Submariners live in isolated, confined, and often crowded conditions when at sea. They must adjust to an 18-h day (6 h on duty and 12 h of training, other related activities, and free time) and are continuously exposed to air contaminants in their environment. To protect submariners from the potential adverse health effects associated with air contaminants, the U.S. Navy has established 1-h and 24-h emergency exposure guidance levels (EEGLs) and 90-day continuous exposure guidance levels (CEGLs) for a number of the contaminants.
In December 1995, the Navy began reviewing and updating submarine exposure guidance levels (Crawl 2003). Because the National Research Council (NRC) Committee on Toxicology (COT) had previously reviewed and provided recommendations for those and other types of exposure guidance levels (NRC 1984a,b,c; 1985a,b; 1986a; 1987; 1988a; 1994; 1996a,b; 2000a,b,c; 2002a,b; 2003), the Navy requested that COT review or if necessary develop EEGLs and CEGLs for a variety of chemical substances. Substances were selected for review on the basis of their presence in the submarine atmosphere, the lack of a recent COT review, their toxicity, or their known or suspected concentrations on board (Crawl 2003). As a result of the Navy's request, the NRC convened the Committee on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants in 2002.
THE COMMITTEE'S CHARGE
Members of the committee were selected for their expertise in inhalation toxicology, neurotoxicology, immunotoxicology, reproductive and developmental toxicology, veterinary pathology, pharmacokinetics, epidemiology, and human-health risk assessment. The committee was asked to accomplish the following tasks:
Evaluate the Navy's current and proposed 1-h and 24-h EEGLs and 90-day CEGLs for acrolein, ammonia, benzene, carbon dioxide, carbon monoxide, formaldehyde, Freon 12, Freon 114, hydrazine, hydrogen, methanol, monoethanolamine, nitric oxide, nitrogen dioxide, 2190 oil mist, oxygen, ozone, toluene, and xylene.
Determine whether the current or proposed guidance levels are consistent with the scientific data and whether the Navy's exposure levels should be changed on the basis of the committee's evaluation.
For two submarine contaminants for which there are no guidance levels—surface lead and 2,6-di-tert-butyl-4-nitrophenol (DBNP)—determine whether sufficient data are available to develop EEGLs and CEGLs and, if so, provide recommendations for guidance levels consistent with the data.
Identify deficiencies in the database relevant to EEGL and CEGL development for the selected contaminants and make recommendations for research as appropriate.
To accomplish its charge, the committee was asked to review the Navy's supporting documentation and other relevant toxicologic and epidemiologic data and publish the results of its evaluations in two reports. This is the committee's second report, and it contains evaluations of EEGLs and CEGLs for 11 chemicals of concern to the Navy.
An estimated 30,000 submariners are on active duty in the U.S. Navy (Cassano 2003). Permanent crew members on U.S. submarines are all male and range in age from 18 to 48 years. Before entry into the submarine service, candidates receive a comprehensive physical and psychologic examination and are rejected if any major medical problems—such as heart disease, asthma, or chronic bronchitis—are noted (U.S. Navy 1992, 2001). Submariners are also required to undergo a complete physical examination every 5 years (Capt. D. Molé, U.S. Navy, personal commun., May 28, 2003). If any medical problems are noted at that time or during active duty, submariners may be disqualified from submarine duty (Cassano 2003). Thus, the population that serves on U.S. submarines is, in general, an extremely healthy one.
Recent studies that have evaluated mortality patterns in U.S. submariners support the conclusion that submariners are extremely healthy. Charpentier et al. (1993) examined a cohort of 76,160 submariners who served on U.S. nuclear-powered submarines during the period 1969-1982. They compared mortality in the submariners with that in the general adult male population of the United States and found that the standardized mortality ratio (SMR) for total mortality was significantly less than 1.1 The SMR was also significantly lower than that
expected in a military population. The SMRs for specific causes of mortality were also less than 1. SMRs approached 1 for only two causes: malignant neoplasms of the central nervous system (SMR, 1.03) and motor-vehicle accidents (SMR, 1.06). The results reported by the study authors were supported by a study of Royal Navy submariners, who must meet stringent physical requirements similar to those of the U.S. Navy (Inskip et al. 1997).
Morbidity patterns in U.S. Navy submariners also indicate a healthy population. Thomas et al. (2000) evaluated the rates of medical events in crews on 136 submarine patrols over 2 years (1997-1998). Injury was the most common medical-event category, followed by respiratory illness (primarily upper respiratory infections) and then skin problems, such as minor infections and ingrown toenails. Other medical events included ill-defined symptoms, infectious disease, digestive disorders, ear and eye complaints, and musculoskeletal conditions. The categories just listed account for about 90% of the 2,044 medical events reported.
Although recent data indicate that U.S. submariners are a healthy population, some might be sensitive to particular air contaminants because of genetic predisposition or conditions arising during active duty. For example, Sims et al. (1999) reported that asthma led to the disqualification each year of 0.16% of the active-duty personnel serving in the Atlantic Fleet Submarine Force (the authors considered the asthma cases to be mild).
Tobacco smokers might be more or less sensitive to some air contaminants. Smoking is permitted only in restricted areas on U.S. submarines. The percentage of U.S. submariners who smoke is difficult to estimate, because no broad survey has been conducted. Sims et al. (1999) estimated a prevalence of smoking of 36% on the basis of data on eight submarines. However, Thomas et al. (2000) estimated that the prevalence of smoking might be as low as 22% on the basis of survey data collected from one submarine in 1997. The Navy has indicated that the percentage of submariners who smoke most likely ranges from 15% to 30% (Cmdr. W. Horn, U.S. Navy, personal commun., August 7, 2003). However, smoking policies on board submarines vary because they are determined by the commanding officer.
THE SUBMARINE ENVIRONMENT
The U.S. submarine fleet is composed mostly of two types of submarines (Thomas et al. 2000). Table 1-1 provides some distinguishing characteristics of the crews and patrols of the two submarine types.
When submerged, a submarine is an enclosed and isolated environment. Submariners work, eat, and sleep in that environment and potentially are ex posed to air contaminants 24 h/day. A submarine differs from typical occupational settings in which workers have respites from workplace exposures at the end of their shifts or workweeks.
TABLE 1-1 Characteristics of Crew and Patrols for U.S. Navy Nuclear-Powered Submarines
Number and Size of Crew
Nuclear-powered attack submarines (SSN)
1 designated crew of 130 men
Irregular intervals between patrols; patrols of variable length
Nuclear-powered ballistic-missile submarines (SSBN)
2 rotating crews of 160 men each
Regularly scheduled patrols; 90-day cycle between ship and shore; patrols over 60 days long
aNote that there are three classes of attack submarines—Los Angeles, Seawolf, and Virginia—and one class of ballistic-missile submarines—Ohio. There are also two deep-diving specialized research submarines (one nuclear-powered and the other diesel-powered) that are in a class of their own (Capt. D. Molé, U.S. Navy, personal commun., January 15, 2004).
Source: Information from Thomas et al. 2000.
Operation of a closed vessel can lead to accumulation of air contaminants (NRC 1988b). Major sources of air contaminants on a submarine include cigarette-smoking, cooking, and the human body. Other sources include control equipment, the power train, weapons systems, batteries, sanitary tanks, air-conditioning and refrigeration systems, and a variety of maintenance and repair activities.
Several onboard methods are used to maintain a livable atmosphere and remove air contaminants (NRC 1988b). Oxygen generators add oxygen to the air by electrolyzing seawater. The hydrogen that is generated in the process is discharged to the sea. Monoethanolamine scrubbers are used to remove carbon dioxide from the air. Carbon monoxide that is generated primarily by cigarette-smoking and hydrogen that is released in battery-charging are removed by a carbon monoxide–hydrogen burner that catalytically oxidizes the two components to carbon dioxide and water, respectively; hydrocarbons are also oxidized by this system. Activated-carbon filters help to remove high-molecular-weight compounds and odorants, and electrostatic precipitators help to remove particles and aerosols. Vent-fog precipitators are used in the engine room to remove oil mists generated there. Other means of minimizing air contaminants include restricting the materials that can be brought on board and limiting the types of activities, such as welding, that can be conducted at sea.
When the submarine is submerged, air is recirculated in a closed-loop system. The system is composed of the forward-compartment air-circulation system and the engine-compartment air-circulation system (R. Hagar, Naval Sea Systems Command, personal commun., April 2, 2003). Figure 1-1 is a generalized schematic of a nuclear-powered attack submarine. The forward-compartment air-circulation system contains most of the air-purification equipment and oxy-
gen generators and is designed to condition the air to 80°F and 50% relative humidity. The forward compartment is divided into zones; the fan room serves as the mixing chamber. Stale air from the boat is exhausted to the fan room, and the fan room supplies treated air to the boat. The engine-compartment air-circulation system provides heating, cooling, and air distribution within the engine room and is designed to maintain its air temperature below 100°F. Electrostatic precipitators and other filters in this room treat its air. Air from the engine room is exhausted directly to the fan room, which supplies air directly to the engine room.
Special variations in the exhaust airflow path described above exist (R. Hagar, Naval Sea Systems Command, personal commun., April 2, 2003). Air discharged from the carbon monoxide–hydrogen burners and the carbon dioxide scrubbers is vented directly to the fan room. Many electronic cabinets have fan systems that also vent directly to the fan room, and air from the laundry dryers passes through lint screens and then to the fan room. About 50% of the air vented to the fan room passes through electrostatic precipitators, and air from the galley, scullery, pantry, and water closets goes through activated-charcoal filters before venting to the fan room. Cooking grease is removed from the range and fryer hoods by centrifugal force.
The central atmosphere monitoring system (CAMS) of the submarine uses an infrared spectrometer to measure carbon monoxide and a mass spectrometer to measure oxygen, nitrogen, carbon dioxide, hydrogen, water vapor, and Freon 11, 12, and 114 (NRC 1988b). A newer version of CAMS also monitors the concentrations of selected trace chemicals in submarine air. Fan-room air is monitored continuously, and air in other onboard locations is analyzed on a rotating basis.
Portable devices are routinely used to monitor submarine air (Hagar 2003; NRC 1988b). Photoionization detectors monitor total hydrocarbon concentrations, although they are not used in submarines equipped with the newer version of CAMS. A portable oxygen detector verifies oxygen concentrations weekly. Colorimetric detector tubes are used weekly to measure concentrations of acetone, ammonia, benzene, carbon dioxide, carbon monoxide, chlorine, hydrazine, hydrochloric acid, methyl chloroform, monoethanolamine nitrogen dioxide, ozone, sulfur dioxide, toluene, and total hydrocarbons. During battery-charging, portable detectors are also used to monitor hydrogen concentrations. Suspected fluorocarbon or torpedo-fuel leaks are assessed with portable devices that have photoionization detectors. Retrospective passive monitoring of the submarine air provides 30-day time-weighted average concentrations of volatile organic compounds, ozone, acrolein, aldehydes, amines, and nitrosamines. Although some monitoring is conducted on submarines, several have reported that it is inadequate and provides few data on overall exposure (for example, see NRC 1988b).
THE COMMITTEE'S APPROACH TO ITS CHARGE
The committee reviewed relevant human and animal data and used data-selection criteria described in the NRC (2001) report Standing Operating Procedures for Developing Acute Exposure Guideline Levels for Hazardous Chemicals. Specifically, the committee's approach to data selection included the following elements:
Whenever possible, primary references (published or unpublished study reports) were used to derive exposure guidance levels. Secondary references were used to support the estimates derived and the selection of critical end points.
Whenever possible, studies that followed accepted standard scientific methods were selected as key studies for deriving exposure guidance levels. Evaluation of study quality required the professional expertise and judgment of the committee.
Inhalation-exposure studies were used to derive exposure guidance levels. Data on other exposure routes were considered, especially when evaluating pharmacokinetics, metabolism, and mechanisms of toxicity.
Human studies were preferred for developing the exposure guidance levels. The committee considered human data from accidental exposures, experimental studies, and epidemiologic studies to be valuable in determining the
effects of chemical exposure. When epidemiologic and human experimental studies were available, a preference typically was given to the latter, because these were conducted in a controlled laboratory setting and allowed measurement of personal exposure and evaluation of end points relevant to derivation of exposure guidance levels. The committee recognizes that one potential problem with experimental studies is the statistical power of a study to detect an effect given the small number of subjects typically used. That design problem often exists in studies using humans or large animal species, such as nonhuman primates and dogs. However, the committee did not set a threshold for statistical power for two reasons. First, data presented in manuscripts or technical reports were often inadequate to allow the committee to perform independent calculations to determine the power of an experiment. Second, derivation of the EEGLs and CEGLs was never based solely on a single study; key studies were always supported by other human experimental studies, epidemiologic studies, or animal studies (see last bullet). To the best of the committee's knowledge, the data used were not obtained from uninformed subjects or by force or coercion.
When high-quality human data were not available, standard laboratory animal studies were used to derive exposure guidance levels. The animal species used were those on which there were historical control data and those which were most relevant to humans. Nonhuman-primate studies were generally preferred but often were not available.
A weight-of-evidence approach was used to select key studies that ensured that selected data were consistent with the overall scientific database and incorporated what is known about the biologic effects of a chemical on pertinent organ systems.
The committee followed basic guidance provided by the NRC (1986b) report Criteria and Methods for Preparing Emergency Exposure Guidance Level (EEGL), Short-Term Public Emergency Guidance Level (SPEGL), and Continuous Exposure Guidance Level (CEGL) Documents but also considered the guidance for developing similar exposure levels provided in more recent reports (NRC 1992, 2001). The committee evaluated chemicals individually and did not address exposures to chemical mixtures. The committee recommends that empirical data that characterize mixtures found in submarine air be evaluated when they become available. The committee considered only health end points relevant to healthy young men on the assumption that women do not serve as permanent crew on board submarines. In deriving EEGLs and CEGLs, the committee assumed that maximal exercise is not achieved because of the confined conditions on a submarine. It also assumed that the submarine is operated at or near a pressure of 1 atm. The specific approaches adopted by the committee for developing EEGLs and CEGLs are outlined below.
Emergency Exposure Guidance Levels
NRC (1986b) defines EEGLs as ceiling concentrations (concentrations not to be exceeded) of chemical substances that will not cause irreversible harm to crew health or prevent the performance of essential tasks, such as closing a hatch or using a fire extinguisher, during rare emergency situations that last 1-24 h. Exposures at the EEGLs may induce reversible effects, such as ocular or upper respiratory tract irritation, and are therefore acceptable only in emergencies when some discomfort must be endured. After 24 h of exposure, CEGLs would apply.
To develop 1-h and 24-h EEGLs, the committee reviewed relevant human and animal toxicity data and considered all health end points reported. The EEGLs were based on acute or short-term inhalation and ocular toxicity data, and the most sensitive end points were emphasized. If extrapolation from one exposure duration to another was required, the committee used the available scientific literature or the guidance provided in Standing Operating Procedures for Developing Acute Exposure Guideline Levels for Hazardous Chemicals (NRC 2001).
In deriving EEGLs, the committee used uncertainty factors that ranged from 1 to 10. Those factors accounted for interspecies differences (extrapolation from animal to human populations, if applicable), intraspecies differences (possible variations in susceptibility that might be applicable to the healthy male population considered), extrapolations from a lowest observed-adverse-effect level to a no-observed-adverse-effect level; and weaknesses or critical gaps in the databases. The committee strove for consistency, but its overarching goal was a thorough case-by-case review of available data. Selection of uncertainty factors for each chemical reflects the committee's best judgment of the data on toxicity and mode of action. Because an uncertainty factor of 3 represents a logarithmic mean (3.16) of 10, the committee considered the product of two uncertainty factors of 3 to equal a composite uncertainty factor of 10, which is consistent with current risk-assessment practices (NRC 2001; EPA 2002).
Continuous Exposure Guidance Levels
NRC (1986b) defines CEGLs as ceiling concentrations of chemical substances designed to prevent immediate or delayed adverse health effects or degradations in crew performance that might result from continuous chemical exposures lasting up to 90 days. To derive CEGLs, the committee used the basic approach outlined for developing EEGLs; relevant data were reviewed, sensitive end points evaluated, and appropriate uncertainty factors applied. The method differed only in that inhalation studies with repeated exposures, when available, were used as the primary basis of CEGL development. The effects of cumulative exposure over time were taken into account by using a weight-of-evidence approach.
For known human carcinogens and substances with suspected carcinogenic activity in humans, the U.S. Department of Defense sets military exposure levels to avoid a theoretical excess cancer risk of greater than 1 in 10,000 exposed persons (NRC 1986b). For chemicals that have been designated as known or suspected human carcinogens by the International Agency for Research on Cancer or by the U.S. Environmental Protection Agency, the committee evaluated the theoretical excess cancer risk resulting from exposure at the 90-day CEGLs. The committee considered deriving the cancer risk resulting from exposure at the 24-h EEGLs but concluded that such estimates would involve too much uncertainty. Additional information regarding cancer risk is provided in individual chapters as appropriate. The committee notes that COT typically has not proposed CEGLs for carcinogenic substances (NRC 1986b) but acknowledges that there is value in conducting such evaluations and has proposed 90-day CEGLs for compounds with known or suspected carcinogenic activity in humans.
Comparison with Other Regulatory Standards or Guidance Levels
The committee considered relevant inhalation exposure standards or guidance levels put forth by NRC and other agencies or organizations in its evaluations. However, it notes that the submarine EEGLs and CEGLs differ from typical public-health and occupational-health standards in three important ways. First, public-health standards are developed to protect sensitive subpopulations—such as children, the elderly, and others with chronic health conditions who might be particularly sensitive—whereas EEGLs and CEGLs are developed for a healthy adult male population with little variation in physical qualifications. Second, occupational-health standards are designed for repeated exposure throughout a working lifetime on the assumption that workers are exposed 8 h/day, 5 days/week for a working lifetime. Submariners can be exposed 24 h/day with no relief from exposure during submergence. In a typical submariner's career, a 10-year assignment to active sea duty would result in about 4.5-5 years of cumulative exposure in the enclosed submarine environment (Capt. V. Cassano, U.S. Navy, personal commun., December 16, 2003). Third, EEGLs allow for the development of reversible health effects that would not prevent the performance of essential tasks; such health effects might not be considered acceptable in setting conventional occupational-health or public-health exposure standards.
The committee considered the submarine escape action levels (SEALs) and the spacecraft maximum allowable concentrations (SMACs) to be useful for comparison with EEGLs and CEGLs. However, SEALs are developed for disabled submarines and allow moderate, rather than minimal, reversible effects (NRC 2002a). SMACs are probably the most comparable with EEGLs and CEGLs because SMACs are developed with similar criteria and address adverse
effects in a healthy population in an isolated and confined environment. However, SMACs are developed for an older male and female population that experiences the conditions of microgravity during exposure.
ORGANIZATION OF REPORT
This report contains the committee's rationale and recommendations for ammonia, benzene, DBNP, Freon 12, Freon 114, hydrogen, 2190 oil mist, ozone, surface lead, toluene, and xylene. Each chapter presents the relevant toxicologic and epidemiologic studies of a substance with selected chemical and physical properties, toxicokinetic and mechanistic data, and published regulatory and guidance levels for inhalation exposure. The committee's recommendations for exposure guidance levels and the research needed to define and support the recommendations are provided. The chemical profiles contained in this report are not comprehensive toxicologic profiles. Only data particularly relevant to the derivation of the EEGLs and CEGLs are discussed. References to recent authoritative reviews of the toxicology of some of the chemicals addressed in this report are provided.
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