Inorganic Arsenic Human Health Toxicity Update: Key Changes and Implications
By Alexandra Meyers, MPH, BCES, Consultant
John R. Griffin, Senior Consultant
Heather Lynch, MPH, DABT, Principal, Toxicology, Health, and Ecological Science
On January 13, 2025, the U.S. Environmental Protection Agency (EPA) released the final Integrated Risk Information System toxicological review of inorganic arsenic. In this report, EPA developed two new toxicity values to evaluate risks and hazards associated with potential exposures to inorganic arsenic: a new oral cancer slop factor (CSF) and oral reference dose (RfD).
Key Changes
The oral CSF increased from 1.5 to 32 per mg/kg-day and the oral RfD decreased from 0.0003 to 0.00006 mg/kg-day. Thus, the oral CSF and oral RfD are now 21 times and 5 times more stringent than they were previously. The key drivers causing the change in toxicity values are related to the choice of endpoints. The previous EPA CSF was based on the observation of skin cancer and the new CSF is based on bladder and lung cancer health outcomes. The previous EPA RfD was based on dermal and vascular effects and the new RfD is based on studies of diabetes and ischemic heart disease.
What is the Impact?
The EPA Regional Screening Levels (RSLs) will likely incorporate the newly revised toxicity values in May 2025, resulting in significant changes. For example, the residential soil RSL would lower from 0.68 mg/kg to 0.032 mg/kg based on an excess lifetime cancer risk of one in a million. Agencies will likely require the use of these newly published toxicity values for all current human health risk assessments.
Arsenic RSLs are well below most naturally occurring sources of arsenic in the environment. The new toxicity updates highlight the importance of collecting background data to assist remedial project managers in distinguishing site-related impacts from anthropogenic and natural sources.
Our team at Integral Consulting Inc. is at the forefront of these regulatory changes and is here to help you navigate these impacts as it affects site investigations and remedial actions.