U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-14-021 Date: January 2014 |
Publication Number: FHWA-HRT-14-021 Date: January 2014 |
Evaluation of exposure pathways allows the risk assessment to focus on the pathways with the most significant potential to contribute to adverse human health effects. Exposure pathways are considered complete when four elements are present: 1) a contaminant source, 2) a transport mechanism to a receptor location, 3) a receptor present at the location, and 4) an exposure pathway to the receptor. Incomplete exposure pathways are eliminated from further consideration. Pathways are categorized as either direct (where the point of exposure is the source of contamination) or indirect (where a transport medium is needed to connect the source to the receptor). Potential exposure pathways are evaluated for each of the exposure scenarios (see figure 13).
The manufacturing worker is exposed to beads daily in the course of production and packaging. The marking crew worker is exposed to the beads daily in the course of product handling, application, and removal. The marking crew worker is exposed in the field during line application; however, based on field observations, the majority of their exposure is likely to be in the storage yard where beads are stored and prepared for use or during line removal without a vacuum. While worker exposure during roadway marking removal is possible, some states require that the removed waste marking material be collected with a vacuum, essentially eliminating exposures in the field. Therefore, the focus of the evaluation of exposure pathways is the bead storage yard for the marking crew worker and the production/blending process for the manufacturing worker.
The residents are assumed to either live adjacent to an active bead storage yard, or under the most conservative scenario, a residential area may also be built on top of a former bead storage area. The residential soil is assumed to be contaminated by past bead releases or from airborne transport of beads from an adjacent bead yard. The residential trespasser is assumed to live adjacent to an active storage yard.
Figure 13. Diagram. Conceptual site exposure model for pavement-marking beads.
Although the inhalation pathway provides the potential for exposure, it is unlikely to be significant. The arsenic and lead are atomically bound to the beads and are not likely to volatilize under ambient conditions. While the beads may be crushed, they are unlikely to be reduced to particles small enough to be entrained in air due to wind scour. Therefore, only the portions of the intact beads that are small enough to be inhaled are used as the inhalation source.
There is a low potential for leaching of metals from beads and subsequent sorption to soil particles that may then be inhaled. The extractable portion of metals from glass beads was shown to be low, and the fraction of leached metals sorbed to soil particles small enough to be inhaled would be minimal. However, the inhalation pathway is included in this assessment to estimate the potential for toxic effects due to silicon exposure.
For dermal pathways, beads are assumed to adhere to skin, similar to soil, resulting in an absorbed dose. Because metals typically result in low dermal exposures relative to the ingestion pathway, the dermal pathway was not quantified in the risk evaluation. Food-related pathways (home-grown vegetables/livestock) are also possible, but are not currently considered a significant source of exposure and were not included in the assessment.
In all cases, exposures are considered to occur from contact with beads and from metals lost from beads into the surrounding media. Laboratory data indicate that arsenic and lead may be leached out of the beads to soil and to groundwater; however, the potential is limited. The ingestion exposure pathway assumed that bead contaminants are absorbed through the gastro-intestinal tract and into the blood stream.
The worker exposure is assumed to occur throughout the work day. The worker is exposed to both direct (ingestion) and indirect (inhalation) pathways. Potential pathways of exposure included in the risk evaluation were as follows:
· Incidental ingestion of beads.
· Inhalation of fugitive bead particulates (if no controls are used).
As indicated previously, the dermal pathway was not included because of the low absorption of metals and uncertainty in the availability of bead contamination for dermal absorption. In addition, the potential for contamination of soil due to migration of contamination from beads is low. Therefore, only direct bead ingestion (and not soil ingestion) is included in the risk evaluation.
These pathways have the potential to be complete exposure pathways based on the four components of exposure outlined previously. Exposure via ingestion assumes that the source of contamination is the beads, the receptor is the worker handling the beads, and the worker may incidentally ingest the contamination if not wearing protective equipment. The inhalation exposure pathway assumes that the source is the spilled beads, the migration pathway is re‑suspension of fugitive dust, the receptor is the worker, and the worker may inhale respirable particles if not wearing protective equipment.
Adult and child residential exposures occur throughout the day both inside a residence and outdoors. The resident would be exposed by direct pathways (ingestion) and indirect pathways (air, water, food). Direct pathways are applicable if the residence is built on top of a former storage yard, otherwise all residential exposures are indirect because a mechanism is required to transport contamination from the adjacent storage yard.
Potential indoor and outdoor exposures included in the risk evaluation were as follows:
· Incidental ingestion of beads.
· Ingestion of groundwater contaminated by bead leachate.
· Inhalation of fugitive particulates.
As indicated previously, the dermal pathway was not included because of the low absorption of metals and uncertainty in the availability of bead contamination for dermal absorption.
The food ingestion pathways were not included in the evaluation because of the low leaching potential identified in laboratory studies. Similarly, the potential for contamination of soil because of migration of contamination from beads is low and soil ingestion is not included in the risk evaluation.
Outdoor exposures occur while residents are playing or working in the bead-bearing soil and are assumed to be limited to ingestion of beads, inhalation of particulates due to wind scour, and ingestion of groundwater contaminated by bead leachate.
