Arsenic Doses from Soil Exposures: Soil Concentrations of Public Health Concern

The adverse health effects of exposure to high concentrations of arsenic have been well documented and include a variety of effects ranging from death to skin, lung, kidney, and bladder cancers, as well as kidney/liver/gall bladder diseases, nausea, cardiovascular disease, developmental and reproductive effects, and skin keratosis and hyperpigmentation. Arsenic is ubiquitous in soils with naturally-occurring concentrations that are log-normally distributed. There are also a number of man-made processes that create elevated localized soil and water concentrations, such as mining and ore-processing, application of arsenical pesticides and runoff or leachate from coal ash and landfills. In order to determine doses likely to produce adverse health effects, potential doses must be calculated from soil concentrations and intake rates. This paper evaluates the potential for adverse health effects from exposures to arsenic in soil using probabilistic arsenic dose calculations and compares these estimated doses with various health effect levels. The primary variables underlying a dose calculation from soil exposure are the contaminant concentration (mg/kg), soil ingestion rate (IR; mg/day), the duration of exposure (or exposure factor; days/day), body weight (kg), and the bioavailability factor (BF; mg/mg). Each of these variables can be represented by a discrete value or by a probability function. In the following calculations, body weight is represented by a log-normal probability distribution, and IR and BF are represented by triangular distributions. Daily exposure is assumed so that the exposure factor is one. For constant daily exposure to a soil with 100 mg/kg As, the resulting 95th percentile dose for a child is 0.0009 mg/kg/day and for an adult the 95th percentile dose is 0.0002 mg/kg/day. These doses are comparable to the average dietary intake of As (average values of 0.0005 to 0.0008 mg/kg/day) and much lower than doses associated with adverse health effects. A number of studies have examined soil As exposure by measurement of urinary As concentrations of residents living on properties with high As soil concentrations. Several of these studies have derived empirical, linear relationships between soil As concentrations and urine As concentrations. These linear soil-urine equations have significantly lower slopes relative to the calculated dose-soil concentration equation. None of these studies have shown a statistically elevated urinary As concentrations (above population norms or control groups) at soil concentrations less than 100 mg/kg. When these dose evaluations are compared with state-mandated As soil clean-up levels that range from 0.14 mg/kg to 250 mg/kg, it is very clear that we are either not uniformly protecting the public, or spending way too much on unnecessary soil remediation.

Contribution proposed for:   oral presentation