post.exposure: post.exposure

In HumanExposure/SHEDSDevel: The SHEDS-HT model for estimating human exposure to chemicals.

Description

Resolves the fate of chemical after initial exposure has occurred. This involves parsing out the amount of chemical removed and amount ultimately contributing to the exposure dose for each person.

Usage

post.exposure(cb, cprops)

Arguments

cb

A copy of the base data set output from the make.cbase function, with columns added for exposure variables.

cprops

The chemical properties required for SHEDS-HT. The default file (the Chem_props file read in by the read.chem.props function and modified before input into the current function) was prepared from publicly available databases using a custom program (not part of SHEDS-HT). The default file contains 7 numerical inputs per chemical, and the required properties are molecular weight (MW), vapor pressure (VP.Pa), solubility (water.sol.mg.l), octanol-water partition coefficient (log.Kow), air decay rate (half.air.hr), decay rate on surfaces (half.sediment.hr), and permeability coefficient (Kp).

Details

This function resolves the fate of chemical after initial exposure has occurred. The same function applies to all exposure scenarios. The dermal exposure is the most complicated, as there are five removal methods. All five are randomly sampled. While the sum of the five means is close to one, the sum of five random samples might not be, so these samples are treated as fractions of their sum. The rem.bath variable is either 0 or 1, so the bath removal term is either zero or the sampled bath removal efficiency. The rem.brush term is also simple. The handwashing (rem.wash) and hand-to-mouth (rem.hmouth) transfer terms are non-linear, because higher frequencies have less chemical available for removal on each repetition. The algorithms in place were fitted to output from SHEDS-Multimedia, which were summed to daily totals. The final removal term is dermal absorption (rem.absorb). The base value is multiplied by the Kp factor from the cprops argument, and divided by the value for permethrin, as the values from SHEDS-Multimedia were based on a permethrin run. The five terms are evaluated separately for each person, as is their sum. Each is then converted to a fraction of the whole. The fractions may be quite different from one person to another. For one, perhaps 80% of the dermal loading is removed by a bath/shower, while for another person it is 0% because they did not take one. For the latter person, other four removal terms are (on average) five times larger than for the former person, because together they account for 100% of the removal, instead of just 20%. The rest of the post.exposure function is mostly a matter of bookkeeping. The hand-to-mouth dermal removal term becomes an ingestion exposure term. Summing exposures across routes is dubious, in part because inhalation exposures use different units from the others, and because much of the dermal exposure never enters the body. In addition, summing dermal and ingestion exposures may double-count the hand-to-mouth term. However, intake dose may be summed. In SHEDS, "intake dose" is the sum of the inhaled dose (which is the amount of chemical entering the lungs in ug/day), the ingestion exposure (which is the amount entering the GI tract in ug/day), and the dermal absorption (the amount penetrating into or through the skin, so it cannot otherwise be removed, in ug/day). The "absorbed dose" is also calculated: for dermal it is the same as the intake dose, but for ingestion and inhalation there is another absorption factor, which was set on the exposure factors input file. An estimate of the chemical in urine (in ug/day) is made. Both the intake dose and the absorbed dose are reported in both (ug/day) and in (mg/kg/day). Note that the latter requires the body weights of each individual. These cannot be obtained from the former just by knowing the average body weight in SHEDS. The above variables for each simulated person are written to the fexp object (the name stands for "final exposure"). Each chemical writes over the previous fexp, so the data must first be summarized and written to an output file.

Value

fexp Absorbed dose and intake dose of a given chemical for each theoretical person being modeled for all exposure scenarios.

Author(s)

Kristin Isaacs, Graham Glen

See Also

make.cbase, Chemprops_small

HumanExposure/SHEDSDevel documentation built on Oct. 30, 2019, 6:49 p.m.