Screening Level Assessment of Arsenic and Lead Concentrations in Glass Beads Used in Pavement Markings

Exposure Assessment

The exposure assessment presented in this section outlines the individuals and pathways by which humans come in contact with contaminants associated with glass beads during the product lifecycle. In the exposure assessment, information regarding the characteristics of bead contamination is combined with assumptions regarding the source of contamination and presence of potential receptor populations, to elucidate the nature of human health exposures. The exposure assessment includes discussions concerning exposure scenarios, pathways of exposure, and migration pathways (used to establish the environmental fate of the beads).

Potential Exposure Scenarios

Populations that could potentially be exposed to glass beads include workers, residents, recreational users, and consumers of agricultural goods. Although some exposures are more plausible than others, all of the potential exposure scenarios listed below were considered for further evaluation. When the field observations indicated a particular scenario was insignificant relative to other scenarios for the same media, that scenario was no longer considered viable and was eliminated from the exposure pathway assessment.

Occupational Exposure Scenarios

The occupational exposure scenario is based on individuals who work on a roadway marking crew or manufacturing workers. The two occupational scenarios are similar, with the primary difference being that manufacturing workers will be exposed to beads only. Roadway marking crews may be exposed to both beads and soil that has been affected by the beads.

Marking crew employees include two distinct populations of workers who experience exposures related to the handling of beads: those who apply markings and those who remove markings. However, the same person or persons may work on all tasks required by a marking crew. Manufacturing workers may also be exposed during the direct handling of the product (either during packaging or transferring of beads).

Field observations indicated that worker exposure during transportation of bead products is minimal because of the handling practices employed to keep the beads free of residual moisture. Therefore, worker exposure during transport is not considered further. However, other exposures associated with the bead lifecycle are further evaluated in this assessment for the marking crew worker and manufacturing worker scenarios.

Residential Exposure Scenarios

The residential scenario is based on an individual property affected as a result of proximity to either a roadside where beads have been applied to markings or a bead storage yard. Field observations indicate that exposures associated with roadway marking application would be less significant than residential exposures due to concentration of beads released to the bead storage yard. Therefore, the residential scenario focused on exposures associated with the bead storage yard of a pavement-marking agency and not on application of the beads along a property line.

Individuals (either adults or children) living on the affected property may be exposed both indoors and outdoors to contaminated soil, water, food, and air originating from a bead storage yard. The resident may be exposed to contamination migrating from a storage yard that is on property adjacent to the residence, or in the most conservative approach, the residence may have been built directly on top of a former bead storage yard.

Where a former storage yard is now a residential area, individuals may contact contaminated soil during daily outdoor activities. A resident may also track contamination indoors, resulting in continued exposure. If beads were left exposed to precipitation in the storage yard, the potential exists for heavy metals to leach to groundwater, which may result in additional exposure pathways (through drinking affected groundwater or irrigating a home garden). In the case of a residence adjacent to a bead storage facility, the possibility exists for metals to migrate into offsite residential soil, particularly in arid climates where wind-generated dust is prevalent. Also associated with the offsite resident is the potential for juvenile trespassing into the adjacent bead storage facility.

For both the resident onsite and offsite scenarios, there is a potential for exposure via ingestion of fish from a contaminated surface water body, consumption of home-grown vegetables (gardening), or consumption of home-raised livestock products (chickens or chicken eggs, for example). Additional residential exposure may occur to individuals near schools where numerous crosswalks are frequently re-marked and, particularly, where streets are curbed allowing beads to accumulate. The sparkling appearance of the beads may draw attention and be attractive to school children, increasing their likelihood of exposure. However, exposures mentioned within this paragraph are considered speculative, and if they occur, they will be infrequent and short in duration. Therefore, to address the most significant and likely scenarios, the residential exposures included in the development of this methodology are those associated with residents living in close proximity or on top of a bead storage yard and the trespasser accessing the storage pile from an offsite residence.

Recreational Exposure Scenarios

The recreational scenario is based on an individual who rides all-terrain vehicles (ATV) along roadways or who fishes from water bodies affected by beads. Where a sufficient amount of beads has accumulated, dust may be inhaled by the ATV rider or taken up by aquatic organisms, resulting in potentially significant exposure. The level of exposure is highly dependent on climate and the fate of beads in the environment.

Because of the expected low concentration of beads estimated and observed for roadside soils, roadside recreational activities are unlikely to result in significant exposures. In arid climates where dust generation is more prevalent, a rider is also more likely to wear a face mask filter to reduce dust inhalation. For areas with higher precipitation, the mass of beads is likely to be insufficient to significantly affect aquatic biota. In the absence of evidence of elevated roadside bead concentrations, the recreational scenario is not evaluated further.

Agricultural Exposure Scenarios

The agricultural scenario is based on a farm situated beside a roadway where beads have been applied. Beads accumulating in farmland may result in uptake into crops or livestock and subsequent human exposure. Although this scenario is possible, the area affected by beads would likely be limited, resulting in contamination of only a fraction of any total farmland. Coupled with the low frequency of bead application (perhaps every 5 years in rural areas), the agricultural scenario is unlikely to result in significant exposures and is not considered further in this assessment.