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Preliminary Evaluation of Soil Vapor Data



This health consultation summarizes the soil gas sampling data that the New HampshireDepartment of Environmental Services (DES) forwarded to the Department of Health andHuman Services (DHHS) to evaluate. As follow up to a chemical release event, soil gassamples were collected in the vicinity of a local high school to determine if the potentialvolatilization of chemical contaminants in groundwater was impacting that facility. Thishealth consultation has been prepared by the New Hampshire Department of Health andHuman Services, Office of Community and Public Health, Bureau of Environmental andOccupational Health (DHHS) through a cooperative agreement with the Agency ForToxic Substances and Disease Registry (ATSDR).

DES is currently investigating the release of fuel products (gasoline) from severalpossible sources near the Hanover High School (HHS). The release has resulted in thecontamination of area groundwater that has migrated to off-site locations. In September,2002, a groundwater monitoring well, located approximately 90 feet up gradient of theHHS was found to contain the chlorinated volatile organic compound (VOC), 1,2-dichloroethane (1,2-DCA) at 125 micrograms per liter or ug/L (1). This concentration of1,2-DCA is above the New Hampshire Risk Characterization Management Policy GW-2standard of 20 ug/L, indicating a potentially significant source of chemical vapors toindoor air if the contaminated groundwater plume is located within 30 ft. of an occupiedbuilding or structure and the average depth of groundwater is 15 feet or less below thebuilding's foundation floor (2). VOCs in groundwater have the potential to infiltrate, assoil vapors, through building foundations and can impact the indoor air of residences orother building types that located above the contaminated groundwater plume. Althoughthis monitoring well was located at a distance greater that what would be necessary toimplement the GW-2 standard, DES requested that additional soil vapor samples becollected in the vicinity of HHS to determine if vapors were present.

Three temporary soil vapor monitoring points were installed around the southeastern cornerof the school at distances of approximately 10 ft. from the HHS foundation. The samplingpoints were installed at a depth of 1.5 - 2.5 feet below the elevation of the building floor. On June 6, 2003, three soil vapor samples were collected.

Results of the sampling are presented below (1):

Concentrations (g/m3) of Chemicals Detected in Soil Vapor Samples Collected on June 6, 2003 at Hanover High School, Hanover, New Hampshire.

Compound VP-1 VP-2 VP-3 Maximum Detected Concentration
Methylene chloride 3.82 4.17 ND (1.74) 4.17
Benzene 1.85 ND (1.60) 2.46 2.46
Toluene 128.14 56.53 35.80 128.14
Ethylbenzene 6.95 5.21 3.95 6.95
Styrene 5.11 ND (2.13) 5.54 5.54
Total Xylenes * 46.89 32.13 19.54 46.89
1,3,5-Trimethylbenzene 10.32 4.72 4.47 10.32
1,2,4-Trimethylbenzene 58.99 23.60 23.11 58.99
Trichlorofluoromethane 6.74 4.16 3.43 6.74
4-ethyltoluene 18.68 8.85 13.27 18.68

* Total Xylenes include the o, m, and p-isomers.
ND indicates non-detect followed by the detection limit in parenthesis.

DES is continuing its remedial investigation of the site. During Fall 2003, it is planning to install permanent monitoring wells in the area to further track the potential movement of contaminants toward the school and other locations.


Exposure Pathways and Contaminants of Concern

Although local groundwater has been contaminated, exposure from either ingestion ofdrinking water or inhalation of VOCs that may volatilize from this source is not an issuesince the school is served by a public water supply (3). Inhalation of contaminated indoorair resulting from the migration of soil vapors into the school building is the potentialexposure pathway of concern for HHS. Indoor air samples from within the school have notbeen collected. Instead, soil gas data collected in the immediate vicinity of HHS has beenused as a surrogate for actual inhalation exposure. An attenuation factor was incorporatedin the indoor air pathway analysis to represent concentrations of chemical contaminants towhich occupants might actually be exposed assuming incomplete migration of VOCs in soilvapor to indoor air.

In evaluating potential exposure to soil vapor, the maximum detected concentration of each chemical detected in the soil vapor samples was used to represent typical contaminant concentrations in this medium. To determine the chemical contaminant concentrations that receptors within the building may have been exposed, DHHS considered using attenuation factors of 1% and 10% to the exposure point concentrations. DHHS has typically used a 1% attenuation factor at other sites (4). DHHS is aware of the recently published Draft Guidance For Evaluating The Vapor Intrusion To Indoor Air Pathway From Groundwater and Soils by U.S. Environmental Protection Agency (EPA), that recommends the application of a 10% attenuation factor to the detected concentrations as being appropriate to determining potential exposure point concentrations (5). As DHHS is using soil vapor data to measure potential exposure for building occupants, it chose to use the 10% attenuation factor proposed by EPA to insure that its evaluation is protective of human receptors. The attenuation factor is a ratio of the indoor concentration to the soil gas concentration – a measure of how much gets inside. An attenuation factor of 10% means that the only one tenth of the original soil gas concentration is entering the home. Another way to look at this is -- an attenuation factor of 10% means that the concentration entering the home is 90% less than the soil gas concentration. These adjusted exposure point concentrations were compared to the following comparison values: ATSDR Cancer Risk Evaluation Guides (CREG), Environmental Media Evaluation Guides (EMEG) and EPA Reference Doses (RfD) for inhalation exposure.

It is important to note that comparison values (CV) are not used to "prove" the existenceof a health hazard, but serve instead as a screening tool in the initial evaluation ofenvironmental data for an exposure pathway of interest. CVs are concentrations ofcontaminants that are considered to be levels at which adverse health effects are notexpected to occur. CVs are derived using conservative (typically "worst case") exposureassumptions, resulting in numerical values that are much lower than the actual exposureconcentrations demonstrated to cause adverse health effects. Comparison values areprotective of public health in most exposure situations, so that, if the concentrations in anexposure medium are less than the CV, the exposures are not considered to be of publichealth concern and further analysis of the chemical for that pathway is not needed. Itshould also be noted, however, that while concentrations below the comparison value arenot expected to result in an adverse health effect, a concentration of a chemical above aCV doesn't automatically indicate that a hazard exists. It means there is a need to furtherexamine the relevant exposure scenario. Depending on site-specific factors (e.g., duration and frequency of exposure) and other factors, such as types of activities, etc.,exposure to levels above the comparison value may not necessarily lead to an adversehealth outcome. Therefore, ATSDR's comparison values and EPA's Reference Doses arenot used to predict the occurrence of adverse health effects; rather they represent amechanism to determine whether further evaluation of the exposure pathway iswarranted.

As previously indicated, DHHS initially adjusted all of the maximum detected soil vaporconcentrations by the 10% attenuation factor to represent estimated exposure pointconcentrations (EPC) for each contaminant in indoor air. As an example, the maximumdetected soil vapor concentration of toluene is 128.14 ug/m3. The EPC for thiscontaminant, therefore, would be 12.8 ug/m3. This represents the theoreticalconcentration to which a receptor would be exposed were this contaminant to be presentin indoor air as the result of soil vapor migrating to the building. DHHS then comparedthese EPCs with their appropriate CVs.

Of the contaminants present in soil vapor, the EPC for benzene (0.2 ug/m3) slightlyexceeded the ATSDR Cancer Risk Evaluation Guide (CREG) for this chemical of 0.1ug/m3 (6). The EPC for methylene chloride (0.4 ug/m3) is below its CREG of 3 ug/m3and the Environmental Media Evaluation Guide (EMEG) for chronic exposure of 300 ppb(approximately 610 ug/m3) (6). Adverse health effects are not expected to occurfollowing exposure to methylene chloride. Likewise, the EPCs for toluene, ethylbenzene,styrene, and total xylenes are below their respective ATSDR comparison values. Adversehealth effects are unlikely to occur following exposure to these contaminants. There areno ATSDR comparison values available for the remaining contaminants.

Public Health Implications of Exposure

To evaluate the remaining contaminants, DHHS developed a conservative exposurescenario for both students and workers of HHS using information provided by the schooladministration (3). In brief, DHHS assumed that a student is exposed for 9 hours per day(8:00 A.M.-3:00 P.M. plus 2 hours for after school activities), 185 days per year for aduration lasting 5 years. For the worker scenario, DHHS assumed that a worker at theschool spends 9 hours per day, 5 days per week (250 days per year) for a duration lasting25 years. As there are no value currently available on the Integrated Risk InformationSystem (IRIS), DHHS evaluated exposure to trichlorofluoromethane using the draftinhalation reference dose (RfD) currently used in EPA Region III (0.2 milligrams perkilogram per day or mg/kg/day) (7). DHHS then evaluated 1,3,5-trimethylbenzene and1,2,4-trimethylbenzene using the provisional value of 0.0017 mg/kg/day proposed by theNational Center for Environmental Assessment (NCEA) for these compounds (8). DHHSwas not able to locate a dose-response value for the compound 4-ethyltoluene from eitherIRIS or NCEA. Considering that this compound is in the same chemical family as 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene, DHHS used the dose-response value forthese two compounds as a surrogate value for 4-ethyltoluene. The estimated contaminantconcentrations of these substances that would be present in indoor air are very low.Estimated doses of these contaminants were well below their respective reference doses. Potential exposure by a receptor (either student or worker) to the estimated EPCs is notlikely to result in adverse health effects.

Similarly, DHHS evaluated the carcinogenic health risk for students and workers utilizingthese scenarios. Again, the estimated levels of contaminants present in indoor air to whichbuilding occupants would be exposed as the result of soil vapor seeping into the building arelow. The theoretical risk of developing cancer for both students and workers, as the resultof these exposures, is very low.

The level of benzene slightly exceeded its CREG. Benzene is widely distributed in theenvironment. Benzene is released from manmade and natural sources. Manmade sourcesinclude motor vehicle exhaust and refueling, cigarette smoke, and industrial emissions. Themost significant source for benzene released to the environment is the combustion ofgasoline in automobiles (9).

The level of benzene in indoor air to which building occupants would theoretically beexposed is 0.2 ug/m3 (equivalent to 0.6 ppb). Although this level slightly exceeds the CREG(0.1 g/m3), it is significantly lower than ATSDR's MRLs for acute and intermediateexposure, 50 ppb and 4 ppb, respectively (6). Non-cancer health effects are not associatedwith exposure to benzene at this concentration.

Some studies in humans indicate that chronic exposure to low levels of benzene may beassociated with leukemia (10). These studies are not conclusive and do not account for otherfactors (exposure to other carcinogenic substances, etc.). A number of these studies indicatethat cancer effects are the result of exposures to concentrations of benzene in excess of10,000 ppb (9). The concentration of benzene in indoor air associated with the potentialmigration of soil vapor is approximately four orders of magnitude lower than the levelstypically associated with leukemia. Cancer effects as a result of chronic exposure areunlikely.


It should be recognized that the unique vulnerabilities of infants and children demand specialemphasis in communities faced with contamination of their water, soil, air, or food. Childrenare at greater risk than adults from certain kinds of exposures to toxic chemical substancesreleased to the environment. They are more likely to be exposed because children spend asignificant amount of their time playing outdoors. They are generally of shorter stature thanadults, which means they breathe dust, soil and heavy vapors close to the ground. Childrenare also smaller, resulting in higher doses of chemical exposure per body weight. Thedeveloping body systems of children can sustain permanent damage if exposures to certaintoxic substances occur during critical growth stages. Most importantly, children dependcompletely on adults for risk identification and management decisions, housing and accessto medical care. With respect to HHS, older teens in the age range of 14-18 yr. representthe most numerous group of individuals who are likely to be exposed to contaminants thatmay migrate to the building. The potential health risk for students was evaluated bycomparing exposure point concentrations and estimated doses with their respectivecomparison values and reference doses, respectively. Students are not at risk from exposure to contaminants that are estimated to be present in indoor air from soil vapor migration.


DES sampled soil vapor near the foundation of the HHS to determine if the occupants ofthe school are at potential risk from contaminated groundwater. Based on a conservativeestimate of indoor air concentrations emanating from soil vapor, the concentrations ofcontaminants found in soil vapor samples at the time of sampling are below levels thatwould constitute a significant health risk for building occupants if they were actuallyinhaling these contaminants in indoor air. According to the ATSDR hazard classificationsystem, potential exposure to these levels of contaminants constitute no apparent publichealth hazard.


Collect additional soil vapor samples to determine if contaminant concentrations areincreasing over time.


The purpose of the Public Health Action Plan is to ensure that this health consultation notonly identifies any current or potential exposure pathways or related health hazards, butalso provides a plan of action to mitigate and prevent adverse human health effectsresulting from exposures to hazardous substances in the environment. The first section ofthe Public Health Action Plan contains a description of completed, ongoing, or plannedactions to mitigate exposures to environmental contamination. In the second section,there is a list of additional public health actions that DHHS recommends be implementedin the future.

Completed Actions

  1. Temporary soil vapor monitoring points have been installed in the vicinity of HHS.

  2. DES has collected soil vapor samples to determine if contaminants volatilizing fromgroundwater pose a threat to building occupants.

  3. DHHS has evaluated soil vapor samples to determine if building occupants arepotentially at risk from contaminants that may enter indoor air from soil vapormigration.

Ongoing or Planned Actions

  1. DES will install permanent soil vapor monitoring points in the vicinity of the school.

  2. DES will continue to monitor soil vapor concentrations that may present a risk to theschool.

  3. DES may conduct sampling within HHS if there are indications that contaminantspose a threat to building occupants.

  4. DHHS will continue to evaluate environmental sampling data to determine ifbuilding occupants are a health risk.


Dennis A. Pinski
Supervisor, Risk Assessment Section
Bureau of Environmental and Occupational Health
New Hampshire Department of Health and Human Services
Concord, New Hampshire
(603) 271-6909
(603) 271-3991 (fax)


  1. GeoInsight, Inc. Report on Chlorinated Volatile Organic Compound Soil VaporTesting for the Hanover High School, Hanover, NH. June 18, 2003.

  2. New Hampshire Department of Environmental Services. Risk CharacterizationManagement Policy (RCMP). 1998.

  3. New Hampshire Department of Health and Human Services. PersonalCommunication with Ms. Deborah Gillespie, Principal, Hanover High School. June27, 2003.

  4. New Hampshire Department of Environmental Services. Draft Residential IndoorAir Assessment Guidance Policy. December, 2002.

  5. U.S. Environmental Protection Agency. Draft Guidance for Evaluating the VaporIntrusion To Indoor Air Pathway from Groundwater and Soils.

  6. Agency for Toxic Substances and Disease Registry. Air Comparison Values(Expires 9/30/2003).

  7. U.S. Environmental Protection Agency, Region III. Risk Based Concentrations Table4/25/03,

  8. U.S. Environmental Protection Agency, Office of Research and Development.National Center for Environmental Assessment (NCEA).

  9. Agency for Toxic Substances and Disease Registry. Toxicological Profile forBenzene. 1996.

  10. American Cancer Society. Cancer Facts and Figures. 1997.


The Health Consultation for the evaluation of soil gas data at Hanover High School inHanover, New Hampshire was prepared by the New Hampshire Department of Health andHuman Services under a cooperative agreement with the Agency for Toxic Substances andDisease Registry (ATSDR). It is in accordance with approved methodology and proceduresexisting at the time that the health consultation was initiated.

Gregory V. Ulirsch
Technical Project Officer, SPS,SSAB,DHAC

The Division of Health Assessment and Consultation (DHAC), ATSDR, has reviewed thisHealth Consultation and concurs with its findings.

Roberta Erlwein

Table of Contents The U.S. Government's Official Web PortalDepartment of Health and Human Services
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Contact CDC: 800-232-4636 / TTY: 888-232-6348

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