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1. ATSDR > Public Health Assessments & Consultations

Draft Preliminary Site Evaluation
Santa Susana Field Laboratory (SSFL)
Ventura County, California

Executive Summary

The Santa Susana Field Laboratory site is in the Simi Hills area of Ventura County, California. Located on approximately 2,668 acres, the site has been used as a rocket engine testing facility since 1948. At that time, few people resided in the area, but in the 1980s and 1990s there was a large increase in population. Approximately 3,100 people reside within one mile of the site boundary, based on spatial analysis of 1990 census data.

In response to a petition request, ATSDR held public availability sessions at three different locations in the area in the fall of 1999 to gather information on the health concerns of community members. More than 250 community members attended those meetings, and other community members have presented ATSDR with written comments and concerns. This report addresses the concerns of the community and is a preliminary assessment of the potential for adverse human health effects from past, present, and future activities at the Santa Susana Field Laboratory site based on currently available information.

ATSDR has also reviewed five epidemiologic studies from the Santa Susana site. Two were detailed health studies of workers at the site, and three were brief evaluations of community cancer registry data conducted in response to concerns that community members expressed about cancer occurrences in the area.

Process operations and activities at the Santa Susana site have resulted in the release of chemicals and radionuclides to the environment. The release of hazardous substances does not necessarily result in harm to humans. There must be human contact with these substances at levels of health concern before there is a potential for exposure-related health effects. Human contact of hazardous substances may occur through the air, soil, water, or food chain. ATSDR has evaluated these pathways relative to chemical and radioactive releases from the Santa Susana Field Laboratory.

This is a preliminary evaluation of the potential exposure pathways and associated health studies which ATSDR has reviewed for the Santa Susana site. Based on currently available data:

• The preliminary results of the exposure pathway analyses for air, ground water and surface water, and soil and sediment indicate that it is unlikely that people living in communities near the site have been exposed to substances from the site at levels that would have resulted in adverse health effects.

• Although chemicals and radionuclides were released from the site, the likelihood of those releases resulting in human exposure is limited by a number of factors, including; 1) the distance from the release sources to the offsite residential areas that results in rapid dispersion and degradation of oxidants and solvents in air; 2) the predominant wind patterns that normally blow away from the nearest residential areas; 3) other meteorological conditions at the site such as the atmospheric mixing height; and 4) drawdowns in ground water levels that reduce the rates of contaminant migration. Considering these factors, it is unlikely that residents living near the site are, or were exposed to SSFL-related chemicals and radionuclides at levels that would result in adverse human health effects. Changes in site operations, such as reduced frequency of rocket engine testing, discontinuation of trichloroethylene use, and shut down of nuclear operations make it unlikely that future exposures to the offsite community will occur.

• A more in-depth evaluation of exposure pathways that addresses past, current, and future exposure to chemicals and radionuclides from the SSFL should be conducted to improve the assessment of potential offsite exposures and public health implications associated with this site. Such an assessment must be facilitated through community outreach and participation and must include health education activities. We further recommend that this assessment address the following related issues:

• More in-depth evaluation of airborne chemical releases from SSFL operations, including air dispersion modeling of past accidents and disposal activities, and compilation and use of a consistent, site-specific meteorological data set to improve the assessment of past exposures to these substances.

• Development of a regional hydrogeological flow model and additional monitoring at down-gradient springs or seeps in Simi Valley and Santa Susana Knolls to evaluate the potential for deep fracture flow and potential future exposure. Also, even though it may not be related to SSFL, additional source characterization of the perchlorate detection in Simi Valley should be conducted.
• Additional radiological characterization of Area IV with more sensitive instrumentation and appropriate grid spacing to assure a lower detection limit.

• A re-analysis of the cancer registry data including additional years of newly available cancer data and updated demographic information should be conducted to see if the apparent increase in the incidence rates of bladder and lung cancers persist. A more in-depth evaluation of cancer data should be conducted that addresses environmental exposures from the SSFL, possible confounding exposures from other nearby contaminant release sources, and residential histories.

Agency for Toxic Substances and Disease Registry

Santa Susan Field Laboratory

Preliminary Site Assessment Team

Mr. Michael Brooks, CHP Health Physicist
Ms. Loretta Bush, Community Involvement Specialist
Mr. Burt J. Cooper, Industrial Hygienist
Ms. Lafreta Dalton, Community Involvement Specialist
Ms. Gwen Eng, Regional Representative
Dr. Mark W. Evan, Environmental Geologist
Ms. Laura Frazier, Environmental Health Scientist
Dr. R. Buck Grissom, Toxicologist
Dr. Deanna Harkins, Medical Officer, Physician
Mr. Joseph Maloney, Health Education Specialist
Ms. Kathy Skipper, Public Affairs Specialist
Dr. Youn Shim, Epidemiologist
Dr. Brenda Weis, Toxicologist
Mr. Tom Wilson, Editor

Introduction

Scope of Report

The Agency for Toxic Substances and Disease Registry (ATSDR) received a petition to conduct a public health assessment of the Santa Susana Field Laboratory (SSFL) site in Ventura, California. To determine whether ATSDR would accept the petition request the agency assembled a team of environmental health professionals to visit the SSFL site and review and evaluate the available environmental and health information. In addition, the National Aeronautics and Space Administration (NASA) has requested that ATSDR conduct a public health consultation regarding ground water contamination at the SSFL site.

ATSDR visited the SSFL site from October 3 to 8, 1999. The purpose of the visit was to see the site operations first-hand, meet with members of the surrounding community, and interview site and regulatory agency personnel. The site team also conducted three public availability sessions to collect information from the community about their health and environmental concerns. Specifically, this report includes (1) observations from the site visit, (2) a preliminary evaluation of available information related to chemicals and radionuclides released from the SSFL site, and an evaluation of the environmental pathways by which the surrounding community may be exposed to these substances, (3) a summary of the health and environmental concerns of the surrounding community, and (4) a preliminary evaluation of the public health implications of the SSFL site on the surrounding community.

This report is comprised of five sections. This introductory section outlines the scope of this report, provides a brief overview of the SSFL site operations and history, and describes the demographic characteristics of the community living adjacent to the SSFL facility. The Community Concerns and Health Studies Section presents a summary of the community's health and environmental concerns that were collected by the ATSDR site team, and a review of health studies that have been conducted for SSFL workers and the surrounding community.

The Exposure Pathways Analyses discusses the chemicals and radionuclides released to air, ground water and surface water, and soil and sediment from the SSFL. It also describes the adequacy or limitations of the data available for evaluating each pathway and the potential for community exposure to contaminants via these pathways. In the Public Health Implications section, ATSDR considers the results of the exposure pathways analyses together with the community concerns and available health studies to determine the potential for health impact to communities surrounding the SSFL. The Conclusions and Recommendations summarize the findings of this preliminary evaluation including the potential for public health hazard posed by the SSFL as well as gaps in the available health or environmental data and information. It also makes specific recommendations for follow-up activities to address the health issues or data gaps identified in this report.

Site Operations and History

The Santa Susana Field Laboratory (SSFL) site consists of four administrative areas used for research, development, and test operations and buffer areas on the southern and northwestern boundaries of the facility. The 2,668 acre site is located in the Simi Hills area of Ventura County and is approximately 30 miles northwest of downtown Los Angeles. SSFL is situated on an east-west trending ridge, between 1640 and 2250 feet (above sea level), overlooking Simi Valley to the north and northwest and the San Fernando Valley to the southeast [Robinson, 1998].

The SSFL is jointly owned by Boeing and the National Aeronautics and Space Administration (NASA), and is operated by the Rocketdyne Propulsion and Power Division of Boeing. A small portion of the Rocketdyne-owned property is leased to the U.S. Department of Energy (DOE). The site is divided into four administrative areas (I, II, III, and IV) and undeveloped buffer properties to the northwest and south, as shown in Figure 1 [Robinson, 1998; Ogden, 1998b].

• Area I consists of 671 acres owned by Rocketdyne and 42 acres owned by NASA (formerly owned by U.S. Air Force) in the northeast portion of the site. Area I contains administrative and laboratory facilities and was formerly used for rocket engine testing. Area I also includes the former Area I Thermal Treatment Facility and three rocket engine test areas, the Bowl, Canyon, and Advanced Propulsion Test Facility (APTF) areas.
• Area II consists of 410 acres in the north-central portion of the site and is owned by NASA and operated by Rocketdyne. Area II contains two formerly-used rocket test firing facilities and two currently operating rocket test firing facilities. Area II includes the Alfa, Bravo, Coca, and Delta rocket engine test areas (Figure 2).
• Area III consists of 114 acres in the northwest portion of the site and is owned and operated by Rocketdyne. Area III includes systems test area (STL-IV) and associated laboratories.
• Area IV consists of 290 acres owned and operated by Rocketdyne and 90 acres leased by the DOE. DOE and its contractors operated several nuclear reactors and associated fuel facilities and laboratories within this area. This area also includes five surface water discharge outfalls monitored under the National Pollutant Discharge Elimination System (NPDES).
• The northwestern and southern buffer areas consist of 175 and 1140 acres, respectively. Industrial activities have never occurred on these naturally vegetated areas. Two NPDES discharge outfalls and drainage channels are located within the southern boundary area. The northern boundary area was recently purchased by Rocketdyne from the adjoining Brandeis-Bardin Institute [GRC, 1999].

Figure 1. Facility Areas and Communities around Santa Susana Field Laboratory

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(Click on thumbnail for larger view)

Figure 2. Area II rocket test stands and surrounding terrain.

SSFL has been used as a rocket engine testing facility since 1948. The principal activity has been testing of large rocket engines and engine components. Other operations include testing small rocket motors; testing and developing water pumps, lasers, liquid metal heat exchanger components, and nuclear reactor research and associated activities. Processes that released substances to the environment at the SSFL can be grouped into four general categories:

• Accidental spills and releases of solvents, rocket fuels, and oxidizers related to rocket engine testing.
• Emissions resulting from the combustion of rocket propellants.
• Volatilization of solvents; Very large volumes of trichloroethylene (TCE) were used to clean rocket engine components before and after testing.

• Accidental releases of radioactive isotopes; Nuclear reactor operations and associated facilities, and reactor accidents resulted in the release of various radioactive isotopes.

Areas I, II, and III. History of Rocket Test Operations

Many thousands of rocket engine tests have been performed at the SSFL over the almost fifty years of operation. The predominant types of propellants used at the SSFL are liquid fuels and associated oxidizers. The primary propellants used at the SSFL are (1) hydrazine-based fuels (including hydrazine, monomethyl hydrazine, and unsymmetrical dimethyl hydrazine) and nitrogen tetroxide (oxidizer), (2) kerosene-based fuels (RP-1 and JP-4) and liquid oxygen (oxidizer), and (3) liquid hydrogen fuel and liquid oxygen (oxidizer). At present, hydrazine- and kerosene-based fuels are being used at the SSFL [CH2M Hill, 1993; Rocketdyne, 1999b]. Table 1 presents an overview of the engine testing programs at the SSFL including the types of fuels and oxidizers used, as well as the duration and location of each program.

During the 1960s, the SSFL also conducted research on solid rocket fuels, many of which contained beryllium. Solid fuels were used on a much smaller scale than the liquid fuels. Very limited information is currently available concerning the historic solid rocket fuel testing conducted at the SSFL. The specific release sources and chemical emissions are presented in the following section on Exposure Pathways Analyses.

Area IV. History of Nuclear Research Operations and Incidents

Area IV was the location of nuclear power development activities from the 1950s until the late 1980s. Facilities utilizing radioactive materials made up less than 5% of the total land area within Area IV. The site has had an ongoing program to monitor and cleanup radiological contamination since operations began.

Radioactivity at the SSFL has resulted from: The operation of ten reactors and seven criticality test facilities, fuel fabrication, reactor and used fuel disassembly activities, small scale laboratory work, and onsite storage of nuclear material. Nine of the ten reactors operated at power levels below one megawatt (1 MW). The ten reactors and criticality test facilities have all been dismantled and removed from the SSFL [Oldenkamp and Mills, 1991].

There have been nine radiological incidents at the SSFL. The August 16, 1959 tetralin explosion, was a non-radiological, chemical explosion in the Sodium Laboratory. Table 2 presents a chronological list of the radiological incidents in Area IV of the SSFL.

Table 1. Overview of Rocket Engine Testing Programs at the SSFL

Key: LOx = liquid oxygen; NTO = nitrogen tetroxide; MMH = monomethyl hydrazine; UDMH = unsymmetrical dimethyl hydrazine

APFT - advanced propellants test facility. LETF - Laser engineering test facility. STL - System test laboratory

* Other types of fuels and oxidizers, such as pentaborane, have been used at the SSFL

Table 2. Chronological List of Radiological Incidents in Area IV of the SSFL.

Notes:

AE-6 was a 2 kilowatt, low power research reactor with a solution of uranyl sulfate in a spherical tank and used as a neutron source.

SRE (Sodium Reactor Experiment) was part of a program with the Atomic Energy Commission to demonstrate the feasibility of a high-temperature, sodium cooled power reactor for civilian application.

SNAP 8 was a small sodium cooled reactor for space applications.

Of all these incidents, only the Sodium Reactor Experiment (SRE) Fuel Damage incident, commonly known as "The Meltdown," resulted in a measurable release of radioactive material into the environment. The SRE was a graphite moderated, liquid sodium metal cooled, 20 MW power reactor (Figure 3). In 1959, a clogged coolant channel resulted in localized melting of 30% of the fuel elements in the reactor core. The fuel elements fell to the bottom of the primary sodium containment vessel and the reactor was shut down. Most of the radioactive fission products were trapped in the sodium coolant or attached to metal components. Only the noble gas fission products made it to the helium cover gas and were held for decay before being vented to the atmosphere [Hart, 1962].

These operations and the resulting releases of chemicals and radionuclides into the environment are discussed in the following section, Exposure Pathways Analyses.

Figure 3. The Former Sodium Reactor Experiment Containment Building

Area Demographics

Figure 4 summarizes the demographic characteristics of the community surrounding the SSFL within a distance of one mile from the facility boundary. Population estimates in Figure 1 are based on census tracts and population data from the 1990 census. As of 1990, there were 1,089 housing units within one mile of the site boundary. The total population was estimated to be 3,062 based on an area-proportional spatial analysis technique. The population is comprised predominantly of Whites (87%), with smaller proportions of Asian/Pacific Islanders (9%), Blacks (2%), and other races (2%). Approximately 10% of the total population was children aged 6 years or younger while less than 7% of the population was adults 65 years or older.

The rapid population increase of the SSFL area during the 1980's and 1990's is documented in Table 3. This table, based on Ventura County census tract 75.03, shows a total population of 3,597 in 1980 which increases to an estimated total of 5,755 in 1998. The location of census tract 75.03 is shown in Figure 4. It encompasses most of the one mile area surrounding the SSFL, except for a small portion to the east of the site, which is in Los Angeles County. Census tract 75.03 also includes a larger portion of the Santa Susana Knolls and Simi Valley areas than the one mile buffer area, and consequently includes a larger total population than the one mile buffer area of Figure 4.

Table 3 also shows that most of the houses in this census tract were built after 1970; the area was very sparsely populated before 1970. Maps created by the U.S. Geological Survey [USGS, 1952; 1967] indicate that fewer than six buildings were present in the area directly bordering the SSFL before 1967. If each of these buildings is assumed to be a residence, the total population adjacent to the SSFL before 1967 was probably less than 20 individuals. Currently, residents live directly adjacent to the eastern and southern site boundaries, and two mobile home parks are located east of the site on Woolsey Canyon Road. According to the maps and observations by the ATSDR site team, there are no schools, nursing homes, or other facilities within one mile of the site boundary that would indicate the presence of persons who may have increased sensitivity to chemicals and radionuclides.

Figure 4. Population Characteristics in One Mile Area Surrounding the SSFL and Location of Ventura County, CA Census Tract 75.03.

(click on thumbnail for larger view)

Data are from 1980 and 1990 census tables. The 1998 data are estimates produced by extrapolation of 1990 data.

Community Concerns and Health Studies

Community Concerns

On October 5 and 6, and November 3, 1999, ATSDR staff held public availability sessions to meet with members of the SSFL community in order to discuss their health concerns. These meetings, in Chatsworth, Simi Valley, and the West Hills area, California, were preceded by mail-outs of the meeting announcements and ATSDR fact sheets to more than 1700 members of the SSFL community. To further assist in informing the public about the availability sessions, press releases and media press kits were mailed to 36 media outlets.

At the public sessions, ATSDR made a brief statement about ATSDR responsibilities and the purpose of these sessions. Following the introductory remarks and public questions, community members had individual discussions with ATSDR site team members. No personal identifiers were collected unless the community members requested follow-up information or indicated that they wished to be placed on the mailing list. More than 250 community members attended the sessions and provided the site team with the comments. Comment cards were also distributed to meeting attendees for written submission of comments and concerns.

The concerns obtained at the public availability sessions and comment cards are summarized in Tables 4 and 5. These concerns fall into two basic groups; health and medical concerns (Table 4) and environmental or exposure concerns (Table 5). These categories are further broken into subcategories based on the relative frequency with which each concern was repeated. It should be noted that for the week preceding the public availability sessions, a local television station ran a series of reports on SSFL activities and community health concerns.

The most commonly expressed health concerns were related to the perceived frequency of several types of cancer, specifically, breast, bladder, lung, and prostate cancers. These concerns were frequently expressed as comments concerning the number of cancers in their neighborhood. Several comments about health concerns were related to the communities adjacent to the Boeing/Rocketdyne Canoga Park and DeSoto facilities. Several comments also addressed the health concerns of former Rocketdyne employees or contractors.

The most commonly expressed environmental concerns related to property values and whether it is safe to construct or buy homes in the areas surrounding the SSFL site. Other common environmental concerns were about the potential for migration of contaminated ground water or surface water from the SSFL site. There were also reports from community members that they were distrustful of the data and results produced from Rocketdyne, and have a general distrust of the government agencies overseeing or regulating environmental operations at the SSFL.

Health Studies

ATSDR reviewed five epidemiologic investigations previously completed in the SSFL area. Of the five investigations, three were brief reviews of cancer registry data conducted in response to community concerns about cancer occurrence surrounding the SSFL. The remaining two were occupational health studies of the SSFL workers. ATSDR has reviewed the methodologies and findings of these reports for two main purposes, (1) as a measure of the health status of the communities surrounding the SSFL, and (2) to determine whether future activities should be considered to address any health issues in these communities.

Community Health Studies

The first of the community-based epidemiological investigations evaluated cancer incidence rates in five Los Angeles County census tracts within a five-mile radius of the SSFL [CDHS, 1990]. The SSFL is located in eastern Ventura County adjacent to Los Angeles County. However, Ventura County was not included in this investigation because the cancer registry had not been established at that time.

The age-adjusted cancer incidence rates in five Los Angeles census tracts were compared to the rates for Los Angeles County as a whole. Two time periods were examined, 1978-82 and 1983-1987. Specific cancers examined included lung, bone, bladder, thyroid, Hodgkin's and non-Hodgkin's lymphomas, and leukemia (including acute lymphatic leukemia, acute non-lymphatic leukemia, chronic lymphatic leukemia, and chronic myelogenous leukemia). The report concluded that a significant increase was observed in bladder cancer during 1983-1987 for one census tract (tract 1132). This census tract adjoins the SSFL site, however it also extends more than five miles to the east, such the individual cases may not be close to the site.

This study has several limitations; most of them inherent to this type of investigation. The accuracy of the population estimates at the census tract level is not known. Although standardized rates are useful as a summary measure, the rates are affected by random variation. Because multiple comparisons were made, the probability of finding a significant association by chance is increased even if there is no association at all. No information was available on actual exposures to contaminants from the SSFL sites: a five-mile radius within the SSFL site is a weak surrogate for exposures and no information is available regarding how long the residents lived in the area. No information was available on any other risk factors. This investigation serves the purpose of generating and refining questions on cancer incidence and cannot assess the cause and effect relationship of potential SSFL exposures.

The second community health study was conducted as a follow-up in response to recommendations made in the 1990 investigation described above [CDHS, 1992]. In this investigation, the 1990 study data were re-analyzed after including a more homogeneous study population (i.e., cases of invasive cancers among non-Hispanic whites only, stratified by sex). Standardized Incidence Ratios (SIR) were calculated adding one more year of newly available cancer incidence data (i.e., 1978-1988). This study also analyzed the cancer data for census tracts where population estimates were not available. Proportional Incidence Ratios (PIRs) were computed using an odds ratio (OR) for Ventura County and other Los Angeles County census tracts at least partially within a five-mile radius of the SSFL site for 1988 and 1989. Comparison groups were the rest of Los Angeles County residents for Los Angeles County and the rest of Ventura County residents for Ventura County. Cancer sites were grouped based on the evidence for radiogenic causes because of radiation exposure concerns. No increase was found in the "very radiosensitive" cancer group (cancers of the thyroid and bone, and all the leukemias except for chronic lymphocytic leukemia). The bladder cancer rate was elevated (SIR:132.6, 95%CI:100.5-171.9) among Los Angeles men living near SSFL during 1983-1988. The odds of having lung cancer among all cancers diagnosed was higher among Ventura men living near SSFL compared to that among the rest of Ventura men (OR:1.66, 95%CI: 1.17-2.37).

The study methodology is generally sound, given the limited data and lack of exposure information. Most of the limitations of the 1990 study also apply to this study and they are acknowledged appropriately. The interpretation of the findings is reasonably cautious because lung and bladder cancers are "strongly associated with other risk factors (smoking and non-radiation occupational exposures), it is important to consider alternative explanations" [CDHS, 1992].

The third community study was a follow-up to the 1990 and 1992 studies. It involved an analysis of the newly available cancer registry data for the years 1988-1995 for the Ventura census tracts that were included in the 1992 study [Tri-Counties Regional Cancer Registry, 1997]. This study calculated SIRs by using the 1990 census data. The Tri-Counties region population served as a comparison group. This preliminary analysis reported a significant decrease in the leukemia incidence in women. A significant increase in lung cancer was also reported for the combined group of men and women. However, this increase was small (SIR:117, 99%CI:100.6-135.6 [Nasseri, 1999]), and lung cancer was not significantly increased in men or women separately. The report acknowledged the lack of appropriate census tract level population estimates. If estimates of the base population are too low, the population-based number of expected cancer cases is also too low, which would lead to an overestimation of SIRs.

Occupational Health Studies

ATSDR reviewed two occupational studies of SSFL workers [Morganstern, et.al., 1997; 1999]. The first of these was a retrospective cohort study to determine whether workers at the SSFL nuclear sites experienced excessive mortality from specific cancers, total cancers, or other causes as a result of their work-related exposures to radiation. The cohort consisted of the SSFL workers enrolled in the Health Physics Radiation Monitoring Program, for external (4,563 workers) and internal (2,289 workers) radiation exposures. The internally monitored group was mostly a subset of the externally monitored group. A fairly long follow-up period is included, extending from 1950 to 1993. The study estimated radiation effects by employing internal comparisons of monitored workers according to level of cumulative radiation doses. Conditional logistic regression was used to examine the dose-response relationships by controlling for potential confounders and effect modifiers. Variables controlled for were (1) the other type of radiation exposure, (2) age at risk, (3) time since first radiation monitoring, (4) pay type, and (5) exposures to asbestos and hydrazine. External comparisons were also conducted by using two external reference populations to describe the mortality experience of the study population. The study found that mortality rates of the study cohort were lower for all causes, all cancers, and heart disease compared to the rates of the general U.S. population. Compared with NIOSH cohort members of similar pay type, the monitored workers experienced lower mortality rates for all causes and heart disease, but similar rates for total cancers. Although none of the 95% confidence intervals exclude the null value, there appear to be some excess mortality from leukemias in the monitored workers compared with either reference population. In the dose-response analyses of monitored workers, external-radiation dose was positively associated with the mortality rate for hemato-lymphopoietic cancers and for lung cancer. For dose levels greater than 200 mSv, the mortality rates for both types were particularly elevated. Increasing trends in mortality rates were found with internal-radiation dose for upper aerodigestive tract cancers and for hemato-lymphopoietic cancers.

This study is well designed and the data analysis is rigorous. The major strength of the study is the ability to examine the dose-response relationships by reconstructing internal and external doses received by the individual workers in the past. The choice of the study cohort and availability of the radiation monitoring records at the SSFL benefitted the study; however, they also pose some problems because of incomplete records. In particular, for internal radiation doses, uncertainty of the estimates appears to be high. The study measured cumulative SSFL exposures, however exposures received before employment at SSFL could not be accounted for because of inconsistency in the recording practice. Although the study attempted to control for the effect of other chemical exposures (i.e., hydrazine and asbestos), misclassification of the chemical exposures is highly likely. The use of the upper aerodigestive tract cancers group is somewhat unusual, although it is meant to take consideration the properties of internally deposited radionuclides. Another problem of the study is the small number of cancer deaths, particularly in the high dose group (e.g., >200 mSv). Most of these limitations are acknowledged appropriately in the report. Given the limitations, the most consistent and biologically plausible finding of the study is the hemato-lymphopoietic cancers. The observed positive relationship between external radiation and lung cancer mortality has not been reported consistently in other studies of nuclear workers.

The second occupational study is part of the 1997 study described above [Morganstern, et.al., 1999]. This addendum report focused on the chemical exposure portion, and included a cohort based on presumed exposure to the hydrazine (6,107 workers with 176,886 person-years) and a cohort with presumed exposure to asbestos (4,563 workers with 118,749 person-years). Employing an internal comparison method described in the 1997 report, this study reported the observed positive association between presumptive exposures to hydrazine and the rates of dying from cancers of the lung.

The weakness of this study mainly stems from the unavailability of adequate information on past exposures for individual workers. Even though the study was able to identify work locations with a high probability of exposure to hydrazine and asbestos at the SSFL site, information was not sufficient to link individual workers with job locations. As a result, the exposure classification was based on job titles. In addition to the possible exposure misclassification, bias may also have been introduced by confounding. Exposure information on other risk factors, such as exposure to other chemicals (e.g., trichloroethylene and nitrosamines) or personal characteristics is not available for the study. There is also a possibility that the radiation exposures are misclassified, hindering the ability to control for confounding by radiation exposures. Despite the limitations, the observed increase in the lung cancer risk associated with presumptive hydrazine exposure is noteworthy. The direction of the bias caused by the exposure misclassification may be toward the null value, because individual subject's exposure classification did not depend on the subject's disease status. This increase is observed after taking into account the effects of other potential confounding factors on which the relevant data were available. The increase is consistent across two hydrazine compounds. Given the uncertainties, the authors' recommendation that the worker group should be followed further is reasonable since the result shows a positive association, and health effects of exposure to these chemicals in humans are not well understood.

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