Air Quality in the Barnett Shale - Part 7: Benzene Exposure and the No Significant Risk Level.

Carcinogens are a bit trickier to deal with in terms of determining a safe level.  This is because there is no concentration other than zero that one can say will present no chance of getting cancer from exposure to it.  Instead of a threshold dose, we look at a exposure level determined to present "no significant risk."

This level is defined as the level which is calculated to result in not more than one excess case of cancer in 100,000 individuals exposed over a 70-year lifetime. In other words, if you are exposed to the chemical in question at this level every day for 70 years, theoretically it will increase your chances of getting cancer by no more than 1 case in 100,000 individuals so exposed. (5)

What it does not mean is that one person per 100,000 individuals will get cancer.  This is the same concept behind rolling a six sided die.  You expect to see a "one" show up every six rolls, but you could roll it 50 times without ever rolling a one.

Basically, the ESL and AMCV are calculated the same way:

• (chronic)ESL = 0.00001 / URF
• (chronic)AMCV = 0.00001 / URF

The URF or "Unit Risk Factor" for carcinogens do not have a threshold dose at which below that concentration no cancer would take place. What this means is that any exposure has the chance to cause cancer and that the more exposures the more chances for cancer to manifest itself result.  This makes exposure to "Benzene, a know human cancer causing agent" a scary thing to read over and over again in a report (7).  This is where context and explanation are critical components to any report involving a complex formula, a number, and the words "know human cancer causing agent."  This is also where Alisa Rich, who - need I remind you again - has a Master in Public Health, failed to convey the risk properly.

Why did Alisa Rich and Wilma Subra fail to put this information in context?  Maybe it's because they lack a basic understanding of how the "no significant risk" is calculated and how to compare the 24 hour values with this risk.  It is quite possible that we receive a much better education in our MPH program at Texas A&M's School of Rural Public Health than one receives at the University of  North Texas, but I know that is not the case here.  So instead I will assume that both of  these experts just don't fully understand the topic they sell themselves as knowledgeable in.

Here is how TCEQ explains it in their 2006 document called "Guidelines to Develop Effects Screening Levels, Reference Values, and Unit Risk Factors" in 1.5.2:

For adverse effects associated with a linear dose-response [such as carcinogens], it is assumed that an effects threshold does not exist. Therefore, a linear extrapolation from the POD [point of departure] to the origin of the inhalation dose-response curve is performed to estimate excess lifetime risk at lower doses. The slope of the line from this linear extrapolation is the inhalation URF, which is defined as the upper-bound excess risk estimated to result from continuous lifetime exposure to an agent at a concentration of 1 μg/m3 in air (i.e., risk estimate per μg/m3). 

Confusing?  Lets look at this in terms of Benzene: (8)

• EPA uses mathematical models, based on human and animal studies, to estimate the probability of a person developing cancer from breathing air containing a specified concentration of a chemical. EPA calculated a range of 2.2 x 10-6 to 7.8 x 10-6 as the increase in the lifetime risk of an individual who is continuously exposed to 1 µg/m3 of benzene in the air over their lifetime.  
• EPA estimates that, if an individual were to continuously breathe air containing benzene at an average of 0.13 to 0.45 µg/m3 over his or her entire lifetime, that person would theoretically have no more than a one-in-a-million increased chance of developing cancer as a direct result of continuously breathing air containing this chemical.
• Similarly, EPA estimates that continuously breathing air containing 1.3 to 4.5 µg/m3 would result in not greater than a one-in-a-hundred thousand increased chance of developing cancer, and air containing 13 to 45 µg/m3 would result in not greater than a one-in-ten thousand increased chance of developing cancer.

So the Benzene values obtained from a single 24 hour sampling event performed by Wolf Eagle Environmental were

Now on first glance, one might think "I'm going to get cancer because of Samples 2, 3,and 4."  But that is not the case.  There are two things necessary for one to develop cancer from breathing this particular air.  The first is you would need to breath that air - at the concentration - for 70 years.  Secondly, your body would have to have develop a cancer from the uptake of that particular contaminant.  

It's a lot like flipping a coin.  If you have a 50% chance of the coin landing on tails - when you flip the coin - either heads or tails can come up.  So if I told you you had a 1 in 2 increased chance in developing cancer, you might not want to be exposed.  Now lets change that coin for a single die with six sides.  If you exceed that level over a lifetime, you roll the die and if the number "one" shows up you get cancer.  That would be a one in six increased lifetime chance.  Now lets change out that 6-sided die for one of those funky die they use when playing Dungeons and  Dragons, only in this case, it will have 100,000 sides to it with the numbers 1 to 100,000.  Now if you continuously breath air containing 1.3 to 4.5 µg/m3 of Benzene for a lifetime, you would roll this 100,000 sided die and if the number "1" came up you would get cancer.

For samples 2, 3, and 4, the benzene level is higher than the amount deemed no significant risk.  In this case, the risk - if one were to be exposed at that level in sample 4 constantly over a 70 year lifetime-  would be between:

• 247.9 x  0.0000022 = 0.0005   and    247.9 x 0.0000078 =  0.002

Or an added - or excess - lifetime risk between 1 in 500 and 1 in 2000.  At these elevated levels, the cancer risk has increased, but that is only if the exposure remains at 247.9 ug/m3 and the body fails to repair itself and develops a cancer.  Obviously the levels of benzene detected are too high and additional investigation should be done to determine the cause and fix the problem.  But even at these levels, cancer from benzene is not inevitable.

So looking at this in context, exposure to benzene has the potential to cause cancer but unless the levels are high and sustained over a lifetime, the risk of cancer is not significant.  You may be the unlucky person that rolls the "1".  This is where evaluating risk comes into play and the concept of accepted risk and risk that is forced upon you.  One may reasonably conclude that any risk is a risk too many.  But is eliminating the potential for a one in 100,000 possibility at something negative happening worth the benefits obtained from having it?  I can't make this decision for someone who is potentially impacted by the risk coming from those benefits.  What I can do is try and put it in perspective.   Because even if you eliminated every gas well and drilling rig from the area, exposure to Benzene will still take place and cancer - from lots of other sources - will still manifest itself.

There are a lot of factors in place for citizens in and around gas production activities.  The fact that we find chemicals in the air on one particular day does not mean that exposure at that level will take place the other 364.  And even if there is exposure at that level, we still do not know if the second to final step in the K. C. Donnelly Risk Paradigm - uptake - takes place.  And even if uptake does happen, the body is very good at dealing with contaminants through a process called pharmacokinetics.  And even if we end up with a negative health effect we have very good medical procedures to treat this.

And even if the die rolls a one, we are still unable to pinpoint the culprit, for we have no background data on what levels of these contaminants were present before the gas operation began.  All of this must be taken into consideration befor you say "yes" or "no" to assuming the risk.

Now lest you think I am giving the oil and gas industry a free ride here, I am not.  I will deal with their issues at the conclusion.  What I am writing about in these posts is how Wolf Eagle Environmental and Wilma Subra have painted a picture that does not represent fully and accurately the reality the citizens of the Town of DISH Texas are facing.  That is unfair to them, unfair to the Oil & Gas industry, and unfair and damaging to my profession which is dedicated to protecting the environmental and public health.

Next post: Air Quality in the Barnett Shale - Part 8: Benzene is like a bull...

Note: 2/16/11 A bio found on the internet lead me to incorrectly assume that Alisa Rich received her MPH and was working on her Ph.D from UT.  An email I received from Barnett Shale News included a Deposition from her stating she got her MPH from UNT's Health Science Center and is working on her Ph.D at UT Arlington.
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