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3.1 Incidental Influences on Total Soil Dust Ingestion: Dr. Natalie Freeman

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Dr. Freeman's presentation focused on incidental ingestion of soil and dust that results from children's typical eating and mouthing behaviors. Though her most recent studies have primarily examined exposures to household dust, Dr. Freeman noted that 50-70% of household dust is believed to come from outdoor soils. Thus, she reports her results as ingestion of "soil/dust," rather than ingestion of strictly household dust.

Dr. Freeman explained that examining incidental soil ingestion among children is important, because national studies have suggested that 87% of children (aged 1-4 years) lie or play on the floor and nearly 50% play on grass or dirt. (Dr. Freeman acknowledged that these summary statistics reflect national trends and may not be representative of regional or local trends.) She added that her own research of children in Newark, New Jersey, has shown that a large proportion of children (aged 1-3 years) regularly engage in activities that can lead to soil ingestion, such as eating most of their food with their hands, eating food dropped on the floor, and putting their fingers in their mouths.

Dr. Freeman then outlined preliminary data she has collected in support of an ongoing "Children's Dietary Lead Study." She specifically addressed the extent to which children's food can be contaminated by their behavior patterns. This study characterized children's exposures to soil and dust in New Jersey urban centers by sampling levels of metals contamination on kitchen floors, children's hands, food that fell on the floor, and food that children handled. Dr. Freeman explained the approach used to attribute measured levels of metals in food to contributions from metals contamination in soils and household dust, and then presented preliminary results from her study. One preliminary result is that, on average (median), 2 mg of dust adheres to a slice of apple dropped on the floor, the maximum being 16 mg. Similarly, 8 mg of dust, on average (median), adheres to a slice of cheese dropped on the floor, the maximum amount being 59 mg. Based on her results for apples, cheese, bananas, and hot dogs, Dr. Freeman estimated that children, on average, may ingest 22 mg of household dust daily just from eating foods from the floor, with a 90th percentile ingestion rate from this activity of 93 mg.

Dr. Freeman also presented preliminary estimates of soil ingestion rates resulting from children's typical mouthing behavior. These estimates were calculated from measurements of the amounts of dusts typically found on children's hands and the assumptions on hand-to-mouth rates, the percentage of the hand that enters a child's mouth, the extent to which the amount of dust on the hand might be replenished between mouthing events, and the number of hours a day in which children engage in mouthing behavior. Not surprisingly, the results varied with the assumptions made in the calculations. For instance, children with a median dust loading on their hands (i.e., 1.5 mg dust per hand) who have a hand-to-mouth rate of 8.5 events per hour were estimated to have an incidental dust ingestion rate--from mouthing behavior alone--of 14 mg per day. At the upper end of the soil ingestion range, children with the maximum dust loading on their hands (i.e., 58.2 mg dust per hand) who have a hand-to-mouth rate of 27.0 events per hour are estimated to have an ingestion rate of 1,800 mg per day. Dr. Freeman noted that this elevated ingestion rate was not actually observed, but was estimated using a statistical analysis of her data. Dr. Freeman emphasized two assumptions that introduce uncertainty into these calculations. First, the dust loading on hands was assumed to fully replenish between mouthing events--an assumption that an observer suggested was conservative for the upper-end exposure scenario--and mouthing activity was assumed to be limited to 8 hours per day. Dr. Freeman explained that little data are available for soil/dust replenishment on hands, for mouthing activities during 24-hour time periods, and for the consistency of these behaviors across days or weeks. She noted that most observational studies are limited to 8 hours or less per child.

In conclusion, Dr. Freeman stressed that considerable amounts of soil and dust ingestion can occur on a daily basis as a result of children's typical behavior patterns, and not necessarily from what have traditionally been considered intentional soil-pica events.

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