Indoor Air Background Subpopulations and Seasonality at the CDOT MTL Site, Denver, Colorado

Stephen J. Foster, Swan-Foster Consulting, P.O. Box 19572, Boulder, CO, 80308, Tel: 303-530-3499, Email: FosterESci@aol.com
Jeffrey P. Kurtz, EMSI, P.O. Box 1572, Longmont, CO 80502, Tel: 303-485-8468, Email: emsi@rmi.net
Anne Woodland, Waterstone Environmental, 1650 38^th Street, Suite 201E, Boulder, CO 80301, Tel: 303-444-1000, Email: Woodland@WaterstoneInc.com

The CDOT MTL Site in Denver, Colorado has been the subject of extensive indoor air sampling for over eight years.  Beginning in October 1998 24-hour SUMMA canister samples were analyzed using TO-15 SIM, with results reported down to the method detection limit (MDL).  While several publications have documented background indoor air concentrations of chlorinated chemicals in residences at this site (Indoor Air 2002, MSRAS 2002, 16^th Int. Conf. Soils, Sediments and Water 2000, etc.), additional results on background data seasonal variability and sub-populations are now available from the several thousand indoor air samples collected on a quarterly basis.  A detailed examination of the variability of background and its relationship to residence specific factors are presented.

Benzene, chloroform, methylene chloride (DCM), tetrachloroethylene (PCE), bromodichloromethane (BrDCM) and 1,2-dichloroethane were routinely measured in all indoor air samples in addition to the groundwater-derived chemicals of interest. Several distinctive results have been found in the background indoor air data from this site. There is a very strong correlation of BrDCM, with chloroform.  This suggests that, similar to chloroform, BrDCM is derived from chlorinated tap water.

There is a highly significant difference in the background indoor air concentration of benzene between single family homes with and without attached garages.  Homes with attached garages have concentrations averaging two times greater (5.5 ug/m3) than those without (2.8 ug/m3).  The other gasoline related hydrocarbons show similar behavior.

There is a notable seasonality to background indoor concentrations of chloroform and methylene chloride.  Chloroform concentrations tend to peak in summer and reach minimums in winter.  DCM concentrations generally peak in summer and fall.   

Context-Based Benchmarks for Indoor Air Evaluation

Stephen G. Zemba, Ph.D., P.E., Cambridge Environmental Inc., 58 Charles Street, Cambridge, MA, 02141, Tel: 617-225-0810, Email: Zemba@CambridgeEnvironmental.com

Evaluation of potential health risks to indoor air contaminants typically entails two risk management issues.  First, assuming that the focus of evaluation rests on a specific source (such as vapor intrusion from subsurface soil or groundwater), it is often desirable to compare source-based contributions against background indoor air quality (i.e., concentrations of the same chemical(s) that would be present in the absence of the contaminant source).  Second, toxicity-based benchmarks must be selected to evaluate potential hazards to health. 

Selecting appropriate benchmarks for comparison relative to both of these objectives involves multi-faceted considerations.  Regarding the selection of a background concentration benchmark:

Should building uses and types be differentiated (e.g., residential v. industrial/commercial)?
Should indoor air sources such as environmental tobacco smoke be considered in establishing background air quality?
Should source-related impacts even be evaluated relative to background (e.g., should modeled values smaller than background be viewed as acceptable)?
In the absence of measurable background levels, should analytical detection limits be used as surrogate measures of background?

Regarding toxicity-based benchmarks:

Should values be selected from traditional risk assessment databases such as the Integrated Risk Information System (IRIS), or should applicable occupational or workplace standards be applied in some settings?
How should significant uncertainties in toxicological values be handled?
Should risk-based limits be established in conjunction with risks associated with background air quality?

Lacking definitive federal guidance or professional consensus, the answers to these questions are debatable.  In some cases, state regulatory programs provide relevant constraints and policies that effectively guide the evaluation process.  Insufficient and uncertain data also make it difficult to answer some of the relevant questions.  However, decisions must be made as indoor air risks become increasingly important in the evaluation of sites with soil and groundwater contaminated with volatile chemicals. This paper will provide suggestions for developing a framework for addressing indoor air risks that selects benchmarks appropriate to the context of the exposure setting.  Examples that reflect current issues of interest will be drawn from project experience.

Top