Air Quality in the Barnett Shale - Part 9: Dose and Dogs

So now back to TCEQ and their statement:

“In response to these concerns, the TCEQ has committed a tremendous amount of time and resources to the issue of Barnett Shale air quality, and we will continue to do so.  After several months of operation, state-of-the-art, 24-hour air monitors in the Barnett Shale area are showing no levels of concern for any chemicals. This reinforces our conclusion that there are no immediate health concerns from air quality in the area, and that when they are properly managed and maintained, oil and gas operations do not cause harmful excess air emissions.”

How can this statement be made when contradictory statements from Wolf Eagle Environmental and Wilma Subra contend:

• Laboratory results confirmed the presence of multiple Recognized and Suspected Human Carcinogens in fugitive air emissions present on several locations tested in the Town of DISH. (1)
• Xylenes (m & p), a neorotoxin, was present in the highest concentration of all stations sampled and exceeded the Long-term ESL by 2 times. (2)
• Benzene, a know human cancer causing agent, was present in the highest concentration of all stations sampled. (2)

It all comes back to the dose.

Lets say that you wanted to know the possible health effects of human contact with different types of mammals.  The first thing you would do is group them.

1. Cats
2. Dogs
3. Hamsters
4. Pigs
5. Goats

Then you would look at each one to see what health effects occur.  For the sake of simpleness, lets pretend the only health hazards we are concerned with are bites, bone breaks, rabies, and death.

Now through research - some good and some poor - we find that not all bites are the same, so we break them down into small bites, medium bites, and large bites.

So our concern with human exposure to mammals has to do with the following health effects:

1. Small bites
2. Medium bites
3. Large bites
4. Bone breaks
5. Rabies
6. Death

The next thing one would do is look to see what type of exposure to one of the five mammal groups causes any of these health effects.  Through research we find that these health effects are dependent on the size of mammal (dose) and the amount of time in contact with the mammal (duration).

As a rule, we find that the bigger the mammal the larger the bite; the more time spent with the mammal the higher the chance of rabies; and in cases where a large number of mammals is encountered at one time, death may occur.

We also find that in most cases nothing happens.  Also, some large mammals can just nip and some small mammals can bite, although the size of the mammal does impact the size of the bite.  Additionally, any one single contact may have the ability to transmit rabies.

So looking at dogs, for example:

Now we need to determine what a safe level of dog exposure is.  For other than rabies, we look to see if there is a non-linear dose-response relationship, which means it has an effects threshold - an amount of dog for which human exposure is not expected to result in any adverse health effects.

We also find that within a human population, some people are much more prone to have negative health effects when exposed to dogs.  So when determining this safe level, it will need to be based on the most sensitive population (children, elderly) and adjusted for uncertainty and variability.

In toxicological assessments, this safe value is usually based on a determination of no-observed-adverse-effect-level - or - NOAEL.  The NOAEL is than divided by an uncertainty factor.  

For example, benzene - which is not a dog but a chemical - has a BMDL (which is similar in scope to the NOAEL) of 1.2 mg/kg/day is divided by an uncertainty factor of 300 for a reference dose (RfD) of 0.004 mg/kg/day.  The reference dose is the safe level one can be exposed to without any adverse health effects.

This uncertainty makes the safe level of benzene 300 times lower than the amount determined to be at the no-observed-adverse-effect-level (NOAEL).  In other words, at 1.2 mg/kg/day we don't expect anything, so at 0.004 mg/kg/day we are pretty dang sure nothing will happen.  Aint science fun!

It is because of this uncertainty that TCEQ can say:

"If predicted airborne levels of a constituent do not exceed the screening level, adverse health or welfare effects are not expected.  If predicted ambient levels of constituents in air exceed the screening levels, it does not necessarily indicate a problem but rather triggers a review in more depth."

So what would be the safe level of dog?  One that would show consistently a no-observed-adverse-effect-level.   That would probably be a puppy.

And because of uncertainty, not just any puppy, but a really young puppy:

So that's basically how dose and this "safe level" of ESL, RfD, and AMCV works.  Exposure to any of the dogs pictured - as well as encountering a pack of dogs - does not necessarily mean you will get bit or harmed.  And the chance of rabies?  Slim.  Yet these health effects - including rabies - are possible any time you are exposed to a dog.

Next post: Air Quality in the Barnett Shale - Part 10:  How not to describe Toluene

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