Trace elements in urban environments: a review - Sedimentology of Aqueous Systems - page 117

particulate materials (i.e. soil, street dust, indoor
dust), driven by the realization that children are the
most sensitive segment of the population to anthro-
pogenic contamination and by the strong indication
that toxic trace elements may reach levels of poten-
tial concern for human health in urban environments
(Evans et al . 1992; Mielke et al . 1999). Specifi cally,
some researchers have concentrated their efforts on
the chemical composition of playground soil and
dust (Anagnostopoulos et al. 1985; Duggan et al .
1985; Wong & Mak 1997; Ng et al . 2003; Ljung
et al . 2006a,b,c), because the exposure of children
to trace elements in this material is particularly high
- relative to other activities and other locations -
during games at school breaks and in public
playgrounds after school (Roscher et al. 1996).
Wong & Mak (1997) performed a simplifi ed risk
assessment, comparing the heavy metal concentra-
tions found in dusts and soils in Hong Kong play-
grounds with the Dutch Soil Investigation Levels and
concluded that Pb and Zn might pose a health hazard
for children. De Miguel et al . (2007) collected
samples from the top 2 cm of the sandy substrate in
Madrid municipal playgrounds. A detailed risk
assessment revealed that the highest risk experienced
by children playing in these playgrounds arose from
the ingestion of soil particles during games (Dudka
& Miller 1999) followed by dermal absorption, and
that the element of most concern among those
present in the sandy substrate of Madrid playgrounds
was arsenic in terms of both carcinogenic and non-
carcinogenic risk (De Miguel et al . 2007).
In playgrounds with chromated copper arsenate
(CCA)-treated equipment, children may experience a
much higher exposure to As than that found in
Madrid, as a result of not only soil ingestion and
dermal absorption but more importantly of direct
oral ingestion of dislodgable arsenic from wood
(Stilwell & Gorny 1997; Hemond & Solo-Gabriele
2004). Whether this elevated exposure might result
in an unacceptable level of risk is debatable: On the
one hand, the default assumptions in the standard
model of risk assessment are quite conservative and
elevated estimates of hazard index or carcinogenic
risk do not imply that exposure to playground sub-
strate alone should result in the advent of adverse
health effects. However, it should be noted that the
children's background exposure to trace elements
(i.e. dietary intake, inhalation of urban aerosol,

indoor dust higher than in outdoor soil, but also
their bioaccessibility was determined to be 1.5-2.5
times higher (i.e. 44% for Cu and Ni, and 65% for
Zn). Most studies of metal bioavailability in soils
have been conducted in locations strongly affected
by industrial or mining operations (Ruby et al. 1996;
Williams et al . 1998; Rodriguez et al. 1999; Schroder
et al . 2003). One of the few studies on uncontami-
nated urban soils was performed by Ljung et al.
(2007) in Uppsala, Sweden, where they collected
samples from the upper 10 cm of playground soil and
analyzed it for aqua regia and bioavailable contents
following the extraction protocol of Oomen et al.
(2003). Bioavailability for the different elements
included in the study followed the order: Cd
(26%) > As (16%) > Pb = Cr = Ni (4%) for the less

than 50- m fraction, considered to represent the par-

ticulate material that children ingest accidentally in
their games.
Despite the variability in extraction protocols,
nature of particulate material, size fraction and esti-
mated percentages of bioaccessibility and bioavaila-
bility, several conclusions can be drawn from the
those studies listed above. Firstly, concentrations of
metals in the neutral pH intestinal phase have been
found to be lower than in the acidic stomach juice,
particularly for those elements which are more easily
re-adsorbed or precipitated under near-neutral con-
ditions (i.e. Pb). The use in a risk assessment of
bioaccessible concentrations of those elements, as
determined in extractions with HCl, would overesti-
mate the risk although not to the same extent as aqua
regia or similar pseudo-total digestions, in which less
than 5% of the resulting concentrations of Pb, Ni,
and Cr may be available for absorption in the intes-
tine. Metals in urban particles of a natural origin are
generally more strongly bound and consequently
exhibit a lower bioavailability than those associated
with anthropogenic sources. Lastly, indoor dust
presents higher concentrations and higher bioacces-
sibility than urban soil. As a consequence, risk
assessments - which normally integrate dust and soil
as one single exposure medium - may gain in accu-
racy if both sources of exposure were decoupled.

μ

4.3.3 Risk in playgrounds

A large body of knowledge has been developed over
the past decades on the exposure of children to urban