Inhalation

One of the routes of exposure to the substance, emitted from the product to the air, is inhalation. The total air concentration is the sum of the concentration of the substance in the gas phase (C_air) and the air concentration of the substance bound to airborne particles (C_air,p). From the total air concentration the daily-inhaled amount A_inh is estimated as

where Q_inh is the inhalation rate (per hour) of the exposed person, f_abs,inhal is the fraction of the inhaled amount which is absorbed, and T_exp is the daily exposure duration (hours per day). In this equation the concentrations C_air and C_air,p are assumed to be essentially constant over the day of exposure, which is reasonable for SVOCs.

Dermal exposure from air

A second route of exposure to the substance from air is the dermal exposure from the air. Weschler and Nazaroff (2012) propose a method to estimate the dermal uptake from substances in the gas phase of indoor air. Following this approach, the amount absorbed through the skin, A_dermal is estimated from the transdermal flux J (mass/surface area/time) as

Here, S_skin is the surface area of the exposed skin.

The transdermal flux is estimated using

Where k_{p_g} is the trans-dermal skin permeability of the substance that describes mass transfer from the bulk indoor air into the epidermal capillaries.

Estimating k_{p_g}

In absence of measured values of the k_{p_g} , the DustEx tool provides a method to estimate it from basic substance properties. This method is described in detail in Weschler and Nazaroff (2012). The method assumes that the k_{p_g} is determined by the mass transfer of the substance from air to the skin, h_m and the permeability of the skin for a substance from air k_{p_b} , which together act as a system of resistances in series.

The trans-dermal skin permeability k_{p_g} is modelled as a system of two transfer coefficients in series: the mass transfer coefficient of the susbtance from air to skin h_m and the permeability of the stratum corneum/epidermis composite k_{p_b}.

k_{p_b} describes the permeability of the the stratum corneum/epidermis composite.

It is modelled as

Here, K_gw is the gas-water partition coefficient of the substance.  It  follows from the molecular properties K_ow and K_oa as

kp_cw is the permeability of the stratum corneum for a substance dissolved in water. DustEx models the kp_cwusing the model by Mitragotri (2002)

This equation takes the molecular weight in g/mol and predicts a kp_cw in cm/s.

Finally, the total transdermal permeability coefficient k_{p_g} is determined by approaching the skin-air system as  two resistors in series, with a resistance of  h_m and K_{p_b} respectively

Ingestion of house dust

Exposure to substances contained in house dust is assumed to arise from the ingestion of dust. The daily intake of a substance with dust A_dust is determined by

Where A_{dust_ingest} is the amount of dust daily ingested and a fraction  f_abs,oral  is actually absorbed.

In the DustEx model dermal exposure to house dust is assumed to be negligible.