Wednesday, January 2, 2013

The Village of DePue: Theoretical Cancer Risk at Background - Part 10

The theoretical cancer slope factor (CSF) that has been set for Arsenic is 1.5 (mg/kg)/day.

So...theoretically:
• The soil screen level that will achieve a risk of one additional cancer in one million for arsenic is 0.39.
• A person exposed for 350 days a year, for 70 years, to 0.39 mg/kg of arsenic in soil would have a 33.3334 percent chance of getting cancer in their lifetime
• A person not exposed to 0,39 mg/kg of arsenic in soil will have a 33.3333 percent chance of developing cancer in their lifetime.
• The difference is 0.0001 percent or a probability of 0.000001 (1 in a million)
This is how we look at chemicals suspected to cause cancer.  There is no "safe" dose, so we calculate the risk based on a chronic exposure to a set amount.

We feel that one in one million is an acceptable risk and the amount of arsenic in soil that would be expected to produce the chronic daily intake (CDI) necessary to show a one in one million risk is 0.39 mg of arsenic per kg of soil.

All of this is theoretical.  But for purposes of setting a cleanup standard, it is the best we have so we will make the assumption that 0.39 mg/kg is the "safe" limit.

That, as we discussed in a previous post, is a value that is unable to be obtained in nature.  That is, there is a natural amount of arsenic that we are exposed to.  We call that "background" and it is set based on what we "normally" see in places where we live.

Illinois has set the background for arsenic in soil at 11.3 in 742.Appendix A, Table G.

11.3 is quite a bit higher than the theoretical "safe" value we want which is 0.39 mg/kg.

There is nothing we can do about background, so we ignore the theoretical what we want, and accept the background as "safe."

So...11.3 mg/kg is "safe."

In OU-4 we have 102 samples that exceed the background concentration that Illinois has said is acceptable.

What do those exceedances  mean in terms of additional risk?

Let's look at this from EPA's Integrated Risk Information System (IRIS) perspective: IRIS
IRIS shows that the concentration of 0.02 μg of arsenic per liter of drinking water is the "safe" amount that will increase the risk of cancer from 33.3333 to 33.3334 percent.

If you increased the concentration 100 fold, from 0.02 μg to 2.0 μg per liter of water, the risk would move from 33.3333 to 33.3433 percent.

100 times more arsenic consumed increases the risk from 33.3334 percent to 33.34 percent.

Okay, so what does this mean for the people who live in DePue?  It depends on how one wants to look at it.

First off, the slope factor used to calculate the risk is very conservative which means it most likely over-estimates the risk.  Second, the amount of arsenic that enters into the body from soil fluctuates wildly.  Our calculations of how much are based on a consistent amount of arsenic entering into the body 350 days a year for 70 years.  Third, we are looking at excess cancer above and beyond the one in three chance of getting cancer in a 70 year lifetime.

But let's ignore all of that and focus solely on establishing a threshold where on one side it is "safe" and on the other side there is "risk."

The Illinois background level for arsenic has been set at 11.3 mg/kg.  This level is a cleanup objective that Illinois assumes presents a "safe" environment if the arsenic is at or below that amount.

For all intents and purposes, we will assume 11.3 or less to present no excess cancers above the norm of a 33.3333 percent chance of getting cancer in a 70 year lifetime.

That assumption means, theoretically, that exceeding the value of 11.3 presents an additional risk.  So...how much are we talking about?

The average exceedance of the background threshold of 11.3 mg/kg was 1.7 times the background for an average amount of arsenic in the soil in OU-4 (where the people live) of  19.6 mg/kg.

19.6 mg/kg exceeds the threshold of 11.3 mg/kg by 8.3 mg of arsenic.  So...8.3 mg of additional arsenic exposure is what we will estimate the additional cancer risk on.

Here is where it gets fun.

The EPA's soil screening level (SSL) for arsenic is 0.39 mg/kg.  0.39 mg of arsenic in the soil is assumed to present a risk of no more than one additional cancer in one million cancers for a person exposed 350 days a year for 70 years.

EPA's IRIS has calculated the chronic daily intake (CDI) for arsenic to be 0.02 μg of arsenic per liter of drinking water for a risk of one additional cancer in one million cancers  .

Since the average person is assumed to consume 2 liters of water per day, the amount of arsenic that will theoretically bring about a risk of one additional cancer in one million is 0.04 μg per day for 70 years.

So...we can assume that exposure to 0.39 mg/kg of arsenic in soil, for 350 days a year, for 70 years, will equate to a CDI of 0.04 μg of arsenic.

0.04 μg of arsenic.consumed each day for 70 years will theoretically result in no more than one additional cancer in one million.

0.39 mg/kg exposure = 0.04 μg of arsenic consumed.  That's our line in the sand.

On average, in OU-4, the soil exceeds background by 8.3 mg of arsenic per kg of soil.

Okay...take a break and relax.  Here is a picture of a kitten and a puppy to help...before we do more math and take on more assumptions. Source
Feeling better?  Okay, let's continue.

8.3 mg is 21 times higher than 0.39.  Therefore, the CDI at 8.3 mg of arsenic exposure would - theoretically - be 21 x 0.04 = 0.84 μg of arsenic consumed.  That's based on the SSL calculations the EPA used.

So what is the risk of consuming 0.84 μg of arsenic per day for 70 years?

In my previous post I showed this calculation:

• The NSRL is the uptake - 0.84 μg which is equal to 0.00084 mg
• The the slope factor (qhuman) is 1.5 (mg/kg-day)-1
• The body weight we use is 70 kg.
Basic algebra here with the math.  We need to solve for the risk "R"

(0.00084 x 1.5) / 70 = 0.00018 or 1.8 excess cancers in 10,000.

Here is what the TACO's fact sheet says about that:
The risk of cancer due to exposure to a contaminant is commonly expressed in exponential terms, e.g., 10-6 and 10-4. These terms equate to a risk of 1 in 1,000,000 and 1 in 10,000 respectively. Adding a 10-6 risk would increase the probability of an individual getting cancer to 0.333334. With the addition of a 10-4 risk, the probability of an individual getting cancer would be 0.333433.
On average, in OU-4, the probability of an individual getting cancer would be 0.333453.

What that means is one's normal probability of cancer in a life-time of 0.333333 becomes 0.333453 if exposed to that soil for 350 days a year for 70 years.  This is based on contact with the soil so that the soil enters into the body.

Although it is possible, it is highly improbable that any one in DePue would have an uptake of soil over a lifetime that would equate to a risk of 1.8 excess cancers in 10,000.

Based on how conservative these formulas are, and how theoretical the slope factor is, I do not see any reason to be concerned about the amount of arsenic in the soil in OU-4.

But that raises and ethical question.

Should the citizens of DePue be exposed to any risk above one in one million?

Next Post: The Village of DePue:  The ethics of drawing a line - Part 11

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