Air Quality in the Barnett Shale - Part 4: Exceeding TCEQs Short-term ESLs

So what do we know so far:

1. Wolf Eagle Environmental and Wilma Subra used sample results collected from one 24 hour sampling event to compare against Long-term ESLs that are used to evaluate modeled 1-year concentrations.
2. For the seven sample locations, there were a total of 13 times where a chemical detected exceeded the TCEQ's Short-term ESL.
3. TCEQ states that exceeding the ESL "does not necessarily indicate a problem but rather triggers a review in more depth."
4. TCEQ states that “[a]fter 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[.]"

Lets assume that the values obtained from this single 24-hour sampling event represent the amount the citizens of DISH are exposed to, day in, day out.  Because this was a sample collected over a 24 hour period, the 1 hour concentration is equal to 1/24 of the concentration detected and reported.  So, if the highest concentration of benzene reported by Wolf Eagle Environmental was 77.7 ppb (Airfield @1:29) the one hour concentration would be 77.7 / 24 =  3.24 ppb.


Note 2/14/11: I looked at it as an average concentration,  The way TCEQ looked at it, was that there was a single one hour level which was diluted with 23 hours of "clean air."  In re-looking at this, because we are comparing a sample concentration to a one hour ESL (the amount that would be the highest concentration in an hour whereby no adverse health effects would be found) the TCEQ assumption fits better than my averaging assumption.

The reason we divide by 24 is because the sample canister was left on for 24 hours. The Short-term ESLs used by the TCEQ are based on the concentration that would be present in a one hour block of time.  Because we use ppb - or parts of contaminant per billion parts of air - to make comparisons, leaving the canister open for more than one hour would take in more parts of contaminant but the total volume of air would remain the same.  

Canister Sampling is  method of taking a sample of air that uses a stainless steel canister that has been scrupulously cleaned and is under a vacuum. The operator opens the valve on the canister, allowing the air sample to rush into the canister to fill the vacuum. When the canister is opened all at once allowing the air to be sampled over the span of a few seconds, [TCEQ] terms this an instantaneous sample.  A controller can be put on the canister sampler controlling (slowing) the flow of air into the canister over a prescribed time, i.e. 30 minutes, 1 hour, or 24 hours. The samples in the canister are then analyzed on another instrument, typically a gas chromatograph (GC).

TCEQ also recognized this flaw in Wolf Eagle Environmental's reporting but gave them the added benefit of conservatively assuming "that the chemical had a maximum concentration for one hour and was not detected for the remaining 23 hours."  In their memo dated October 29, 2009, TCEQ, in order to compare Wolf Eagle Environmental's 24-hour concentrations with a 1 hour value, multiplied them by 24.  In this manner, the TCEQ compared reported concentrations of the ambient air at the Town of DISH that were 24 times higher than actually detected!  They did, however, note: "This may or may not represent actual conditions."

So now we can add one more to the list of what we know so far

5. Sampling and analysis reported 24 hour concentrations which were compared to ESL values that represent the concentration collected in a one hour period of time.

 Ready for another one?  Yes, there is a number 6.  One more error in their reporting of certain chemicals exceeding TCEQ's ESLs.  The ESLs they used to compare with in their "Revised Air Study Documents" were the wrong type of values to use.


According to the TCEQs AMCV document called "Uses of Effects Screening Levels (ESLs) and Air Monitoring Comparison Values (AMCVs) Revised (May 2010)" Although the same Short-term and Long-term were used for both air permitting and air monitoring, air concentrations of chemicals collected in air monitoring samples represent emissions from multiple chemicals and from different facilities and sources.  For review of air monitoring data, the health-based ReV is appropriate.

ReVs [reference values] are designed to protect the most sensitive individuals in a population by inclusion of uncertainty/variability factors (UFs). UFs account for differences between study animal and human species, variability within the human species, and uncertainties related to the applicability and completeness of the available data. Since UFs are incorporated to address data gaps, variability, and other uncertainties, exceeding the ReV does not automatically indicate that an adverse health effect would occur.

For air permit applications, site-wide modeled concentrations for one chemical at a time are evaluated.  Therefore, for air permitting, an additional buffer is applied to the acute or chronic ReV to calculate the acute and chronic ESLs. The final acute and chronic ESLs developed using the Guidelines are 70% lower than the respective acute and chronic ReV.
  

Because there are "significant differences between performing health effect reviews of air permits and the various forms of ambient air monitoring data," the [TCEQ] has begun using the term “air monitoring comparison values” (AMCVs) in evaluations of air monitoring data.  

AMCVs are chemical-specific air concentrations set to protect human health and welfare. Exposure to an air concentration at or below the AMCVs is not likely to cause adverse health effects in the general public, including sensitive subgroups such as children, the elderly, pregnant women, and people with preexisting health conditions. AMCVs are a collective term that refer to all values used by [TCEQ] staff to review ambient air monitoring data.

The use of different values and different terminology is appropriate because the air monitoring and air permitting programs perform different functions in the protection of human health and welfare."

• The short-term AMCV, based on acute exposure health and welfare data, is compared to monitored concentrations that can be instantaneous or up to 1-hour, which represent a point in time for a specific location.
• The short-term ESL, based on acute exposure health and welfare data, is compared to the modeled 1-hour Maximum Ground Level Concentration (GLCmax).
• The long-term AMCV, based on chronic health and welfare data, is used to evaluate annual averaged monitored concentrations or annual concentrations averaged over multiple years (if available), which represent multiple points in time for a specific locations.
• The long-term ESL, based on chronic or lifetime exposure health and welfare data, is compared to the worst-case annual GLCmax.

So basically, the work performed by Wolf Eagle Environmental looked at the contaminants detected and reported them as if they were being addressed for an air permit.  Lets assume that the values obtained did represent the concentration in air that a citizen of the Town of DISH was going to be exposed to.  What then?

Getting back to benzene and xylene (m & p), how many times was there an exceedence?  And by how much?

Now remember, there are two things in play here.  1st, the ppbv results shown are for 24 hours worth of sampling and could be 24 times higher than what you would have seen if the sample was collected for just one hour.  And 2nd, the ESLs are 70% lower than the reference values.  So, because none of these values exceed the AMCV, we can safely say that exposure to an air concentration at or below the AMCV is not likely to cause an adverse health effect in the general public, including sensitive subgroups such as children, the elderly, pregnant women, and people with preexisting health conditions.

Ignoring the incorrect sampling...ignoring the incorrect comparison, and assuming that these values represent the amount a citizen of the Town of DISH is exposed to, will there be any negative health concerns from these contaminants?

Here is what the TCEQ states in their memo:

The highest potential 1-hour maximum benzene concentration [actual x 24] is below the health effects level observed in short-term animal and human studies; however, it is possible that adverse health effects could occur from exposure to this concentration. It was not possible to determine if residents were exposed to this concentration of benzene based on the information provided.

Here is what I say - and I am not going to far out on a limb: Not likely - because even if you multiplied the contaminate concentrations by 24 like the TCEQ, and even if they exceeded the ESL and the AMCV, there is a margin of safety - uncertainty - built into these comparison values.  It all comes down to how we calculate risk and the value we use in the calculation for the ESL and AMCV.  It's all about the reference value - the ReV.

Air Quality in the Barnett Shale - Part 5:  'n' is for sample size.

.