Diet Study to estimate Mn intake via food items but this study may not be representative of Mn levels in food consumed by our population, however, the primary source of dietary intake in our population was from prenatal vitamin supplements. Strengths of this study include extensive prenatal questionnaire data and prenatal house dust samples with measured Mn concentrations and loadings for a subset of participants. We measured Mn levels in dentin for specific prenatal time points using knowledge of tooth mineralization instead of digesting the entire tooth and combining prenatal and postnatal exposures. Previous studies have used measurements in tooth enamel to estimate Mn exposure; however measurements in enamel cannot be readily linked to developmental timing of exposure because, unlike dentin, initial deposits of enamel matrix are not completely mineralized immediately but rather more slowly and diffusely during maturation. An additional strength of our study is the availability of prenatal latitude and longitude coordinates which allowed the use of GIS methods and publically available data on agricultural pesticide use, drinking water, hazardous air pollutants and traffic density resulting in limited exposure information bias. We were also able to evaluate a comprehensive set of exposure predictors including occupational information, household and demographic characteristics, dietary intake, drinking water consumption, outdoor air concentrations and house dust levels. In future analyses, we will evaluate measurements of MnPN for children newly enrolled in the CHAMACOS study at 9-years of age, utilizing multilevel Bayesian measurement error models to improve exposure estimates.We will also evaluate the relationship between Mn levels in teeth and neurodevelopment in the CHAMACOS cohort. In conclusion, we found that exposure variables related to Mn containing fungicides are related to higher levels of Mn body burden in children. Further, deciduous teeth are relatively easy to obtain and store and measurements in dentin provide a unique opportunity to retrospectively assess prenatal exposure. Do farm workers’ work histories affect their current wages? Based on search theory, we argue that workers with history of unemployment cannot afford to search as long as other workers and, as a result, obtain lower wages. Thus, an unemployed worker suffers from reduced income at the time of unemployment and lower wages in the future.