Reference Dose: The foundation of Risk

I am getting ready to prepare my presentation "Apples, Arsenic, and Risk" for the upcoming 2012 AHMP conference in Alaska September 9 -13 in Anchorage. I wrote a lot on this topic in previous posts and need to pare it down (pun intended).

One of the most difficult parts of preparing a presentation is finding the starting point from which to tell your story. I can't just stand up there and whine about Dr. Oz and Chuck Norris. Nor can I present a bunch of facts and figures to prove my case. Boring. I get one shot at this, and I need to make it work.

So, where to start and what to build is where I am in July. I know the story's end, I know the moral, how do I tell it for Environmental Health Safety Professionals who depend on the very information I am going to present but know little about how it works without being condescending, preachy, or uninteresting.

Sure they understand risk, and regulatory compliance concentrations, and clean up levels, but do they understand why other than that is what they are told - or - what some formula spits out?  Why do we set the limit at 10µg/L for arsenic in our drinking water, 5 mg/L for the level of arsenic in our waste, 0.018 µg/L for ambient water quality criteria, 0.3 mg/L for arsenic in storm water, and 23 µg/L for arsenic in our fruit juice?

We are dealing with the same chemical, yet depending on where we find it, the values assigning the risk change.  As well it should. It is deemed "safe" if it is below the threshold depending on where we find it and what receptor (animal/plant/human) will be exposed to the arsenic, how much (dose), and length of time exposed (acute/chronic).

We draw a line in the sand and say "safe" or "presents a risk."  That line in the sand is drawn from a formula that looks at the length of time exposed as well as how the chemical will enter into the receptor. Since I am interested in protecting human health, the receptor will be based on that.

That formula - for humans - is based on the concept that there is a daily level at which we do not expect harm to take place over a lifetime.  This is what we call the "reference dose" or "RfD."

RfD: An EPA estimate, with uncertainty or safety factors built in, of the daily lifetime dose of a substance that is unlikely to cause harm in humans.

The foundation for risk for all chemicals that are suspected to cause harm to human health are based on this concept.  You must accept that concept - that there is an amount that we can consume every day for a lifetime - of a risk that is unlikely to cause harm in humans.

I've discussed this concept a number of times in previous posts.  This is how we do it in the United States. We set thresholds based on a formula of what we think might take place using a value that we have determined is a "safe" level if there is uptake into the receptor.

When the FDA says that 23 µg/L for arsenic is the maximum "safe" level in our fruit juice it is basing that level on the amount of juice to be consumed that would be put into the human receptor.  Therefore, if consuming juice - normally - over a lifetime, it would be “unlikely” to cause harm from arsenic.

When folks like Dr. Oz, Consumer Reports, and Chuck Norris compare the amount of arsenic found in apple juice to the amount of arsenic deemed safe for drinking water - the MCL - they are forgetting that the MCL and FDA concentration deemed "safe" are based on the same foundation; the RfD.  What is different in these two values is what is considered to be "normal" uptake. We drink more water than apple juice therefore we can consume more arsenic in the apple juice and remain safe.

Why are the two values different?

Because the RfD states that there is "daily lifetime dose of a substance that is unlikely to cause harm in humans."  Consume less than the RfD and there will "unlikely" be harm.  It is all about the RfD and the dose that delivers that RfD, and the length of time you will be exposed to that dose.

Chemical Exposure is similar to wealth, only the inverse.  If it takes $2.00 to buy a cup of coffee and you have no money or $1.99, you are in the same predicament - you will have no coffee.  Same goes with chemicals.  If the RfD concept is to be our foundation, then we have to move away from "less is better" and move toward a threshold based on uptake into the receptor.  Below that concentration in the matrix we are exposed to, we are fine.

Which brings us to another question.  If the foundation is the RfD, then how confident are we in the RfDs we have established?


Next Post: RfD and Consensus


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