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The volume over which the substance distributes after emission. May vary from the size of the room where the product is introduced up to the size of the entire residence.
Input is restricted to the range 10 – 1000 m³.
The product surface area designates the area of the product that is in contact with air, i.e. the surface area from which emission through evaporation may take place.
Input is restricted to the range 0.01 – 1000 m².
The total volume of the product. This, in combination with the initial concentration of the substance in the product determines the available mass of the substance.
Input is restricted to the range 0.001 – 1 m³.
(Weschler and Nazaroff 2008) approximate the partition coefficient between dust and air by
Koa and the organic matter content of house dust fom,dust, as
Kd=fom,dust × Koa
Based on a review of available literature 0.2 was found to be a representative value of fom,dust.
The organic matter content of house dust is highly variable, however. Values up to 0.7
have been reported (Sukiene et al. 2016). For the default, (Weschler and Nazaroff 2008) is
followed and 0.2 is chosen.
Input is restricted to the range 0.1 – 1.
The amount of dust per unit surface area. (Batterman et al. 2009) report measurements of
dust surface loading 0.2 to 0.3 g/m³. The default was set to 0.3 g/m².
Input is restricted to the range 0.01 – 2 g/m².
The total volume of dust Vd in a residence is estimated
from the dust surface loading (σ), the mass density of dust and the total surface area of dust
Vd = σ × Sd / ρdust
Following (Weschler and Nazaroff 2008), the mass density of dust
was assumed to be 2.0 g/cm³.
Input is restricted to the range 0.5 – 5 g/cm³.
Dust is continuously removed from the residence by vacuum cleaning
and through resuspension into air and removal by ventilation. Various studies report estimates of
half-lives of house dust in the indoor environment. Allot et al. (1993) estimated a dust elimination
half-life of 29 days. More recent studies by Qian et al. (2008) and Layton and Beamer (2009) report
values of 81 and 61 days, respectively. From the elimination half-life λ, the elimination rate is
The experimentally determined half lifes correspond to a range of elimination rates of ~3 to ~9 times
per year. As a scenario default a value of 0 (no elimination) was adopted as a conservative lower bound.
Input is restricted to the range 0 – 20 (times) per year.
The substance's ocatonl/air partition coefficient determines
the potential of volatilization of the substance and its tendency to transfer into dust, skin
and onto surfaces.
Input is restricted to the range 7 – 13 on a ¹⁰Log scale.
The product/air partition coefficient Kma depends
both on the substance and the material of the product. In general, this parameter is unknown.
Methods to estimate Kma have been proposed (US-EPA 2015; Little et al. 2012), but the accuracy of
these methods is unknown. (Salthammer and Uhde 2009) found that for a number of phthalates Kma is
well approximated by the Koa.
Although it is not clear to what extent this finding may be
extrapolated to other substances, in case the parameter is unknown it is suggested to assume that
Kma is proportional to Koa, i.e. Kma= α × Koa. With α in a range of 0.01 to 1. Based on the
observations made by (Salthammer and Uhde 2009) it can be assumed by default that Kma equals Koa.
Input is restricted to the range 7 – 13 on ¹⁰Log scale.
The mass transfer coefficient hm is the rate at which
the substance is transferred from the product surface into the residential air. The mass
transfer coefficient depends on the roughness of the product surface, the air velocity over
the product surface and the diffusivity of the substance in air.
Several estimation methods for the mass coefficient exist (e.g. Delmaar et al. (2010)). Xu et al. (2009) report a range of
3.6-28.8 m/h for typical indoor conditions. Weschler and Nazaroff (2008) estimate that a value
of 3 m/h is representative of realistic circumstances.
Input is restricted to the range 0.1 – 100 m/h.
According to (Weschler and Nazaroff 2010) dermal
absorption of a substance in air is determined by the indoor air transdermal permeability
coefficient kpg of the substance. They provide estimates of the kpg for an array of substances
commonly found in the indoor environment. Reported values range from 0.08 to 6 m/h.
Input is restricted to the range 0.0 – 100 m/h.
The total surface area of house dust. This area
determines the contact and exchange between a substance as a gas in air and dust.
It includes all surfaces indoors that are dust-laden and in contact with indoor air.
The area should cover horizontal surfaces of flooring, furniture, electronic equipment
Input is restricted to the range 10 – 1000 m².
Total surface area onto which the substances may absorb.
Should be estimated on the basis of the room volume. Typically, sorptive surface areas are
in the order of 1-5 m²/m³ (that is, surface area per m³ of room volume) (see for example,
Won et al. (2001)).
Input is restricted to the range 0 – 1000 m².
To estimate the surface-air partition coefficient that describes the adsorption and desorption kinetics, DustEx provides two different conceptual models. The first (1) follows the approach proposed in (Weschler and Nazaroff 2008): sorption of SVOCs to indoor surfaces is conceptualized as sorption into a thin layer of organic material on these indoor surfaces. The sorption is then mainly driven by partitioning into organic material or by the substance’s Koa. The thickness of the organic film determines the absorption potential of the surface. (Weschler and Nazaroff 2008) estimate the thickness of this layer to be between 10 and 100 nm.
Input is restricted to the range 0.001 – 1 µm. The default in the web application is 0.01 µm.
The second model to describe surface kinetics (2) assumes that SVOCs adsorb directly onto the surface. This process is described with a surface-air partition coefficient Ksa [m]. The Ksa is given by the ratio of the surface concentration Cs [mg/m²] and the air concentration Cair [mg/m³] at equilibrium.
It is expected that method (1) is more representative of realistic indoor, residential conditions, in which surfaces tend to be covered by organic films. Method (2) may be more useful in laboratory settings with clean, pristine surfaces.
Substances in indoor air may sorb to airborne particles. The total capacity of airborne
particles to take up substance depends on the(mass) concentration of these particles.
(Weschler and Nazaroff, 2010) suggest a value 20 µg/m³ as a typical indoor value.
restricted to the range 1 – 200 µg/m³
Mass density of airborne particulate matter. A typical value of 1 g/cm³is
suggested by (Weschler and Nazaroff, 2008).
Input is restricted to the range 0.1 – 10 g/cm³.
Determines the transfer between gas phase substance and
airborne particles. Transfer is rapid under typical indoor conditions. Mass transfer from air
to particles is inherently faster than mass transfer to flat surfaces (Weschler and Nazaroff,
Input is restricted to the range 1 – 1000 m/h.
Following (Weschler and Nazaroff 2010) the partition coefficient Kpart between airborne
particles and air is assumed to be determined by the organic matter content of the airborne
particles fom,part and the substance Koa as:
Kpart = fom,part × Koa
(Weschler and Nazaroff 2010)
suggest a value of 0.4 for fom,part.
A comprehensive evaluation of dust ingestion rates can
be found in the exposure factors handbook (EPA, 2011). Ingestion rates there are estimated by
biomarker measurements (e.g. lead concentration in blood) or modelled according to human
activity pattern data from observational studies or responses to the questionnaires. For
example, the recommended values of ingested indoor dust (including soil and suspended
particulate matter) for adults and children are 50 mg/day and 100 mg/day, respectively and
represent conservative defaults. However, dust ingestion rates are highly uncertain as the
methods used to derive the values have certain limitations.
Wilson et al. (2013) have developed an alternative, indirect method to estimate indoor dust
ingestion. The ingestion rates for the Canadian context were calculated using measures of
loading to hands, hand surface area, hand area that is mouthed, frequency of mouthing events,
saliva extraction factor and exposure time. Using this approach, the estimated values are lower
than values recommended in previous literature and vary from 2.2 mg/day to 41 mg/day for
teenagers and toddlers, respectively.
Input is restricted to the range 0 – 1000 mg/day.
The skin surface area determines the area over which dermal absorption of the
substance(in gas phase) from air takes place. Based on the work of (Morrison et al., 2016) it is assumed that
clothing does not form an efficient protective barrier for dermal uptake of substances from air.
Input is restricted to the range 0.2 – 5 m².
The period over which emission, transfer and exposure
are calculated. Starts with the introduction of the product in the indoor environment and ends
when the exposure stops. Typically SVOC emission and distribution processes are slow and take
months or years to reach(quasi-) steady states.
Simulation durations should be chosen between 10 and 999 days.
The DustEx model calculates concentrations in indoor air
compartments air, airborne particles and dust. Exposure is evaluated by combining the simulated
concentration data with the presence of a person in the environment. This presence is specified
by the exposure frequency: the number of times a person enters the indoor environment starting
with the ‘start of exposure’. Exposure is assumed to take place at regular times separated by a
Input is restricted to the range of 1 to 365 per year.
The first time a person enters the indoor environment after introduction of the product.
The number of hours a person spends indoors on the day of exposure.
The number of iterations to be performed.
Select a number between 100 and 10 000.
Estimate transdermal permeability coefficient
The mass transfer coefficient is the rate at which the
substance is transferred from the residential air to the skin. The mass transfer coefficient to skin
is typically an order of magnitude higher than the mass transfer coefficient for indoor surfaces.
(Weschler and Nazaroff, 2012).
Indicative indoor values are 10 - 30 m / h.
Estimate Kma from saturated vapour pressure
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The Kma is estimated on the basis of the saturated vapour pressure of the substance Ps, and the activity coefficient of the substance in the matrix γ.
The Kma follows from the relation between y0, the air concentration directly above the material surface and C0, the concentration of the substance in the material. This relation is assumed to be linear, the Kma being the coefficient of proportionality, i.e.:
y0 = C0/Kma
The Kma is related to the saturated vapour pressure ps of the compound as:
with ρ the density of the material.
The method is based on the research described in Eichler et al. 2018. 'Equilibrium Relationship between SVOCs in PVC Products and the Air in Contact with the Product', Environmental Science and Technology, 52: 2918-25.
The authors of this work found an empirical value for the activity coefficient of 5.12 for phthalates in PVC products, to relate C0 (given as a weight fraction) to the vapor pressure. This value is used as a default.
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