Exposure Through Soil and Dust Ingestion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 274

An important limitation of tracer studies is that it is not possible to distinguish
between soil and dust ingestion. However, this distinction is important because of
differences in concentrations and bioavailability and for adequate design of Risk
Management solutions. Estimates that were made by some authors (e.g., Davis et al.
1990 ) provide assumptions about how the ingestion rate is distributed over soil and
dust, but data to support these assumptions are limited. The contribution of soil and
dust in the overall ingestion rate may be not only related to the time children spent
outdoors and indoors, but also to the playing pattern and the higher oral availability
in the human body of soil material compared to indoor dust material.
6.2.2 Alternative Approaches for Estimating Soil
and Dust Ingestion
Estimates of soil and dust ingestion based on data of hand-loading and mouthing
behaviour of children require detailed knowledge on time activity patterns, fre-
quency of hand to mouth contact, age specific hand-mouth contact area, soil/dust
hand-loading and an estimate of transfer efficiencies, i.e. the degree to which soil or
dust adheres to skin and the percentage of soil adhering to the hand that is ingested
during a single hand to mouth contact. An advantage of hand-loading studies is that
ingestion rates can be derived separately for outdoor and indoor activities and thus
for soil and dust ingestion
Finley et al. ( 1994 ) and Van Holderbeke et al. ( 2008 ) give an overview of studies
that have been conducted to estimate soil/dust hand-loading for children of different
ages, locations (indoor and outdoor) and sampling methods (gravimetric and wipe
method). Finley et al. ( 1994 ) interpreted these studies using different assumptions
such as surface area of hands, percentage of children's hand covered with soil, effi-
ciency of the methods and concentrations in soil and house dust. From these studies
it can be concluded that dermal hand-loadings for children generally range from 0.1
to 1.0 mg soil or dust/cm 2 .
It is difficult to quantify the hand-to-mouth transfer of soil, dust or contaminants.
Not only the duration and type of contact (licking, sucking, et cetera), but also the
polarity and affinity of contaminants to saliva and skin are important. Kissel et al.
( 1998 ) reported the results of a laboratory-based examination of hand-to-mouth
transfer of soil. The mass fraction of total soil load on the hand recovered from
mouths following thumb sucking, palm licking or mouthing three fingers above the
first knuckle ranged from 10.1 to 21.9%. It should be noticed however that this
study was conducted with adults and does not take into account the difference in
hand/finger proportions that exists between adults and toddlers. These data provide
the soil mass transferred to the mouth divided by the soil mass on the entire hand,
rather than the part of the hand that is in contact with the mouth. Therefore, they
can be used to estimate an extreme lower bound of the hand-to-mouth transfer effi-
ciency factor (OEHHA 2007 ). Most authors investigated the transfer efficiency of
contaminants such as pesticides (Camann et al. 2000 ) or riboflavin (Cohen Hubal
et al. 2005 ). Values in these papers range from 5 to 100% extraction. Nearly all
authors use 50% as the efficiency of the hand-to-mouth transfer.