Arsenic in Rice: Part 7 - EPA and IRIS

So what do we know so far...

• Consumer Reports compares the amount of arsenic found in rice to the New Jersey threshold for arsenic in drinking water.
• New Jersey set its arsenic level based on the "limits of medical, scientific and technological feasibility."
• The New Jersey limit is based on data presented in the National Academy of Sciences (NAS) report titled Arsenic in Drinking Water: 2001 Update
• The NAS report is based on a health risk assessment performed by the EPA in 1996.
• The EPA had proposed an arsenic standard for drinking water of 5 μg/l
• The EPA set an MCL standard of 10 μg/L on January 22. 2001 based on dose-response models and extrapolation from a cancer study of Taiwanese population exposed to high concentrations of arsenic in its drinking water.

We went from a proposed threshold of 5 μg/L to a threshold of 10 μg/L.  New Jersey, in 2004 takes that information and ends up back at 5 μg/L. Interesting...

Which brings us to present time.  February 19, 2010 EPA places in the Federal Register a notice for public comment the "2010 draft document titled, Toxicological Review of Inorganic Arsenic: In Support of the Summary Information on the Integrated Risk Information System (IRIS)":

The purpose of this Toxicological Review is to provide scientific support and rationale for the hazard and dose-response assessment in IRIS pertaining to chronic exposure to inorganic arsenic.

In the document, the EPA states:

Quantitative risk estimates may be derived from the application of a low-dose extrapolation procedure. If derived, the oral cancer CSF (CSF) is a plausible upper bound on the estimate of risk per mg/kg-day of oral exposure.

 Heading on down to page 151, I read, in bold print, this:

In keeping with EPA policy, the combined oral CSF for women (25.7 per mg/kg-day) is appropriate for use in establishing health criteria, since, based on the available data, women appear to be the more sensitive group.

Wow, that's a significant difference in the Slope Factor New Jersey used which was 11.6 mg/kg-day.  At that Slope Factor, New Jersey determined that the drinking water concentration that results in a one-in-one-million excess lifetime risk of lung and bladder cancer for United States populations was an estimated 0.003 μg/L (or three nanograms per liter or three parts per trillion).

With a Slope Factor of 25.7 per mg/kg-day that estimated μg/L will be even lower.  So lets look at estimating what exactly that risk would be.  I'll use the IRIS calculation for that:

IRIS - Draft

With that formula in Equation 5-4, we can calculate the excess risk for one μg/L of arsenic at a Slope Factor of 25.7 per mg/kg-day.

25.7 x 0.001 x (1 / 70) = 3.67 excess cancers in 10,000 per μg/L of arsenic.

Let's put this into play.  If Consumer Reports believes that exceeding 5 ppb of arsenic is "troubling," "worrisome," "cause for concern," or "potentially harmful," then the risk they are assuming what is "safe" will present a risk of  8 excess bladder and lung cancers per 10,000 if the Slope Factor is 11.6 mg/kg-day as New Jersey used.

If the 2010 EPA IRIS Slope Factor of 25.7 mg/kg-day is used, 5 ppb would present a risk of  2 excess bladder cancers per 1,000.

You should start to see the problem we produce when we draw a line in the sand and claim "safe" on this side and "troubling," "worrisome," "cause for concern," or "potentially harmful," on the other.  It is only safe if the starting point for where "safe" begins is agreed upon.

Because Consumer Reports is using 5 ppb based on the New Jersey standard, it is claiming that "safe" means no more than 8 excess bladder and lung cancers per 10,000.  If New Jersey was wrong, and the EPA is correct, what Consumer Reports considers "safe" is now 2 excess bladder cancers per 1,000.

If you think about it, that number "5" does not really mean "safe" anyway - nor does it mean "troubling," "worrisome," "cause for concern," or "potentially harmful."  It is a theoretical number based on a model that spits out a value we say defines the risk.  The question becomes is how close to reality is that theoretical number?

To use "5" - as Consumer Reports does - is to say definitively that 5 is "safe."  "5," however, is just a threshold established by the EPA and New Jersey based on a theoretical calculation of a cancers potency (the Slope Factor).  That calculation, along with our technological ability to remove arsenic down to that level, is what sets the level which, for New Jersey, becomes known as "most protective."

If arsenic is a carcinogen, then less is better.  Period.  But with that in mind, exceeding a threshold based on a theoretical calculation does not make the product "troubling," "worrisome," "cause for concern," or "potentially harmful."

What does it make it then?  Good question.  Thanks for painting me into a corner.

All I can tell you is that exceeding 5 ppb does not make it "troubling," "worrisome," "cause for concern," or "potentially harmful."  Does that clear things up?  Now you see my problem with reports such as these.  They elude to something dangerous, when all that happened is that in some samples the threshold was exceeded.

That number detected - that concentration - would need to be significantly higher than 5 ppb before it got anywhere close to being "troubling," "worrisome," "cause for concern," or "potentially harmful"  So what would be considered significantly higher to warrant those concerns?  Good question, at least, in my opinion, rice that result in an 70 kg adult uptake of 56 μg of inorganic arsenic per day.  That would make me concerned (0.8 x 70 = 56 IRIS).  That would be at least six (6) servings of the highest concentration (9.4 μg/L) of inorganic arsenic Consumer Reports found in the rice.

But let's say the real hazard for arsenic is cancer, and that 10 ppb is not protective enough based on the Slope Factor's calculated.  In this case, we will need to look deeper into at how that Slope Factor was determined.  Is 5 ppb a good threshold?  Too restrictive?  Not protective enough?  We'll need to look at how they came up with it to make that call.


Next post: Arsenic in Rice:  Part 8 - How Low Can You Go?


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