Apples, Arsenic, and Risk - Part 2: EPA's Proposed Cancer Slope Factor

In the January 2012 Consumer Report's article "Arsenic in your juice," they write:

Debate over that [new arsenic] standard is likely to begin anew. The agency's [EPA] latest draft report, from February 2010, proposes that the number used to calculate the cancer risk posed by ingesting inorganic arsenic be increased 17-fold to reflect arsenic's role in causing bladder and lung cancer.

The proposal "suggests that arsenic's carcinogenic properties have been underestimated for a long time and that the federal drinking-water standard is underprotective based on current science," says Keeve Nachman, the Johns Hopkins scientist.

What the EPA proposes in their final draft is to base the "safe" level of arsenic exposure on is an Oral Cancer Slope Factor (SFo) of 25.7 per mg/kg-day.  25.7 is a 17 fold increase over the SFo currently used by IRIS which is 1.5 per mg/kg-day. (3).  In this draft, EPA is basing their risk on work that theoretically determines a cancer potency 17 times more potent than what has been established and used to determine the MCL for drinking water of 10 ppb.

With this new and increased potency, Johns Hopkins' Dr. Nachman is now emboldened to tell Consumer Reports that the current 10 ppb standard for drinking water is "unprotective."  And, as I quoted in my last post, Consumer Reports is able to make their case for concern by reporting that "For water with 10 ppb of arsenic, the excess cancer risk is one in 500."

The current acceptable limit for arsenic in drinking water is 10 ug/L (10 ppb) based on an "oral reference dose" (RfD) of 0.0003 mg/kg-day which was derived from data showing a "no observable adverse effect level (NOAEL) of 0.0008 mg/kg-day and a "lowest observed adverse effect level" (LOAEL) of 0.0014 mg/kg-day. (1)

Here is what all of that RfD, NOAEL, and LOAEL means:

NOAEL: The highest exposure level at which there are no biologically significant increases in the frequency or severity of adverse effect between the exposed population and its appropriate control; some effects may be produced at this level, but they are not considered adverse or precursors of adverse effects. (2)

LOAEL: The lowest exposure level at which there are biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control group. (2)

RfD: The reference dose is based on the assumption that thresholds exist for certain toxic effects such as cellular necrosis. It is expressed in units of mg/kg-day. In general, the RfD is an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. (1)

The "deleterious effects during a lifetime" for arsenic is currently based on this possible health effect above the RfD of 0.0003 mg/kg-day:

Hyperpigmentation, keratosis and possible vascular complications.

The EPA, in 2010, proposes to change this from the amount of arsenic that might cause "hyperpigmentation, keratosis and possible vascular complications" to one that uses a cancer slope factor (SFo) based on:

Lung and bladder mortality data from Morales et al. (2000) for the dose-response assessment for the final draft SFo. (3)

This is where it gets a bit, well, complicated.

Next post:  Apples, Arsenic, and Risk - Part 3: EPA's Black Diamond Cancer Slope


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