Although many contaminants are applied as liquids and produce dermal exposures as dried residues, some chemicals are applied in the particle phase or move in that direction through absorption/adsorption onto the surfaces of dust particles. The particle phase injects additional physical mechanisms into the dermal exposure process that can substantially alter mass transfer factors and important source/sink relationships. Residues deposited on particle surfaces can facilitate movement within a microenvironment, as well as foster a large sink capacity within the dust pore structure long after the surface residues have evaporated. Particle-phase contaminant movement between surfaces and within microenvironments are strongly particle size dependent, and influenced by factors that are not necessarily relevant to residues. One of the most important factors is contact surface charge which can provide bonding forces that limit the release of the smallest particle sizes to the skin during dermal contact, to vacuum cleaner collection, or to resuspension while walking. Combining this bonding force with the very limited fiber surface that actually contacts the skin, substantially reduces mass transfer factors as compared to smooth surfaces - often 1000x less or smaller. It is currently not clear whether wetted dermal surface sufficiently drain away residual surface charges, facilitating enhanced transfer.

Smooth surfaces can be important sources for dermal exposures, with relatively large mass transfer factors that leave little residual material available for transfer in subsequent-contact events. Surface roughness reduces the actual contacted surface area, and adds reservoirs that can serve as contaminant sinks. While carpeting is often characterized as an areal source, it is actually the ultimate "depth" source - exhibiting a huge internal surface area (and volume, depending on the pile height). Much of the fiber surface area and backing are essentially unavailable for dermal contact, and serve as an enormous sink for dusts and particle-phase contaminants. Apparently, only the upper fiber surfaces actually take part in dermal press or smudge contact events, thus providing a limit to the mass of material actually available for transfer during repeated dermal contact events.

Research studies conducted at the Research Triangle Institute, and supported by both HUD and EPA during the past 3 years have added greatly to our understanding of particle phase dermal transfers from both smooth and carpeted surfaces. Because of its prevalence in both residential and commercial buildings, carpeted surfaces can play important intermediary roles in multi-pathway risk analyses (especially for children) and can serve as both a sink and a source. The transferability of particles from smooth and carpeted surfaces are beginning to be characterized during controlled studies to allow the application of microactivity dermal exposure models. The use of Pb-dust contaminated carpet samples for testing provided a tracer species (lead) that was easily detectable at low concentrations (by ICP-AES) and very stable (no vapor pressure issues).

Key findings will be summarized, supporting data summarized, and the implications discussed, including that:
(1) the mass of Pb particle contaminants found in the backing and the base of fibers are essentially unavailable for exposure (but may become available through a high energy event),
(2) contact mass transfer rates of particles to both dermal and wipe surfaces are almost solely associated with carpet fiber loadings,
(3) areal mass transfer factors are much less than 1% of the contaminant loading found on the fibers,
(4) contaminant particle sizes smaller than about 1 micrometer (e.g. very prevalent for Pb-contaminated carpeting) are held tightly to fibers by surface charges and are minimally available for air resuspension or transfer to either skin or wipes,
(5) particle-phase contaminant degradation indoors may include eventual oxidation to submicron sizes,
(6) repeated contacts of the same contaminated carpet surface can rapidly remove the easily dislodged particles, substantially reducing the mass available for transfer relatively quickly,
(7) the areal particle loading capacity of the skin is finite, such that successive contacts exhibit exponentially smaller mass transfer rates, and
(8) carpet fibers represent a very finite source for particles (and probably residues) such that the source may become depleted within only a few dermal contacts (e.g. total fiber loadings cannot be used to estimate dermal exposures without applying a depletion factor).

Current research uncertainties include the amount of energy that must be imparted to the carpeting (e.g. vacuuming, walking) to replenish fiber loadings from the carpet backing reservoir, after contact or cleaning events, and the role of wet contact surface films (e.g. hands, wipes) in altering the transfer of surface charges that are so dominant in bonding particles to the surfaces.