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

Section 2. Proposed Model for the Assessment of Human Health Risks Associated with Glass Beads Used for Pavement Marking

Introduction

The use of glass beads in pavement markings presents the potential for contaminant release to the environment and subsequent human exposure. This section presents the proposed modeling methodology used to estimate the potential for adverse human health effects associated with arsenic and lead in glass beads used for pavement marking.

Specifically, this section presents the development of the human health risk model, including the following:

The risk evaluation methodology is based on observation of human interaction with the workflow of glass beads in the environment. The workflow includes interactions during manufacturing, transportation, storage, application, and disposal of old marking residues. The exposure assessment considered human exposures over a range of potential scenarios that could lead to contact with beads. The scenarios that captured the most likely and reasonably conservative exposures based on field observations were included in the exposure model development. The process of developing the model identified a number of important parameters. The required parameters were assembled from literature or laboratory measurement of glass beads where applicable. For some parameters, however, literature was either not available or demonstrated significant variability among the available sources of data. Parameters that were considered key to the development of the risk model and which indicated significant variability in parameter values are specifically highlighted in the text.

The field observations of glass bead production, storage, application, and removal were conducted in an arid environment during warm, dry, and low wind conditions. Roadway marking operations are generally conducted during periods of low winds (< 15 mi/h) to minimize bead and paint loss. Although observations were made in an arid environment, this assessment considers a range of environmental conditions. Additional information used in this assessment was obtained through interviews with individuals involved in the glass manufacturing and highway marking industries.

The laboratory investigations presented in section 1 were used to establish the total, extractable, and bioaccessible arsenic and lead content used in the model. Because of uncertainty regarding the source of the beads used in the metals analysis in experiments prior to the September 2012 TTI/TAMU report, the risk evaluation presented in this report focuses on arsenic and lead using the most recently available bead analysis.

The bead workflow assessment was conducted using English units because those are most commonly used in the United States for the applicable processes. The remaining portions of the document use SI units because English units are not used for the chemical analyses that support development of the risk-assessment models.