PRINCIPLES OF TOXICOLOGY - Biology East Borneo

19.5 RISK ASSESSMENT FOR ARSENIC 487Arsenic occurs naturally in air, water, soil, and food in low concentrations. Thus, daily exposure tovery low amounts of arsenic is unavoidable. Thus, risk assessments of arsenic must often deal with“background” exposure from everyday living in addition to exposures resulting from occupational orenvironmental sources.Inorganic forms of arsenic are known to be carcinogenic to humans. Since 1888, elevated arsenicexposure has been associated with an increased incidence of skin cancer. Arsenic exposure has alsobeen linked to lung, bladder, and liver cancer. Although high levels of arsenic exposure are indisputablycarcinogenic to humans, there is growing evidence of an apparent threshold for arsenic carcinogenicity.A number of epidemiologic studies indicate that arsenic may cause cancer by a nonlinear or a thresholdmode of action. In large part, this nonlinear action may explain the lack of association betweenrelatively low levels of arsenic exposure and the development of skin, bladder, or other cancers. Anonlinear carcinogenic relationship to dose indicates that the carcinogenic response induced by thechemical decreases more than a linear relationship to dose. In other words, dose-response is sublinearat low doses.A risk assessment for arsenic using USEPA default exposure factors is presented below. However,the impact of the bioavailability of arsenic in soil is included as an important modifying factor in theUSEPA risk assessment process. The impact of these default factors and the adjustment for soilbioavailability is evaluated in this arsenic risk assessment example.Consider the case of a medium density residential development being built on top of fill partlycomposed of mining waste containing elevated concentrations of arsenic. Investigation of the site soilindicated surface soil arsenic concentrations ranging from 12 to 140 mg/kg with a mean concentrationof 90 mg/kg. The family living in the residence includes both adults and young children. Possiblepathways of exposure to arsenic in soil include incidental ingestion of arsenic in soil, absorption ofarsenic into the skin from soil adhering to the skin, inhalation of arsenic-containing dust, and ingestionof arsenic taken up from the soil by home-grown produce. Since a residential housing developmentoffers very limited space to plant a garden, ingestion of home-grown produce is not considered relevantfor this site.The USEPA soil screening level (SSL) for arsenic is 0.4 mg/kg. The arsenic SSL is based oningestion of soil and an added lifetime cancer risk of 1 × 10 –6 . As a first tier risk-based screening level,use of the USEPA SSL is problematic since the average background concentration of arsenic in soilin the United States is about 5 mg/kg. Nonetheless, the mean arsenic concentration exceeds the SSLand the typical background concentrations, indicating that a higher tier of risk assessment is neededto address potential health risk at the site due to arsenic.With the exception of arsenic bioavailability in soil, default USEPA assumptions used to evaluatearsenic exposure due to ingestion, skin contact, and inhalation of soil particles are presented in Table19.5. The bioavailable fraction of arsenic from soil was assumed to be 0.28 based on studies inmonkeys. This is below the typical USEPA default bioavailability of 0.8–1. The exposure equationsused to perform these calculations are presented in Table 19.10, later in this chapter.The following average daily intakes (ADIs) were calculated for a child and adult resident exposedto arsenic in soil. Lifetime ADIs are also calculated to assess the added lifetime cancer risk associatedwith exposure to arsenic in soil. These calculations are presented in Table 19.6.The noncarcinogenic risks associated with exposure to arsenic in soil are assessed using the hazardquotient (HQ) method. As discussed earlier in this chapter, the hazard quotient (HQ) is calculated bydividing the ADI by the reference dose (RfD). For arsenic, only an oral RfD is available. However,because skin absorption and inhalation may add to overall exposure, hazard quotients may also becalculated for these routes of exposure using the oral RfD (0.0003 mg/kg/day). The sum of the HQsis known as the hazard index (HI). The HI for the ingestion, skin absorption, and inhalation soilexposure pathways for the child is thus calculated as3.22 × 10 −4 mg / kg⋅day3 × 10 −4 mg / kg⋅day + 1.15 × 10−6 mg / kg⋅day3 × + 1.07 × 10−7 mg / kg⋅day10 −4 mg / kg⋅day 3 × 10 −4 mg / kg⋅day