Estimating Contaminant Dose for Intermittent Dermal Contact: Model Development, Testing, and Application

Abstract

Assessments of aggregate exposure to pesticides and other surface contamination in residential environments are often driven by assumptions about dermal contacts. Accurately predicting cumulative doses from realistic skin contact scenarios requires characterization of exposure scenarios, skin surface loading and unloading rates, and contaminant movement through the epidermis. In this article we (1) develop and test a finite-difference model of contaminant transport through the epidermis; (2) develop archetypal exposure scenarios based on behavioral data to estimate characteristic loading and unloading rates; and (3) quantify 24-hour accumulation below the epidermis by applying a Monte Carlo simulation of these archetypal exposure scenarios. The numerical model, called Transient Transport through the epiDERMis (TTDERM), allows us to account for variable exposure times and time between exposures, temporal and spatial variations in skin and compound properties, and uncertainty in model parameters. Using TTDERM we investigate the use of a macro-activity parameter (cumulative contact time) for predicting daily (24-hour) integrated uptake of pesticides during complex exposure scenarios. For characteristic child behaviors and hand loading and unloading rates, we find that a power law represents the relationship between cumulative contact time and cumulative mass transport through the skin. With almost no loss of reliability, this simple relationship can be used in place of the more complex micro-activity simulations that require activity data on one- to five-minute intervals. The methods developed in this study can be used to guide dermal exposure model refinements and exposure measurement study design.

Young children's hand contact activities: An observational study via videotaping in primarily outdoor residential settings

“…Microlevel activity time series (MLATS) data capture the sequence and duration of contact events and allow contact frequency to be calculated (Ferguson et al, 2005). MLATS data can be combined with environmental concentrations to create sequential time exposure profiles that help researchers determine contact activities that lead to peak exposure (Ferguson, 2003;Riley et al, 2004).…”

Spatial modeling of personalized exposure dynamics: the case of pesticide use in small-scale agricultural production landscapes of the developing world

“…In short, exposure occurs if a person is at a location where there is some contaminant concentration in the environment and if there is a contact between the human body and the contaminant e.g., caused by a particular activity. The intensity of exposure of one individual can vary depending on attributes that indicate safety measures taken or protection equipment used [ 45 ] but also on frequency and duration of contact with the active ingredient [ 47 ]. Consequently, the assessment of individual exposure to pesticides requires an individual-based framework, which incorporates a social and an environmental component, as well as defined parameters and decision rules.…”

Dynamics of diffusion with reversible binding in microscopically heterogeneous membranes: General theory and applications to dermal penetration

“…We focus attention on the example of dermal absorption through the SC, and carry it along throughout our analysis, for three reasons, namely because: (i) it represents a biological tissue for which spatial periodicity in one dimension (Yotsuyanagi and Higuchi, 1972) is not just an idealized model but actually an almost literal descriptor of the actual microstructure (Scheuplein and Blank, 1971;Montagna and Parakkal, 1974;Roberts and Walters, 1998), which alternates regularly between corneocyte and intercellular lipid layers; (ii) it has high relevance to important biomedical and occupational safety applications, including transdermal drug delivery (Siddiqui, 1989;Wiechers, 1989;Hadgraft and Guy, 2003;Cross and Roberts, 2004;Ho, 2004) and risk assessment of chemical exposure (Poet and McDougal, 2002;Schuhmacher-Wolz et al, 2003;Fitzpatrick et al, 2004;Ho, 2004;Riley et al, 2004); and (iii) our results directly impact the very latest (and very fruitful) direction taken in the dermal absorption literature, namely that of explicitly considering solute binding (Anissimov and Roberts, 2009).…”

“…In the dermal context, for instance, diffusion rates are of great interest for such diverse applications as transdermal drug delivery (Siddiqui, 1989;Wiechers, 1989;Hadgraft and Guy, 2003;Cross and Roberts, 2004;Ho, 2004) and risk assessment of chemical exposure (Poet and McDougal, 2002;Schuhmacher-Wolz et al, 2003;Fitzpatrick et al, 2004;Ho, 2004; Riley et al, 2004). Other membranes present equally compelling motivations for understanding the transport processes that occur within them.…”