On January 13, 2025, the United States Environmental Protection Agency (USEPA) released an update to its toxicological assessment of inorganic arsenic (CAS Number 7440-38-2) on its Integrated Risk Information System (IRIS)1. This update incorporated new developments in epidemiological research on inorganic arsenic for non-cancer effects of the circulatory system, diabetes, fetal, newborn, and infant health outcomes, and neurodevelopmental effects.
As outlined in the final assessment, USEPA concluded that:
- Robust evidence demonstrates inorganic arsenic causes diseases of the circulatory system (ischemic heart disease and hypertension, atherosclerosis) and diabetes (type 2) in humans.
- Moderate evidence demonstrates fetal, newborn, and infant effects (mortality and inverse fetal and post-natal growth, length of gestation, or birth weight) as well as neurodevelopment effects (cognitive and behavioral deficits in children and adolescents) in humans.
Inorganic arsenic had been previously classified by USEPA as “carcinogenic to humans.” The final assessment also included updated epidemiological research for known cancer outcomes (i.e., bladder and lung cancer) associated with inorganic arsenic exposure.
As a result of the epidemiological evidence incorporated into the updated assessment, USEPA revised the non-cancer and cancer oral toxicity values. As detailed in the table below, the revised values are 5-fold and 32-fold lower than the prior toxicity values, for non-cancer and cancer respectively.
https://iris.epa.gov/ChemicalLanding/&substance_nmbr=278
| Timeframe | Non-cancer Oral Reference Dose (mg/kg-day) | Cancer Oral Slope Factor (per mg/kg-day) |
|---|---|---|
| Current (January 2025) | 0.00006 | 32 |
| Previous (November 2024) | 0.0003 | 1.5 |
How Do These Changes Impact Your Sites?
With this update, we would expect both federal and state agencies to revise their current screening levels for inorganic arsenic in the near future as well as begin the rulemaking process needed to revise promulgated inorganic arsenic standards or cleanup levels.
EHS Support derived screening levels using the USEPA Regional Screening Level (RSLs) calculator2 and determined that the update to the inorganic arsenic oral toxicity values would result in a roughly 20-fold decrease in the soil and water screening levels for residential and industrial land uses, as shown in the table below.
| Timeframe | Resident Soil (mg/kg) | Industrial Soil (mg/kg) | Tap Water (μg/L) |
|---|---|---|---|
| Current (January 2025) | 0.032 | 0.14 | 0.0024 |
| Previous (November 2024) | 0.68 | 3 | 0.052 |
These updated screening levels, as well as the previous screening levels, are below average background concentrations reported for the conterminous US (approximately 7 mg/kg)3, underscoring the need for a robust conceptual site model that adequately characterizes upgradient (background) groundwater conditions and establishes representative site-specific or regional background soil concentrations.
How Can EHS Support Help?
EHS Support provides clients with focused human health risk assessment strategies to quantify risks in a cost-effective manner and to support technically sound and protective risk management decision-making. Reach out to Dana McCue today for more information.
- https://iris.epa.gov/ChemicalLanding/&substance_nmbr=278 ↩︎
- Screening levels based on USEPA default exposure assumptions with a target cancer risk of one in one million (1×10-6) and a hazard quotient of 1. RSL calculator is available here: https://www.epa.gov/risk/regional-screening-levels-rsls ↩︎
- Shacklette, H.T. and J.G. Boerngen. 1984. Elemental Concentrations in Soils and Other Surficial Material of the Conterminous United States. U.S. Geological Survey Professional Paper 1270. United States Department of the Interior. ↩︎
