It is only appropriate to compare these one-hour values from the permanent air monitors to the short-term comparison value (AMCV). As outlined by the TCEQ, “long-term values should only be used in conjunction with monitoring data averaged over a full year or more.” Until the monitors collect one year’s worth of data in April 2011, and an annual average can be calculated, short-term values are the appropriate comparison values.

The Texas Commission on Environmental Quality (TCEQ) has developed Air Monitoring Comparison Values (AMCVs) to evaluate air monitoring data such as those being collected at the DISH and other automated gas chromatograph (auto-GC) monitors throughout the State of Texas. “AMCV” is a new term, introduced to designate health-based concentrations developed for comparison with monitoring data.  

Chemical effects depend on both the amount and duration of exposure. (Good sources of general information about toxicology are websites of the Society of Toxicology (SOT) and National Library of Medicine Environmental Health & Toxicology Specialized Information Services (NLM)). 

Because effects caused by short-term (acute) exposures are typically quite different from those that may occur after long-term (chronic) exposures, TCEQ has developed two types of AMCVs, short-term (or “acute”) AMCVs and long-term (or “chronic”) AMCVs, specifically designed for comparison with short-term and long-term monitoring data, respectively. 

Acute AMCVs are intended to protect against the adverse effects of short-term exposures – defined as exposures occurring over a period of hours to a few days. Examples of acute effects are dizziness and direct tissue damage. Per TCEQ’s Guidelines, acute AMCVs are intended to be compared to short-term sampling results such as the hourly auto-GC data.

Chronic AMCVs are intended to protect against the adverse effects of long-term exposures – continuous or repeated over a long period of time. Chronic effects include various kinds of systemic toxicity such as liver damage and cancer. For carcinogenic chemicals, long-term AMCVs represent an exposure level that corresponds to a theoretical increased cancer risk of 1 in 100,000 (0.00001). (Data from the American Cancer Society) Per TCEQ’s Guidelines, acute AMCVs are intended to be compared to long-term sampling results such as annual average concentrations (calculated by averaging all measured concentrations together).

Both types of AMCVs are defined as concentrations of chemicals in air at or below which no adverse health effects are anticipated for the specified exposure duration (short or long). The defined risk management objective for both types of AMCV is no significant risk of adverse health effects, even for sensitive subgroups (which may include children, the elderly, pregnant women, and people with preexisting conditions). They are calculated using the latest scientific data and methods, described in TCEQ’s guidelines.

Both types of AMCVs are based on the most sensitive adverse health effect relevant for humans reported in the medical and toxicological literature, adjusted to protect the most sensitive individuals in the population by the inclusion of uncertainty/variability factors. Since substantial margins of safety are incorporated, exceeding either type of AMCV does not automatically indicate an adverse health impact. Rather, it would trigger a more in-depth review. 

The short-term AMCV for benzene is 180 parts per billion (ppb), or 0.000018% -- the equivalent of around one minute in 10.6 years. The difference between the TCEQ’s short-term Ambient Monitoring Comparison Value (AMCV) for benzene (180 ppb) and the highest observed one-hour benzene level detected so far at the new air monitoring station in DISH (0.85 ppb) is 212.  The long-term AMCV for benzene is 1.4 ppb, or  0.00000014% – the equivalent of around one minute in around 136 centuries. As of June 21, 2010, the annual average benzene concentration measured at the DISH auto-GC was 0.12 ppb, 11.7 times lower than the AMCV.