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

Section 3. Model-Derived Human Health Screening Levels For Arsenic And Lead In Glass Beads

Introduction

Parameter Data Available for Development of the Modeling Method

The calculations outlined in section 2 are the basis for development of the human health screening levels. Each of the calculations contains parameters that must be defined before using the developed model. This section assigns a representative value to parameters used to determine intake or exposure concentrations and, in some cases, discusses the range of parameter values considered. The screening levels are then recommended based on the developed model run with the identified parameters. The developed CSEM is used to determine the arsenic screening level, and the ALM and IEUBK models are used to determine the screening level for lead based on each evaluated exposure scenario.

It is important to acknowledge the risk associated with the uncertainty inherent in the risk assessment and to explicitly state that any preliminary calculation of human exposures to metals in glass beads released to the environment will necessarily be based on numerous assumptions and generalizations. The use of generic input parameter values provides results that might be applicable across a range of possible site locations with region-specific considerations (e.g., weather, geology). In addition, available data derived from laboratory investigations are also the source of variability in the estimated screening levels. Where applicable, key uncertainties requiring additional sources of data (or further refinement) prior to using the presented modeling methodology are discussed. Parameters specific to individual receptors, contaminants, or the beads are also presented in the following subsections.

Receptor-Specific Exposure Parameters

Each receptor has particular metabolic or behavioral parameters that must reflect the activities associated with each scenario, and these parameter values are specific to each receptor under evaluation. The population of individuals representing the receptor group is incorporated into the values selected for these parameters to provide a conservative but reasonable estimate of exposure. General parameter values available from EPA’s risk assessment guidance documents that are used to determine arsenic screening levels are summarized in table 18.

Table 18. Exposure parameters for glass bead assessment due to direct exposures.

Notes: Exposure parameters are consistent with EPA guidance document for ingestion and inhalation exposures.

Bioavailable fraction of 10 percent is a conservative estimate based on bead measurements (see table 9).

The worker exposure is assumed to represent light industrial activities, which excludes any heavy excavation or similar work, but does include contact with the bead/soils. The residential exposure is based on an adult and child who spend time both indoors and outdoors. While outdoors, the child/juvenile is assumed to play in the soil and beads. The adult is assumed to work in the beads and soil doing landscaping or similar activities. The following subsections describe receptor-specific parameters used in the assessment, including body weight, exposure duration, exposure frequency, ingestion rates, skin surface area, and soil/bead adherence factors.

Body Weight

The body weight used in the analysis for the worker is based on a conservative estimate for adults of 80 kg.^([18]) Similarly, the adult resident is based on an average adult of 80 kg. The body weight of the child is assumed to be 45 kg, which is an average of both sexes from ages 6 to 18 years old.^([19]) The juvenile trespasser is assumed to be exposed from age 8 to 18 years, so a body weight of 45 kg is also representative of this receptor.

Exposure Duration

The exposure duration for the worker of 25 years is based on a conservative estimate of the length of employment for an adult.^([20]) Although it is likely that the occupational exposure duration would be significantly shorter, the conservative value is assumed to reduce the potential for underestimates of risk in the assessment.

The residential adult exposure duration is based on a conservative estimate of the length of time spend in one residence of 30 years.⁽⁴⁾ The residential child is assumed to live in one household from ages 6 to 18, an exposure duration of 12 years. The trespasser exposure occurs for a period of 10 years, assuming that older teenagers are less likely to continue trespassing into the storage yard. Similar to the worker exposure, the conservative values are proposed for the resident to provide protective results.

Exposure Frequency

The frequency of exposure for a marking crew worker is based on a typical schedule of 5 days per week for a total of 250 days in a year (assuming 2 weeks of vacation). However, the application of roadway markings is a weather -dependent job, and work is only conducted on days with low wind and no precipitation. In some areas of the country, weather conditions may prevent work on all but approximately 120 days each year. On days that are not suitable for applying markings, workers may be employed with the removal of old markings. Therefore, marking crew workers would be on the job for 250 days each year; however, the balance between the time spent applying lines and removing old lines would differ regionally.

A resident is typically assumed to be at home for all but 2 weeks each year, or a total of 350 days annually. Although it is likely that the exposure frequency would be significantly shorter, the conservative value is proposed. The trespasser is assumed to access the bead storage yard five times each week, for a reasonably conservative exposure frequency of 250 days each year.

Ingestion Rate

The ingestion rates of soil and water are specific to each receptor. The soil pathway applies to both marking crew worker and residential exposures, but not to the manufacturing worker. The water ingestion pathway is only applied to the residents who drink contaminated well water (assuming workers drink from municipal water systems).

Soil/Bead Ingestion: The soil/bead ingestion rate is based on the amount of soil/beads adhered to the hands, which is subsequently ingested throughout the day. It is assumed that beads behave similarly to soil, and that the concentration of contaminants in soil/beads that are ingested are consistent with the relative concentrations in soil (i.e., no preferential ingestion of highly contaminated particles).

For workers and adult residents, the average ingestion rate is assumed to be 50 mg/day.⁽¹⁸⁾ It is assumed that the worker is engaged in light industrial activities and does not do any excavation work that might increase contact with soil and thereby increase the ingestion rate. Residents contact soil/beads while engaged in landscaping or other similar activities.

The average child ingestion rate of 200 mg/day is used in the assessment.⁽¹⁹⁾ A resident child may exhibit pica, which is a condition that results in excessive soil ingestion (approximately 2,000 mg/day). However, the pica receptor is highly unlikely and, therefore, is not used as the basis for child soil ingestion.

Water Ingestion: The water ingestion rate applies only to residents who use potable water from a well that extracts groundwater that has been affected by a bead storage yard. The resident adult is assumed to ingest groundwater at a rate of 2.7 L/day on average.⁽¹⁸⁾ Similarly, it is assumed that the child ingests water at the same rate as an adult.⁽¹⁹⁾