Exposure Through Soil and Dust Ingestion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 266

on soil concentrations and the estimates based on dust concentrations (US-EPA
2008 ). By making assumptions on the contribution of soil and dust to the over-
all ingestion rate, some authors attempted to estimate a combined soil and dust
ingestion rate.
Initially the tracer element methodology did not account for the contribution of
tracer elements from non-soil sources (food, medications, and non-food sources
such as toothpaste), e.g., Binder et al. ( 1986 ). Later studies generally account for
tracer element contributions from these non-soil sources (Calabrese et al. 1997 ;
Davis et al. 1990 ).
A number of key studies have been conducted to estimate soil ingestion rates
using the tracer element methodology (Binder et al. 1986 ; Calabrese et al. 1989 ,
1997 ; Clausing et al. 1987 ; Davis et al. 1990 ; Van Wijnen et al. 1990 ). In the second
instance, the experimental data of many of these studies were given new interpre-
tations (Calabrese and Stanek 1992 , 1995 ; Calabrese et al. 1996 ; Davis and Mirick
2006 ; Sedman and Mahmood 1994; Stanek and Calabrese 1995a , b , 2000 ; Stanek
et al. 1999 , 2001a ; Thompson and Burmaster 1991 ). Most of these studies have esti-
mated soil ingestion rates for children. Data on ingestion rates for adults are scarce.
Chapter 5 of US EPA's Child-Specific Exposure Factors Handbook (2008) and Van
Holderbeke et al. ( 2008 ) present a good overview of the relevant studies for children.
Details on the available studies are presented in Tables 6.2 and 6.3 .
Differences exist between the above listed studies in respect to the correction that
is made for ingestion of tracers from non-soil related sources (food, toothpaste, et
cetera) and the tracers that are measured. In addition, the interpretation of the results
between studies differs. Some authors use the Limited Tracer Method (LTM), in
which for every sample the lowest of all tracers is used (e.g., Binder et al. 1986 ;
Clausing et al. 1987 ; Van Wijnen et al. 1990 ). In contrast, Calabrese et al. ( 1997 )
estimated the soil ingestion rate by each tracer element using the Best Tracer Method
(BTM), which allows for the selection of the most recoverable tracer on the basis of
the food/soil ratio for a particular group of subjects (Stanek and Calabrese 1995b ).
Some authors adjust their estimated soil ingestion rates to account for the potential
contribution of tracer elements found in house dust, as well as in soil (Davis et al.
1990 ).
In comparison with children, data on soil ingestion for adults are much more
limited. Details on the relevant studies are given in Table 6.3 .
Soil and dust ingestion rates derived from tracer studies are subject to uncertain-
ties, e.g., the experimental uncertainty in the measurements by only partial sampling
of the food and faeces, or transit time differences, using standard stool weights and
not actual stool weights. Another uncertainty is the uptake of tracers by the human
body: the assumption that tracer elements are not absorbed is not entirely true. The
methodology assumes that children ingest predominantly soil from their own yards.
In reality, however, children ingest also soil from other locations. Other limitations
are inherent to soil sampling (e.g., homogeneity of the soil samples) and laboratory
analytical techniques. In addition, the true soil and/or dust ingestion rate is probably
somewhere between the estimates based on soil concentrations and the estimates
based on dust concentrations.