DustEx

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 K_oa and the organic matter content of house dust f_om,dust, as

K_d=f_om,dust × K_oa

Based on a review of available literature 0.2 was found to be a representative value of f_om,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.

• Weschler CJ, Nazaroff WW. 2008. Semivolatile organic compounds in indoor environments. Atmos. Environ. 42 (40): 9018–9040.
• Sukiene V, Gerecke AC, Park YM, et al. 2016. Tracking SVOCs’ transfer from products to indoor air and settled dust with deuterium-labeled substances. Env. Sci. Technol. 50(8): 4296-4303.

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.

• Batterman SA, Chernyak S, Jia C, Godwin C, Charles S. 2009. Concentrations and emissions of polybrominated diphenyl ethers from U.S. houses and garages. Environ. Sci. Technol. 43(8): 2693–2700.

The total volume of dust V_d in a residence is estimated from the dust surface loading (σ), the mass density of dust and the total surface area of dust S_d:

V_d = σ × S_d / ρ_dust

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.

• Weschler CJ, Nazaroff WW. 2008. Semivolatile organic compounds in indoor environments. Atmos. Environ. 42 (40): 9018–9040.

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 derived as:

k_el,dust=ln₂/λ

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.

• Allott RW, Kelly M, Hewitt CN. 1993. A model of environmental behaviour of contaminated dust and its application to dust fluxes and residence times. Atm. Env. 4: 679–687.
• Layton DW, Beamer PI. 2009. Migration of contaminated soil and airborne particulates to indoor dust. Supporting information. Environ. Sci. Technol. 43: 8199–8205.
• Qian J, Ferro AR, Fowler KR. 2008. Estimating the resuspension rate and residence time of indoor particles. J. Air Waste Manage. Assoc. 58(4): 502–516.

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 K_ma depends both on the substance and the material of the product. In general, this parameter is unknown. Methods to estimate K_ma 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 K_ma is well approximated by the K_oa.

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 K_ma is proportional to K_oa, i.e. K_ma= α × K_oa. 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 K_ma equals K_oa.

Input is restricted to the range 7 – 13 on ¹⁰Log scale.

• US EPA, 2015. AMEM-ADL Polymer Migration Estimation Model, www.epa.gov/tsca-screening-tools/amem-adl-polymer-migration-estimation-model-download-and-install-instructions
• Little JC, Weschler CJ,et al. 2012. Rapid methods to estimate potential exposure to semi-volatile organic compounds in the indoor environment. Environ. Sci. Technol. 46(20): 11171–11178.
• Salthammer T, Uhde E. 2009. Organic indoor air pollutants. occurance, measurement, evaluation. Second edition. Wiley: Weinheim. ISBN: 978-3-527-31267-2.

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.

• Delmaar JE. 2010. Emission of chemical substances from solid matrices. A method for consumer exposure assessment. RIVM Report 320104011.
• Xu Y, Little J. 2006. Predicting emissions of SVOCs from polymeric materials and their interaction with airborne particles. Environ. Sci. Technol. 40 : 456–461.
• Weschler CJ, Nazaroff WW. 2008. Semivolatile organic compounds in indoor environments. Atmos. Environ. 42 (40): 9018–9040.

According to (Weschler and Nazaroff 2010) dermal absorption of a substance in air is determined by the indoor air transdermal permeability coefficient k_pg of the substance. They provide estimates of the k_pg 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.

• Weschler CJ and Nazaroff WW. 2010. SVOC partitioning between the gas phase and settled dust indoors. Atmospheric Environ. 44: 3609-3620.

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 et cetera.

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.

• Won, D., Sander, D. M.,et al. (2001). Validation of the surface sink model for sorptive interactions between VOCs and indoor materials. Atm Env, 35(26), 4479-4488).

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 K_oa. 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 K_sa [m]. The K_sa is given by the ratio of the surface concentration Cs [mg/m²] and the air concentration C_air [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.

• Weschler CJ, Nazaroff WW. 2008. Semivolatile organic compounds in indoor environments. Atmos. Environ. 42 (40): 9018–9040

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.

Input is restricted to the range 1 – 200 µg/m³

• Weschler CJ and Nazaroff WW. 2010. SVOC partitioning between the gas phase and settled dust indoors. Atmospheric Environ. 44: 3609-3620.

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.

• Weschler CJ, Nazaroff WW. 2008. Semivolatile organic compounds in indoor environments. Atmos. Environ. 42 (40): 9018–9040.

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, 2008).

Input is restricted to the range 1 – 1000 m/h.

• Weschler CJ, Nazaroff WW. 2008. Semivolatile organic compounds in indoor environments. Atmos. Environ. 42 (40): 9018–9040.

Following (Weschler and Nazaroff 2010) the partition coefficient K_part between airborne particles and air is assumed to be determined by the organic matter content of the airborne particles f_om,part and the substance K_oa as:

K_part = f_om,part × K_oa

(Weschler and Nazaroff 2010) suggest a value of 0.4 for f_om,part.

Input is restricted to the range 0.1 – 1.

• Weschler CJ and Nazaroff WW. 2010. SVOC partitioning between the gas phase and settled dust indoors. Atmospheric Environ. 44: 3609-3620.

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.

• EPA (United States Environmental Pretection Agency), 2011. Exposure factors handbook: 2011 edition. Office of Research and Development, Washington, DC 20460.
• Wilson, R., Jones-Otazo, H., Petrovic, S., Mitchell, I., et al., 2013. Revisiting dust and soil ingestion rates based on hand-to-mouth transfer. Hum. Ecol. Risk Assess. 19: 158–188.

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.

• Morrison GC, Weschler CJ, Bekö G, Koch HM, Salthammer T, Schripp T, et al. 2016. Role of clothing in both accelerating and impeding dermal absorption of airborne SVOCs. JESEE 26(1): 113-118.

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 constant interval.

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.