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

Modeling Methods

Based on the completed exposure assessment, the risk evaluation modeling methods focused on the following three particular exposure scenarios:

·         Scenario 1—Worker: roadway marking crew employee exposed through incidental ingestion and inhalation of fugitive dust emissions. (Inhalation scenario assumes no protective masks are worn.)

·         Scenario 2—Adult Resident: adult living in close proximity to an active bead storage yard or on top of a former storage yard exposed through ingestion of contaminated drinking water, incidental ingestion of beads, and inhalation of fugitive dust emissions.

·         Scenario 3—Child Resident: child living in close proximity to an active bead storage yard or on top of a former storage yard exposed through ingestion of contaminated drinking water, incidental ingestion of beads, and inhalation of fugitive dust emissions.

A trespassing juvenile exposure scenario was considered but not evaluated because the resident exposure was considered to be more limiting. An occupational exposure scenario for the manufacturing worker was also considered but not included in the risk evaluation. It was assumed that occupational exposures would be controlled through the use of dust suppression methods and/or use of personal protective equipment where necessary.

The proposed modeling framework focuses on developing quantitative measures to evaluate potential risk and develop screening level concentrations of metals in the glass beads that are protective of human health. The quantitative assessment requires calculation of the following two components:

·         The level of metal uptake or air concentration as a function of each individual exposure route.

·         The permissible level of exposure (screening level) due to either cancerous or non-cancerous end points, considering the combined intake from the multiple routes of exposure affecting a single receptor.

The calculations are based on EPA guidance documents addressing human exposure to soil, water, air, and food.⁽⁴⁾ The contaminant exposures calculated for each receptor, environmental medium, and pathway combination are the basis for estimating the potential risk or hazard to exposed individuals.

Equations Used to Calculate Exposure to Heavy Metals for Each Receptor Modeled in the Exposure Scenarios

Exposure equations are specific to each environmental medium (soil, water, and air) and pathway of exposure. The calculations of intake for each receptor due to ingestion of beads or bead-impacted soil, inhalation of particulate matter associated with beads or bead-impacted soils, and ingestion of bead-impacted groundwater are presented within this subsection. The developed equations apply to each scenario, and all direct solid matrix exposures are assumed to occur from the top 150-mm layer of soil or from the glass bead product itself. The quantified exposures are then compared with the toxicity values to determine the potential for adverse health effects.

Lead does not have established toxicity data for the evaluation of risk; instead, the exposures are related to blood lead levels to determine the potential for adverse health effects. The risk evaluation for beads used the U.S. EPA models, Adult Lead Model (ALM), and the Integrated Exposure Uptake Biokinetic Model (IEUBK) for children to estimate human health risks from lead exposure.

Because this risk evaluation focuses on arsenic and lead, the equations used are based on metals exposures for the exposure pathways described below. Because lead is evaluated with specific EPA models, the following equations are applied to exposures to arsenic.

Intake Due to the Incidental Ingestion of Beads or Bead-Impacted Soil

Figure 14 presents the equation to estimate the intake of metals from beads or bead-impacted soil due to incidental ingestion.

Figure 14. Equation. Formula to calculate intake of metals due to incidental ingestion.

Where:

C_s = exposure concentration in solid matrix (mg_metal/kg_matrix).

IR_s = ingestion rate of solid matrix (kg_matrix/day).

EF = exposure frequency (days/year).

ED = exposure duration (years).

FI = bioavailable fraction (unitless).

BW = body weight (kg).

AT = averaging time (days) for carcinogens or non-carcinogens.

Exposure Concentrations Associated with Inhalation of Beads or Bead-Impacted Soils

The inhalation pathway is based on an air concentration representing an average over the exposure duration. Figure 15 presents the equation used to calculate exposure concentrations for inhalation of metals from small beads, bead dust, or from bead-impacted soils.

Figure 15. Equation. Formula to calculate exposure concentrations for inhalation of metals.

Where:

C_s = exposure concentration in solid matrix (mg_metal/kg_matrix).

EF = exposure frequency (days/year).

ED = exposure duration (years).

VF = chemical‑specific volatilization factor (m³/kg_metal).

PEF = particulate emission factor (m³/kg).

AT = averaging time (days) for carcinogens or non-carcinogens.

Note that chemical-specific VF is only applicable for soil contaminants that volatilize significantly. Because metals do not generally volatilize, the VF^-1 term in figure 15 is 0.

Intake Due to Ingestion of Bead-Impacted Groundwater

Figure 16 presents the equation used to estimate the intake of metals from groundwater used as a drinking water source.

Figure 16. Equation. Formula to calculate intake of metals due to ingestion of bead-impacted groundwater.

Where:

C_w = exposure concentration in water (mg_metal/L).

IR_w = ingestion rate of water (L/day).

EF = exposure frequency (days/year).

ED = exposure duration (years).

BW = body weight (kg).

AT = averaging (days) for carcinogens or non-carcinogens.

Risk Evaluation and Calculation of Permissible Levels of Exposure (Cancerous and Non-Cancerous Human Health Screening Levels)

The final step in the risk evaluation combines the exposure assessment and toxicity data to estimate human health risks and generate screening levels. Potential human health effects are characterized as either carcinogenic or non-carcinogenic when calculating the screening levels for each constituent via each exposure pathway.

The probability of cancer effects is assumed to be linearly related to the exposure level of a human receptor to a contaminant in an environmental matrix. An increased probability of cancer effects is assumed to occur with any increased exposure, regardless of magnitude. In other words, there is no threshold for cancer effects and some impact, however small, is expected at any level of exposure. In contrast, non-cancer effects are based on the concept of a threshold of exposure. Below the threshold, no adverse effects are expected; however, exposures exceeding the threshold only indicate an increased likelihood of the occurrence of the adverse health effects.

For this evaluation, carcinogenic effects are defined as an increased probability of cancer incidence, or an Incremental Lifetime Cancer Risk (ILCR). An acceptable ILCR of 1 additional cancer per 100,000 exposed individuals (also expressed as 0.00001 or 1E-05) is proposed as the target risk level for determining screening levels of individual metals in glass beads associated with carcinogenic effects.

Non-carcinogenic effects may be manifested in any number of health impacts (including skin lesions, reproductive effects, and kidney damage) and are represented by a Hazard Quotient (HQ). An HQ is a ratio of the intake of contaminants (or exposure concentration for inhalation) to a reference value. The reference concentration is considered a threshold level below which there is a low probability of adverse health effects. For this evaluation, an HQ of 1.0 is proposed as the target level for determining the screening levels of individual metals in glass beads associated with non-cancer effects.

The target ILCR and HQ are selected considering the acceptable target risk range established by the EPA.^([17]) The upper end of the risk range is an ILCR of 1 in 10,000 or 1E-04, with a lower threshold of 1 in 1,000,000 or 1E-06. For non-carcinogenic effects, a Hazard Index (HI) of 1 is the point of departure for considering mitigation of exposures. The HI is the sum of HQs for individual metals over all pathways.

The screening levels established for bead exposure must account for the multiple contaminants present in the beads and multiple pathways of exposure. An individual would experience an increased risk from each constituent and protective screening levels account for the exposure from the combination of contaminants.

Calculating Direct-Contact Cancer Screening Levels

Figure 17 presents the equation for calculating the ILCR for direct contact with beads or bead-impacted soils through the ingestion exposure pathways.

Figure 17. Equation. Formula to calculate ILCR for direct contact through ingestion.

The equation in figure 18 is used to calculate the ILCR for inhalation exposures.

Figure 18. Equation. Formula to calculate ILCR for inhalation exposures.

ILCRs are calculated specific to a particular contaminant, exposure pathway, and receptor. The slope factor relates the intake level to the probability of increased cancer. The cancer slope factors are determined from animal experiments and data from accidental human exposures where available

The overall ILCR for an individual constituent is then calculated as the sum of the ILCRs for the exposure pathways (ingestion, inhalation). The ILCRs for the constituents are then summed to provide an overall ILCR for the exposure scenario.

If the ILCRs are calculated for a unit concentration in the exposure media (e.g., 1 mg/kg), the ILCR can be used to calculate the cancer screening level (CSL) for protection of human health with the equation in figure 19.

Figure 19. Equation. Formula to calculate CSL for protection of human health.

The resulting screening level is protective of human health for cancer effects from the exposure pathways included in the assessment. The equation in figure 19 can also be used to generate screening levels representative of a single exposure pathway (such as ingestion, inhalation) by substituting the calculated ILCR for the individual pathway for the calculated ILCR for all pathways.

Calculating Direct Non-Cancerous Screening Level

Figure 20 presents the equation for calculating the HQ for direct contact with beads or bead-impacted soils through the ingestion exposure pathways.

Figure 20. Equation. Formula to calculate HQ for direct contact.

Figure 21 presents the equation for calculating the HQ for inhalation.

Figure 21. Equation. Formula to calculate HQ for inhalation.

As indicated, the equation and parameters are specific to the pathway under consideration. The reference dose is generally expressed in mg_metal/kg-d and the reference concentration in mg_metal/m³. These reference values are based on animal experiments or exposure to humans.

The overall HQ for an individual constituent is then calculated as the sum of the HQs for ingestion and inhalation exposure. The sum of the HQs for all constituents considered in the exposure is the HI and reflects the overall potential for toxic effects from bead exposure.

If individual HQs are calculated assuming a 1 mg/kg media concentration, then the results can be used readily to determine the non-carcinogenic screening level (NCSL) using the equation in figure 22.

Figure 22. Equation. Formula to calculate NCSL.

The resulting screening level is protective of human health for non-cancer effects via the pathways considered in the assessment. The above equation can also be used to generate screening levels that are representative of a single exposure pathway (such as ingestion or inhalation) by substituting the calculated HQ for the individual pathway for the calculated HQ for all pathways.

Calculating Indirect Contact Screening Levels: Ingestion of Bead-Impacted Groundwater

The development of the screening level for consumption of bead-impacted groundwater is based on permissible levels of metals in groundwater for residential use, in this case the EPA Regional Screening Levels (RSL). The screening levels for protection of groundwater are based on an ILCR of 1E-06 or an HQ of 1 used for individual contaminants.

The concentration of contaminants in beads is then related to the acceptable groundwater concentration based on the assumed leach rate of metals from beads and dilution of contaminants leaching into the affected aquifer (which is the DAF). Figure 23 presents the equation used to calculate the bead groundwater screening level (GW SL).

Figure 23. Equation. Formula to calculate GW SL.

Final Bead Screening Level

The overall screening level for any medium from direct and indirect exposures is determined by comparing the screening levels based on carcinogenic effects with those for non-carcinogenic effects. The lowest value for each medium is selected as the final screening level for protection of human health within a given scenario. By evaluating each medium independently, combined exposures to soil and water are not represented in the screening levels. Because of the conservative assumptions incorporated in the evaluation, a combined exposure using the current model would lead to a highly conservative screening level. Where combined exposures to soil and groundwater are considered feasible, refinements to the modeling assumptions could provide more representative screening levels.

The calculated screening levels for each of the scenarios evaluated will include:

·         Scenario 1—Worker: roadway marking crew employee exposed through incidental ingestion and inhalation of fugitive dust emissions.

The overall soil screening level for individual constituents is the lowest value of:

o   CSL (ingestion, inhalation pathways combined).

o   NCSL (ingestion, inhalation pathways combined).

·         Scenario 2—Adult Resident: living in close proximity to an active bead storage yard or on top of a former storage yard exposed through ingestion of contaminated drinking water, incidental ingestion, and inhalation of fugitive dust emissions.

The overall soil screening level for individual constituents is the lowest value of:

o   CSL (ingestion, inhalation pathways combined).

o   NCSL (ingestion, inhalation pathways combined).

The overall groundwater screening level for individual constituents is the lowest value of:

o   CSL (ingestion pathway).

o   NCSL (ingestion pathway).

·         Scenario 3—Child Resident: living in close proximity to an active bead storage yard or on top of a former storage yard exposed through ingestion of contaminated drinking water, incidental ingestion, and inhalation of fugitive dust emissions.

The overall soil screening level for individual constituents is the lowest value of:

o   CSL (ingestion, inhalation pathways combined).

o   NCSL (ingestion, inhalation pathways combined).

The overall groundwater screening level for individual constituents is the lowest value of:

o   CSL (ingestion pathway).

o   NCSL (ingestion pathway).

The equations used to determine the screening level concentrations for each route of exposure within each scenario are the same; however, the parameters used for each scenario are different based on how the receptor interacts with the beads (or bead-impacted media) within each exposure pathway. Section 3 details available data included in the exposure assessment and reports modeling effort results used to set the screening levels for arsenic and lead in glass beads.