Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 501

(in mg/kg body weight /d), the daily intake of contaminants via inhalation of dusts may
be calculated from the inhalation rate and the occupancy period and compared with
the RfD value.
There has been some discussion about whether the extrapolation of the toxicity
of a substance via the oral intake route to the inhalation intake route is valid. The
method relies on the assumption that the route of intake is irrelevant to the dose
delivered to the organ where effects occur. This assumption is not supported by the
available data, and is particularly irrelevant for substances inducing local effects and
for which route-to-route extrapolation has been found to be unreliable (Rennen et al.
2003 ), Extrapolation from the oral route to the inhalation route has been suggested
to be unreliable, especially for less volatile contaminants (US EPA 1996b ; Wilschut
et al. 1998 ). There are limited data available on which to base conclusions about
route-to-route extrapolation. It is not clear what factors, beside bioavailability are
responsible for the difference in toxicity between routes.
Generally, RfC-values are unavailable for the less-volatile substances. If expo-
sure by inhalation (e.g. of dusts) occurs for these substances, route-to route
extrapolation may be the only means of accounting for the exposure pathway. In
future, the problem of route to route exposure may be avoided by developments in
the use of pharmokinetic models, and the expression of contaminant dose in mg/g
tissue in target organs, regardless of exposure pathway.
In some cases adjustments are made to account for limited availability of a con-
taminant for uptake. This is normally done by applying a bioavailability factor. This
relates the availability for uptake in the lungs of a contaminant in respired dust to the
availability of the contaminant used when determining the Toxicological Reference
Values. However, methods for testing the bioavailability of respired particles are still
under development and not yet standardised and furthermore, the actual bioavail-
ability of the reference contaminant used for setting the Toxicological Reference
Values is often not very well known. Therefore, in most models a bioavailability
factor of 1.0 is used.

11.6.3 Input Parameters

11.6.3.1 Dust Concentrations in Air

The dust concentration in air varies widely between different sites and also
over time. In the Swedish countryside, for example, an annual average PM10-
concentration is 8-16 g/m 3 (Socialstyrelsen 2005 ). In urban environments, the

background concentration is 14-20

μ

g/m 3 , but can be much higher in streets with
heavy traffic, in many places exceeding the Environmental Quality Standards of a
maximum annual average value of 40

μ

g/m 3 .
In the Dutch CSOIL model (Brand et al. 2007 ; Van den Berg 1991 / 1994 / 1995 )
the applied dust loading factors are 53 and 70

μ

g/m 3 for indoor and outdoor air,
respectively. This is based on Hawley ( 1985 ), who further assumes that the con-
centration of suspended dust in the air inside homes is 75% of that in the outside
air. Paustenbach et al. ( 1997 ) recommended an indoor dust level of 50

μ

g/m 3 μ for