Exposure Through Soil and Dust Ingestion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 265

Concentrations of contaminants in dust can be measured or can be estimated from
concentrations in soil and the fraction of soil in indoor dust. When making measure-
ments, the risk assessor should be aware that the concentrations observed are not
necessarily solely from a soil origin. Distinguishing between soil-derived and non-
soil-derived concentrations is often a challenge in site-specific Risk Assessments.
Estimating the concentration in dust that is derived from soil is uncertain as well, as
enrichment and dilution processes can be highly variable.
6.2 Quantification of Soil and Dust Ingestion Rates
Estimates of soil and dust ingestion can be based both on Macro and Micro activity
data. The Macro activity approach is based on tracer studies. The Micro activity
approach focuses on hand-loading studies.
In the Macro activity approach exposure is estimated individually for each of the
micro-environments where a child spends time, e.g., a living room, courtyard, et
cetera, and for each Macro activity a child conducts within that micro-environment,
e.g., playing, reading, et cetera). In this case exposure is considered as the aggregate
exposure of a series of contacts with the contaminated medium (soil or dust). In
the Micro activity approach exposure is modelled as a series of discrete transfers
resulting from each contact with the contaminated medium.
Other, far less used methodologies can be applied to estimate the amount of soil
and dust ingested by children, such as the Biokinetic model comparison methodol-
ogy, the Lead isotope ratio methodology, the Survey response methodology, and the
Empirical relations methodology (US-EPA 2008 ; Van Holderbeke et al. 2008 ).
6.2.1 Tracer Element Methodology
The tracer element methodology attempts to quantify soil ingestion rates outdoors
and soil tracked into the indoor environment. In this methodology, both samples of
soil from children's residences and samples of the children's excreta (faeces and
sometimes also urine) are analysed for the presence and quantity of soil borne
tracer elements, mainly, aluminium (Al), silicon (Si), titanium (Ti), and yttrium
(Y). Ideally, tracer elements are not metabolised or absorbed from the gastroin-
testinal tract in significant quantities, and their presence in soil, faeces and urine
can be used to estimate soil ingestion rates using a mass balance methodology.
This methodology assumes that the amount of soil (outdoors and tracked indoors)
ingested in grams equals the quantity of a given tracer element present in the fae-
ces and urine (in mg), minus the quantity of that tracer element, present in food
and medicine (mg), divided by the tracer element's soil concentration (in mg/g)
(US-EPA 2008 ). Some authors substitute tracer element soil concentrations with
tracer element dust concentrations, which results in a dust ingestion estimate. The
tracers in soil are present in soil outdoors and in soil in house dust and therefore
the amount of non-food quantity ingested is a combination of soil and dust. The
true soil and/or dust ingestion is probably somewhere between the estimates based