Indoor Air Quality Evaluation
PHILIP SERVICES CORPORATION
(a/k/a BURLINGTON ENVIRONMENTAL INCORPORATED GEORGETOWN FACILITY)
SEATTLE, KING COUNTY, WASHINGTON
This health consultation summarizes the Washington State Department of Health's (DOH's) evaluation of indoor air studies conducted at homes and businesses located near the Philip Services Corporation (PSC) facility in the Georgetown neighborhood of Seattle, King County, Washington. The evaluation was conducted to determine whether residents and workers, who are located in the vicinity of the PSC site, are being exposed to hazardous chemicals released into soil and groundwater. Of primary concern is the potential for volatile organic compounds (VOCs) to move from contaminated groundwater into indoor air. The soil to indoor air pathway was also evaluated for those buildings located immediately west of the PSC facility, where elevated levels of soil contaminants have been detected or potentially exist. DOH conducts health consultations under a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR).
The PSC facility is currently a temporary storage facility for industrial and household hazardous waste, located at 734 S. Lucile Street (Figure 1). The facility receives, packages, and ships hazardous waste for off-site treatment and/or disposal. In the past, some of the hazardous waste was treated at the facility. Leaking underground storage tanks and other past releases appear to be the source of contaminants detected in soil and groundwater at and downgradient of the facility. These contaminants include volatile organic compounds (VOCs) such as trichloroethylene (TCE), vinyl chloride, methylene chloride, and petroleum.1
The underground tanks were reportedly removed in 1987 along with a limited amount of contaminated soil.2,3 A soil vapor extraction (SVE) system was installed at the facility as an interim measure in 1994 to reduce the concentration of VOCs remaining in soil near the former tanks. The SVE system continues to operate today.4 Its effectiveness, however, cannot be determined because soil sampling has not been conducted.
Although some VOCs have been removed by the SVE system, soils on the PSC property contain elevated VOC levels. VOCs have also been detected in soil on the commercial and industrial properties located between the PSC property and Denver Avenue South. All of these properties are also underlain by significant concentrations of groundwater contaminants. Groundwater concentrations at some of the monitoring wells located on these properties suggests that nonaqueous phase liquids (NAPLs) may also exist in the underlying aquifers. NAPLs also likely exist in vadose zone soils on the PSC property.1,5
Groundwater contaminant plumes extend west from the facility to the Duwamish River and underlie a large residential, commercial, and industrial area between South Bennett Street to the north and South Fidalgo Street to the south.1,5,6 PSC has surveyed homes and businesses in the area and determined that the contaminated groundwater is not used for domestic purposes (drinking water or other household uses), industrial use, or irrigation. Water is supplied to homes and businesses by the City of Seattle.7 As a result, area residents and workers are not expected to be exposed to VOCs through ingestion or dermal contact. VOCs dissolved in shallow groundwater, however, can volatilize and move up through the overlying soil column and potentially travel into indoor air where they can be inhaled.
From October 1999 to the March 2001, four environmental studies were conducted in the area immediately west of the PSC property to determine whether VOCs were migrating from the contaminated groundwater to indoor air. The purpose and locations of these studies are summarized below; detailed information about the studies is provided in Appendix A.
- PSC, Soil Gas and Soil Study, October 1999
- PSC Indoor Air Study, August 2000
- DOH/Environmental Protection Agency (EPA) Indoor Air Study, August 2000
- DOH Indoor Air Investigation and Concurrent PSC Soil Gas, Groundwater, and Ambient Air Study, March 2001
The October 1999, PSC study was conducted to determine whether groundwater contaminants had volatilized into the overlying soil in the vicinity of three residences along the west side of Denver Avenue South (Residence 0, Residence 1, and Residence 3)(Figure 2). VOC concentrations in groundwater near these homes were among the highest detected anywhere west of Denver Avenue South. Soil samples were also collected to determine whether contaminated soil gas was affecting soils as the gas migrated toward the ground surface (Personal communication with Ed Jones, Ecology, October 24, 2002). Soil and soil gas samples were collected near the three homes to support the study.8
PSC conducted another study in August 2000 so it could evaluate whether VOCs found in groundwater were entering homes and if so, whether this was occurring at levels of health concern. Indoor air samples were collected at two of the three residences previously evaluated in October 1999 (Residence 1 and Residence 2) (Figure 2). PSC also collected groundwater, soil gas, and outdoor air samples during this study to help determine whether contaminants detected in indoor air were associated with contaminated groundwater or outdoor air.9
DOH and EPA conducted indoor air sampling approximately one week after PSC's August 2000, indoor air sampling. The DOH and EPA study was also conducted to evaluate whether VOCs found in groundwater were entering homes and if so, whether this was occurring at levels of health concern. DOH sampled the two homes previously tested by PSC (Residence 1 and Residence 2) along with Residence 3 (Figure 2). A sample was also collected by DOH at Business 1. EPA collected indoor air samples at Residence 2 and Business 2 and also collected an outdoor air sample near Residence 3.10
Based on the findings from the August 2000 indoor air sampling, DOH recommended that additional indoor air sampling be conducted during the winter months to determine whether there were seasonal differences in indoor air quality associated with the contaminated groundwater.10 Two of the residences previously sampled in August 2000 were resampled in March 2001 (Residence 1 and Residence 2). One of the businesses sampled in August 2000 (Business 2) was also resampled in March 2001; the sample location at that business, however, was changed. Three new residential locations (Residence 4, Residence 5, and Residence 6) and one new business location (Business 3) were added to the March 2001 sampling round. While DOH conducted the indoor air sampling, PSC concurrently sampled groundwater and soil gas in the city right-of-way near the homes and businesses to determine whether there was a correlation with the indoor air results.11
Indoor air samples were collected from seven homes and three businesses located near the PSC facility between October 1999 through March 2001. These homes and businesses were selected for sampling because of their proximity to elevated levels of volatile organic compounds (VOCs) in shallow groundwater. The samples were analyzed for VOCs to help determine whether the VOCs detected in groundwater were migrating through soil into the indoor air. A description of each study along with DOH's findings is presented in Appendix A.
It should be noted that the data obtained from these indoor air studies only provide a few snap shots of a complex, dynamic indoor air environment that can be influenced by contaminant levels in soil, groundwater, outdoor air, and indoor air. How these potential sources contribute to indoor air contamination at homes and businesses near the PSC site depends on meteorological (e.g., temperature, barometric pressure, and rainfall); hydrogeological (i.e., vadose zone soil characteristics, groundwater levels); and building conditions (e.g., building materials, foundation type (i.e., basement, slab on grade construction, and crawlspace); heating and ventilation system operation; air exchange rates between indoor and outdoor air; and maintenance activities (e.g., use of cleaning products, paints, and solvents).
Whether the conditions observed at the sampled homes and businesses are representative of all the homes and businesses located over the groundwater contaminant plumes or whether the types and concentrations of contaminants will change significantly over time because of VOC degradation is unknown. Only limited information is available about the structural integrity of buildings located over the plumes and the underlying shallow subsurface soils conditions, which play a significant role in the migration of VOCs from contaminated soil or groundwater to indoor air. In addition, groundwater contaminant concentrations will likely change over time as the plumes migrate and the contaminants degrade. The rates and types of contaminant changes that will occur cannot be predicted. Consequently, the potential risks to indoor air at homes and businesses overlying the plumes may also change. The indoor air sampling results, therefore, should be used with caution when evaluating long-term exposures to indoor air contaminants. Additional groundwater and indoor air monitoring is necessary to supplement this data.
The following section summarizes the data evaluation process used by DOH to identify chemicals of health concern and determine their potential sources.
Data Evaluation Summary
Indoor air VOC results obtained during the August 2000 through March 2001 sampling rounds were evaluated to identify chemicals of potential health concern (COPC) associated with the PSC facility (Appendix A, Tables A-1 to A-3). Some of the chemicals were detected in indoor air. However, a number of them were either not detected or only detected infrequently. Because many of these nondetected chemicals are potentially associated with releases from the PSC facility, one-half of the laboratory reporting limit or practical quantitation limit (PQL), was assigned to these results rather than a value of zero. The reporting limit or PQL is the lowest level at which a chemical can be accurately quantified. Using one half of the PQL is a conservative, but common approach for evaluating chemicals that may be present but not detected.12
The VOCs results were first compared to health comparison values, which are conservative screening values. It should be noted that contaminants that exceeded their respective comparison values do not signify that a public health threat exists but rather indicate that the chemical be further evaluated as a COPC. The health comparison values used for this health consultation are provided in Appendix A, Tables A-1 to A-3, and described in Appendix B.
VOCs are commonly found in outdoor and indoor air in urban and rural environments. Urban areas like Seattle, where significant automobile and industrial activity occurs, generally have higher outdoor air VOC levels than rural areas. Because outdoor air moves into buildings through windows, vents, and doors, some of the VOCs detected in indoor air may be associated with outdoor sources. To evaluate the outdoor air contribution, the COPCs were compared to the outdoor air sampling results obtained during the indoor air sampling events. Outdoor air sampling results are described in Appendix A and listed in Tables A-1 to A-3. It should be noted that although some of the outdoor air samples were collected within the boundaries of the groundwater contaminant plumes, it was assumed that the VOCs migrating from the contaminated groundwater would not significantly affect outdoor air quality since they would be quickly diluted when released from the soil. Outdoor air literature values were reviewed and compared to the outdoor air sample results obtained over the contaminated groundwater, which confirmed this assumption. COPCs with levels less then the respective outdoor air results, therefore, were assumed to be associated with some outdoor air source and eliminated as a COPC. Chemicals results that exceeded health comparison and outdoor air results are identified in Appendix A, Tables A-1 to A-3, as bolded, shaded values.
Site groundwater and soil gas data were also reviewed to assess whether chemicals that exceeded health comparison values and outdoor air levels were associated with contaminated groundwater. If these chemicals were also found in groundwater or soil gas, they were retained as COPC (Table 1).
|COPCs||Indoor Air Literature|
NA - not available
Indoor air studies conducted in homes and businesses around the U.S. indicate that background VOC levels in indoor air (i.e., levels of chemicals that are present in the environment due to man-made sources, unrelated to a contaminated site) often exceed outdoor air.13,14 Building materials and the use of common cleaning products, paints, solvents, and other chemicals are the typical types of man-made sources associated with these elevated indoor VOC concentrations. To conservatively estimate the influence of man-made sources on indoor air quality at the sampled homes and businesses, the maximum concentration of each COPC was compared to its respective indoor air literature value (Table 1). The literature values were obtained from peer-reviewed studies, one of which was based on a large national indoor air database developed by EPA; indoor air values were also obtained from EPA's Urban Air Toxic Monitoring Program.14,15,16,17 Median, rather than average, values were selected from these studies, when available, to reduce the bias created by outliers. Only single indoor air literature values, rather than ranges, were generally available.
All the COPCs, except carbon tetrachloride, exceeded the indoor air literature values, which suggests that contaminated groundwater is contributing some level of the COPCs to indoor air. The maximum level of carbon tetrachloride was 2.5 times lower than its respective average indoor air literature value suggesting that although groundwater and soil gas contain some carbon tetrachloride, contaminated groundwater may not be the source of this chemical in indoor air. However, it was detected above outdoor air levels at three homes and one business. Carbon tetrachloride was used in the past as a propellant for aerosol cans, degreasing agent, and refrigerant.18 Although its use has been phased out, it is possible that the levels of carbon tetrachloride detected slightly above outdoor air levels are associated with old appliances. Because there is some uncertainty about the potential sources of carbon tetrachloride, it was retained as COPC.
Benzene and tetrachloroethene just slightly exceeded their respective median indoor air literature values (i.e., by factors of 1.02 and 1.28, respectively). Like carbon tetrachloride, they were retained as COPC because there is some uncertainty about the sources of these chemicals. Benzene was detected above outdoor air levels at only two homes. Only one of these two homes slightly exceeded the median indoor air literature value. These two homes, like the other homes in the studies, are located in an area where low levels of benzene have been detected in groundwater.11 However, both homes were occupied by cigarette smokers. Cigarette smoke is a source of benzene and is another possible cause of the elevated concentrations of benzene detected in indoor air at these two homes.19 Tetrachloroethene, a common solvent, was only detected at one business above outdoor air levels. Although the tetrachloroethene level at this business only slight exceeded the median indoor air literature value, it was retained as a COPC because the indoor air sample was collected near a sump in the vicinity of the shallow tetrachloroethene groundwater contaminant plume.
It should be noted, that this final COPC list (Table 1) is only applicable to the data sets evaluated for this health consultation. Other COPCs may be identified during subsequent indoor air sampling. In addition, the COPC list developed by DOH may differ somewhat from the COPC list developed by Ecology for making cleanup decision under the Model Toxics Control Act (MTCA) because different factors (e.g., frequency of detection) may used when selecting COPCs.
Indoor air samples were generally taken at the homes and businesses located over the portions of the groundwater plumes where the highest VOC concentrations had been detected. Most of the VOCs detected are not considered COPC. However, the presence of some VOCs in indoor air above health comparison values, outdoor air levels, and indoor air literature values suggests that VOC contaminated groundwater may be posing a potential health risk.
- Noncancer Health Effects Evaluation
To estimate the potential for noncancer health effects, concentrations detected in indoor air were compared to EPA inhalation reference concentrations (RfCs). RfCs are concentrations of a chemical in air below which adverse noncancer health effects are not expected based on continuous (i.e., 24-hour per day) exposure.20 RfCs are set well below the actual toxic effect levels (i.e., lowest observed adverse effect level ( LOAEL) or no observed adverse effect level (NOAEL) found in studies upon which they are based). This approach provides additional health protection to account for the uncertainty involved in setting these ôsafeö levels of exposure. For chemicals with no available RfC, such as methylene chloride, a dose was calculated based on continuous exposure and compared to the oral reference dose (RfD). The RfD is based on oral exposure and its use for comparison with inhalation exposure adds additional uncertainty.
Maximum levels of COPCs detected in indoor air did not exceed any respective RfCs. Only the dose estimated from exposure to the maximum level of methylene chloride (330 ug/m3) exceeded its RfD (Table 2). The RfD for methylene chloride is based on liver toxicity in rats exposed to very high doses over an extended period of time. While the exposure dose is 3-times higher than the RfD, it is still approximately 2 times lower than the level of exposure that shows no adverse effects. It is also important to note that the comparisons made during this health consultation assume a continuous, 24-hour exposure to maximum detected concentrations, which likely overestimates actual exposure. Therefore, exposure to methylene chloride or any of the chemicals detected in indoor air is unlikely to result in any adverse noncancer health effects.
|COPCs|| Maximum |
| RfC |
| RfD |
- Cancer Risk Evaluation
To estimate the cancer risk associated with the contaminants detected above health screening values and indoor/outdoor air background values, concentrations were compared to EPA unit risk values. A unit risk value represents the estimated lifetime cancer risk estimated to result from continuous exposure to 1 ug/m3 of contaminant in air.21 For chemicals with no available unit risk value, such as trichloroethene, a dose was calculated based on a continuous exposure and compared to the slope factor. The slope factor provides an estimate of the cancer risk from lifetime exposure to a contaminant.21 The contaminants that contribute the most cancer risk are 1,2 dichloroethane, methylene chloride, and trichloroethene (Table 3). The estimates generated by this approach, however, are theoretical and are associated with much uncertainty. Actual cancer risks associated with low level exposure to these contaminants may be lower and could be zero.
|COPCs||Maximum Concentration (ug/m3)||Inhalation Unit Risk (per ug/m3)||Slope Factor (mg/day/kg)-1||Cancer Risk Max|
Comparison with Background Indoor Air
The presence of VOCs in urban indoor air has been well established. Therefore, it is important to consider the background risks associated with typical indoor air when evaluating whether indoor air VOCs levels near the PSC facility are contributing additional health risks.
The cancer risk levels and hazard quotients (i.e., ratio of the dose of a single chemical over a specified period of time to its reference dose) associated with the maximum concentrations of each COPC, where a corresponding indoor air literature values was available, were summed to conservatively estimate the health risks associated with the indoor air COPCs (Appendix D, Table D-1). Cancer risks and hazard quotients were also calculated and summed for the available indoor air literature values (Appendix D, Table D-2). While there is some uncertainty associated with this approach, it does provide a method for estimating the overall COPC cancer and noncancer health risks relative to background indoor air.
As noted in Table 4, the cancer risk associated with exposure to the maximum concentration of the specified COPCs is approximately an order of magnitude greater than would be expected in typical indoor air while the noncancer health risk is about 2 times greater than typical indoor air. It is important to note that no building contained all of the highest concentrations during a sampling round. Consequently, the health risk associated with exposure to these chemicals is likely overestimated.
|Indoor COPCs near PSC site*||Indoor Air Literature Values|
|Cancer Risk||3 in 1,000||3 in 10,000|
* Note: Only those COPC that had corresponding indoor air literature values were used to calculate a┬ cancer risk and hazard index.
All the VOCs evaluated during this health consultation, including those tested but not exceeding health comparison or outdoor air values, contribute to the overall health risk. To further estimate the indoor air health risks, the maximum and average concentrations for each of these chemicals were summed for each sampling round. The results of this additional evaluation are summarized in Table 5.
|Cancer Risk┬||2 in 1,000||5 in 10,000||3 in 1,000||6 in 10,000|
The estimated cancer risks associated with the maximum and average concentrations of these VOCs in indoor air near the PSC site are similar to upper bound estimates of cancer risk associated with background exposure to volatile organic compounds, which have been estimated as high as 1 in 1,000.17,22 The estimated noncancer risks for the maximum contaminant concentrations slightly exceed those estimated for the indoor air COPCs, described above. However, the noncancer risks for the average contaminant concentrations are slightly less.
The indoor air results obtained from the sampled buildings were also evaluated to assess whether there are possible seasonal differences in indoor air quality. Only Residence 1 and Residence 2, however, were sampled during different seasons (August and March). Although less than an order of magnitude difference exists between contaminants detected in the two sampling rounds at each building, the contaminants detected during the August sampling round were slightly higher than the March sampling results (Appendix A, Tables A-1 to A-3). These findings appear reasonable given that little precipitation occurs in the late summer months to occupy soil pore spaces and dilute groundwater VOC concentrations at the water table, which fluctuates very little at the PSC's site, thereby allowing more VOCs to migrate up through the soil column into indoor air. Existing sampling data, however, are not sufficient to clearly define seasonal effects on the groundwater to indoor air pathway.
It was recently discovered that petroleum compounds, particularly gasoline range compounds, were released to groundwater at the PSC facility. Medium to high levels of gasoline have been detected in shallow groundwater in the northern portion of the PSC site, east of Denver Avenue South. The boundaries of the gasoline plume has not been defined. However, the plume extends at least to the west side of Denver Avenue South.4,6,23,24 Like the other VOCs, these gasoline range compounds are contaminants of concern for the indoor air pathway and need to be evaluated during subsequent indoor air sampling. The boundaries of the petroleum plume will also need to be defined to ensure that appropriate buildings are tested.
The PSC site is located in a residential area where children potentially could be exposed to site contaminants through the indoor air exposure pathway. Children can be uniquely vulnerable to the hazardous effects of environmental contaminants. Children breathe more air per pound of body weight than do adults resulting in higher levels of exposure to contaminants in air. For these reasons, child exposures were considered when assessing health risks posed by this site.
Exposure to detected indoor air contaminants were evaluated as described in the discussion section, above. The doses calculated for individual chemicals are not expected to result in adverse health effects for children, or adults, based on comparison with toxicity values. The assessment did find that chronic exposure to multiple chemicals over many years (for example, 30 years) does indicate a slight increased cancer risk.
Levels of volatile organic compounds (VOCs) found in indoor air near the PSC site pose no immediate or short-term health concern. Many of the detected chemicals are present below levels of potential health concern. Levels of some VOCs in indoor air are above those normally found in an indoor air environment and some of these chemicals were also found in groundwater and soil gas. Consequently, part of the estimated health risk associated with indoor air is likely related to VOCs migrating from contaminated groundwater or soil. Long-term exposure to those chemicals indicates only a slight increased risk for cancer and non-cancer health effects.
The site is categorized as an indeterminate health hazard because it is not known whether the levels of VOCs detected at homes and businesses sampled in August 2000 and March 2001 are representative of all the homes and businesses located over the groundwater contaminant plumes. Several factors can affect sampling results including seasonal change, soil type and differences in building structure. In addition, the chemical composition of the plume is likely to change over time as contaminants migrate and degrade. Consequently, additional VOC monitoring (i.e., groundwater and indoor air) is needed to accurately assess long-term exposure related to the groundwater to indoor air pathway.
Additionally, a plume of dissolved gasoline with undefined boundaries is located in the northeastern portion of the PSC site and may be contributing additional VOCs to indoor air. However, no gasoline analysis has been conducted at the potentially affected homes. The potential exposure to gasoline needs to be further evaluated.
- Further evaluation of the indoor air pathway should be conducted at those homes and businesses overlying the VOC contaminated groundwater plumes associated with the PSC facility. More sampling and/or modeling data is needed to adequately characterize and evaluate the groundwater to indoor air pathway.
- Indoor air testing should be conducted at the most vulnerable buildings overlying the gasoline plume to evaluate whether gasoline is a significant indoor air contaminant at these buildings.
- DOH should review future project plans and reports related to the indoor air pathway.
PSC should continue evaluating the indoor air pathway using the modeling procedures described in its Revised Inhalation Pathways Interim Measures Work Plan, dated August 12, 2002, and any subsequent approved revisions to that plan. Indoor air at occupied buildings between Denver Avenue South and the PCS facility that are underlain by contaminated groundwater and soil, as well as a few buildings west of Denver Avenue, should be sampled periodically to confirm that the modeling accurately predicts indoor air concentrations.
PSC should conduct indoor air sampling above the gasoline plume concurrent with the next groundwater sampling round. Ambient air samples should also be collected and analyzed for gasoline compounds.
PSC should provide appropriate plans and reports to DOH for review.
Barbara J. Trejo
Washington State Department of Health
Office of Environmental Health Assessments
Site Assessment Section
Robert Duff, Manager
Site Assessment Section
Office of Environmental Health Assessments
Washington State Department of Health
ATSDR Technical Project Officer
Agency for Toxic Substances and Disease Registry
Division of Health Assessment and Consultation
- Philip Services Corporation. Draft Comprehensive RFI Report. June 29, 2001.
- Philip Services Corporation. Georgetown Community Newsletter. August 3, 2000. Number 1.
- Philip Services Corporation. Technical Memorandum VI, Summary of the Supplemental Offsite Characterization, PSC Corporation, Georgetown Facility, Seattle, Washington. April 20, 2001
- Philip Services Corporation. Quarterly Report April - June 2002, Q202. August 20, 2002.
- Philip Services Corporation. Draft Comprehensive RFI Report, Draft Human Health and Ecological Risk Assessment. August 10, 2001.
- Philip Services Corporation. Quarterly Report, January - March 2002. June 15, 2002.
- Philip Services Corporation. Draft annual letter to Georgetown residents. June 10, 2002.
- Philip Services Corporation. Technical memorandum, off-site soil gas study results. April 6, 2000.
- Philip Services Corporation. Indoor Air Analysis Report, Philip Services Corporation, Georgetown Facility, Seattle, Washington. September 2000.
- Washington State Department of Health. Draft Health Consultation, Evaluation of Indoor Air Sampling Near the PSC Corporation, Seattle, King County, Washington. November 29, 2000.
- Philip Services Corporation. Technical Memorandum: Soil gas investigation, March 2001 results. April 13, 2001
- Environmental Protection Agency. Risk Assessment Guidance for Superfund, Volume 1, Human Health Evaluation Manual (Part A), December 1989).
- Wallace LA and Pellizzari ED. Total Exposure Assessment Methodology(Team) Study: Personal exposures, indoor-outdoor relationships and breath levels of volatile organics in New Jersey. Environment International. 1986; 12:369-387.
- Shah JJ and Singh HB. Distribution of volatile organic chemicals in outdoor and indoor air. Environmental Science and Technology. 1988; 22:1381-1388.
- U.S. Environmental Protection Agency. 1997 Urban Air Toxics Monitoring Program. January 1999. EPA-454/R-99-036.
- Pellizzari ED and Hartwell TD. Comparison of indoor and outdoor residential levels of volatile organic chemicals in five U.S. geographical areas. Environment International. 1986; 12:619-623.
- Wallace LA. Comparison of Risks from Outdoor and Indoor Exposures to Toxic Chemicals. Environmental Health Perspectives. 1991: 95:7-13.
- Agency for Toxic Substances and Disease Registry. Toxicological Profile for Carbon Tetrachloride. May 1994.
- Agency for Toxic Substances and Disease Registry. Toxicological Profile for Benzene. September 1997.
- U.S. Environmental Protection Agency. Integrated Risk Information System (IRIS2). July 1999.
- U.S. Environmental Protection Agency. Integrated Risk Information System web site. Sept 2002.
- Hoddinott, KB and Lee AP. The use of environmental risk assessment methodologies for an indoor air quality investigation. Chemosphere. 2000: 41:77-84.
- Washington State Department of Health. Comments on draft inhalation pathway interim measure work plan. July 1, 2002
- Washington State Department of Health. Comments on final inhalation pathway interim measure work plan. September 2, 2002.
- U.S. Environmental Protection Agency. Region III risk based concentration table. December 18, 2001.
- Agency for Toxic Substances and Disease Registry. Air Comparison Values. April 12, 2002. [26a- ATSDR EMEG; 26b-ATSDR CREG].
- U.S. Environmental Protection Agency. Region IX risk based concentration table. November 20, 2001.