Submariners live in isolated, confined, and often crowded conditions when at sea. They must adjust to an 18-hour (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 those 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) has 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 develop when necessary, 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 Subcommittee on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants in 2002.
THE SUBCOMMITTEE’S CHARGE
Members of the COT subcommittee were selected for their expertise in inhalation toxicology, neurotoxicology, immunotoxicology, reproductive
and developmental toxicology, veterinary pathology, pharmacokinetics, epidemiology, and human-health risk assessment. The subcommittee 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 the following substances: 2190 oil mist, formaldehyde, acrolein, ozone, monoethanolamine, nitric oxide, nitrogen dioxide, oxygen, carbon dioxide, carbon monoxide, methanol, ammonia, benzene, hydrazine, Freon 12, Freon 114, hydrogen, toluene, and xylene.
Determine whether the current or proposed guidance levels are consistent with the scientific data and whether any changes to the Navy’s exposure levels should be made on the basis of the subcommittee’s evaluation.
For two submarine contaminants for which no guidance levels exist—surface lead and 2,6-di-t-butyl-4-nitrophenol—determine whether sufficient data are available to develop EEGLs and CEGLs, and if data are available, 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 future research, when appropriate.
To accomplish its charge, the subcommittee 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 separate reports. This is the subcommittee’s first report, and it contains the EEGL and CEGL recommendations for 10 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. Prior to entry into the submarine service, candidates receive a comprehensive physical and psychological 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). Therefore, the population that serves on U.S. submarines represents, in general, an extremely healthy population.
Recent studies that have evaluated mortality patterns in U.S. submariners support the conclusion that submariners represent an extremely healthy population. Charpentier et al. (1993) examined a cohort of 76,160 submariners who served on U.S. nuclear-powered submarines during the period 1969-1982. The study authors compared the mortality rates of the submariners with those of the general adult male population in the United States and found that the standardized mortality ratio (SMR) for total mortality was significantly less than one.1 The SMR was also significantly lower than the SMR expected for a military population. The SMRs for specific causes of mortality were also less than one. SMRs approached one for only two causes (malignant neoplasms of the brain and central nervous system [SMR = 1.03] and motor-vehicle accidents [SMR = 1.06]). The results reported by the study authors are 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 illdefined symptoms, infectious disease, digestive disorders, ear and eye complaints, and musculoskeletal conditions. The categories listed here represent about 90% of the 2,044 medical events reported.
Although recent data indicate that U.S. submariners are a healthy population, some members of this population might be particularly sensitive to certain air contaminants because of either genetic predisposition or conditions arising during active duty. For example, Sims et al. (1999) reported an annual incidence of asthma leading to the disqualification of 0.16% of the active duty personnel serving in the Atlantic Fleet Submarine Force. However, the authors considered the asthma cases to be mild.
Tobacco smokers are another subset of the population that might be more or less sensitive to certain air contaminants. Smoking currently is permitted 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 from 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 from 1997 collected from one submarine. 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, the smoking policies on board submarines vary, as they are determined by the commanding officer.
THE SUBMARINE ENVIRONMENT
The U.S. submarine fleet is composed primarily of two types of submarines (Thomas et al. 2000). Table 1-1 provides some distinguishing characteristics of the crews and patrols for the two submarine types. The nuclear-powered attack submarines have a designated crew of about 130 men who are deployed at irregular intervals for varying lengths of time. The nuclear-powered ballistic missile submarines have two crews that rotate between ship and shore duty on a 90-day cycle.
When submerged, a submarine is an enclosed and isolated environment. Submariners work, eat, and sleep in that environment and potentially are exposed to air contaminants 24 h per day. The submarine differs from typical occupational settings where workers have respites from workplace exposures at the end of their shifts or workweeks.
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 through the electrolysis of 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 from cigarette smoking and hydrogen that is released while charging the batteries are removed using a carbon monoxide-hydrogen burner that catalytically oxidizes the two components to carbon
TABLE 1-1 Characteristics of Crew and Patrols for U.S. Navy Submarines
Number and Size of Crew
Nuclear-powered attack submarines (SSN)
1 designated crew, 130 men
Irregular intervals between patrols; patrols of variable length
Nuclear-powered ballistic missile submarines (SSBN)
2 rotating crews, 160 men per crew
Regularly scheduled patrols; 90-day cycle between ship and shore; patrols >60 days in length
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.
dioxide and water, respectively. Hydrocarbons are also oxidized by this system. Activated carbon filters help 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 while at sea.
When the submarine is submerged, air is recirculated in a closed-loop system. This system is composed of the forward compartment air-circulation system and the engine room air-circulation system (R. Hagar, Naval Sea Systems Command, personal commun., April 2, 2003). Figure 1-1 provides a generalized schematic of a nuclear-powered attack submarine. The forward compartment air-circulation system contains most of the air-purification equipment and oxygen 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 serving as the mixing chamber. Stale air from the boat is exhausted to the fan room, and treated air is supplied by the fan room to the boat. The engine room air-circulation system provides heating, cooling, and air distribution within the engine room and is designed to maintain room air temperature below a maximum of 100°F. Electrostatic precipitators and other filters in this compartment treat the engine room air. Air from the engine room is exhausted directly to the fan room, and the fan room supplies air directly to the engine room.
Special variations in the exhaust airflow path 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 vent directly to the fan room, and air from the laundry dryers passes through lint screens prior to discharge into the fan room. About 50% of the air vented into the fan room passes through electrostatic precipitators, and air from the galley, scullery, pantry, and water closets go through activated charcoal filters before venting into the fan room. Also, cooking grease is removed from the range and fryer hoods using centrifugal force.
The submarine atmosphere is monitored with the central atmosphere monitoring system (CAMS), which 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 levels, although that method is not used in submarines equipped with the newer version of CAMS. A portable oxygen detector verifies oxygen levels weekly. Colorimetric detector tubes are used weekly to measure concentrations of the following compounds: acetone, ammonia, benzene, carbon dioxide, carbon monoxide, chlorine, hydrazine, hydrochloric acid, nitrogen dioxide, ozone, sulfur dioxide, toluene, total hydrocarbons, methyl chloroform, and monoethanolamine. During battery charging operations, 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 for volatile organic compounds, ozone, acrolein, aldehydes, amines, and nitrosamines. Regardless of the frequency and type of monitoring that is conducted on submarines, NRC (1988b) concluded that “monitoring on submarines does not provide quantitative analysis of all submarine air contaminants and provides only sparse data on exposure.”
THE SUBCOMMITTEE’S APPROACH TO ITS CHARGE
In conducting its evaluations, the subcommittee 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 subcommittee’s approach for data selection included the following elements:
Whenever possible, primary references (published or unpublished study reports) were used to derive the 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 (studies used to derive the exposure guidance levels). Evaluation of study quality required the professional expertise and judgement of the subcommittee.
Inhalation exposure studies were used to derive the exposure guidance levels. Data on other exposure routes were incorporated into the analyses when they provided useful information on pharmacokinetics, metabolism, or mechanisms of toxicity.
Human studies were preferred for developing the exposure guidance levels. The subcommittee 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 human experimental studies because they were conducted in a controlled laboratory setting and allowed measurement of personal exposure and end points relevant for derivation of the exposure guidance levels. To the best of the subcommittee’s knowledge, it did not consider data obtained from uninformed subjects or by force or coercion.
When quality human data were not available, standard laboratory animal studies were used to derive the exposure guidance levels. The animal species used were those that had historical control data and the most relevance to humans. Nonhuman primate studies were generally preferred but often were not available.
A weight-of-evidence approach was used to select the key studies, thus ensuring 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.
For derivation of the EEGL and CEGL values, the subcommittee 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 NRC reports (NRC 1992, 2001). The subcommittee evaluated chemicals individually and did not address exposures to chemical mixtures. When empirical data that characterize mixtures found in submarine air become available, the subcommittee recommends that those data be evaluated. The subcommittee considered only those health end points relevant to healthy young adult men on the assumption that women do not serve as permanent crew on board submarines. In deriving the EEGL and CEGL values, the subcommittee assumed that maximal exercise is not achieved due to the confined conditions on the submarine. The subcommittee also assumed that the submarine is operated at or near 1 atmosphere pressure. The specific approaches adopted by the subcommittee for developing EEGLs and CEGLs are outlined in the sections that follow.
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 lasting 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, the CEGL would apply.
To develop the 1-h and 24-h EEGLs, the subcommittee reviewed relevant human and animal toxicity data and considered all health end points. The basis for the EEGLs was 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 subcommittee used the available scientific literature or the guidance pro-
vided in Standing Operating Procedures for Developing Acute Exposure Guideline Levels for Hazardous Chemicals (NRC 2001).
In deriving EEGLs, the subcommittee used uncertainty factors that ranged in value from 1 to 10. These factors accounted for interspecies differences (extrapolation from animal to human populations, if applicable); intraspecies differences (possible variations or susceptibilities that might be applicable to the healthy male population considered); extrapolations from a lowest-observed-adverse-effect level (LOAEL) to a no-observed-adverse-effect level (NOAEL); and weaknesses or critical gaps in the databases. The subcommittee did strive for consistency; however, its overarching goal was a thorough case-by-case review of available data. Selection of uncertainty factors for each chemical reflects the subcommittee's best judgment of the data on both toxicity and mode of action. Because uncertainty factors of 3 each represent a logarithmic mean (3.16) of 10, the subcommittee 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 the 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 subcommittee used the basic approach outlined for developing EEGLs. Thus, relevant data were reviewed, sensitive end points evaluated, and appropriate uncertainty factors applied. The method differed only in that, when available, inhalation studies with repeated exposures were used as the primary basis for CEGL development. The effects of cumulative exposures over time were taken into account 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 greater than 1 in 10,000 exposed persons (NRC 1986b). For those 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 subcommittee evaluated the theoretical excess cancer risk resulting from exposures at the 90-day CEGLs. The subcommittee considered deriving the cancer risk resulting from exposures at the 24-h EEGLs, but concluded that such estimates would involve too much uncertainty. Furthermore, the chemicals evaluated in this first report are not suspected of causing cancer after a single exposure from 1-24 h. Additional information regarding cancer risk is provided in individual chapters, when appropriate. The subcommittee notes that COT typically has not proposed CEGLs for carcinogenic substances (NRC 1986b). However, the subcommittee acknowledges that there is value in conducting these evaluations, and it has proposed 90-day CEGLs for compounds with known or suspected carcinogenic activity in human beings.
Comparison to Other Regulatory Standards or Guidance Levels
The subcommittee considered relevant inhalation exposure standards or guidance levels from NRC and other agencies or organizations in its evaluations. However, the subcommittee notes that the 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 exposure standards are designed for repeated exposure throughout a working lifetime assuming that workers are exposed 8 h per day, 5 days per week for a working lifetime. Submariners can be exposed 24 h per 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 to 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. Those health effects may not be considered acceptable when setting conventional occupational or public-health exposure standards.
The subcommittee 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 to the EEGLs and CEGLs because SMACs are developed with a similar criteria and address adverse effects for 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 THE REPORT
This report contains the subcommittee’s rationale and recommendations for the following substances: acrolein, carbon dioxide, carbon monoxide, formaldehyde, hydrazine, methanol, monoethanolamine, nitric oxide, nitrogen dioxide, and oxygen. Each chapter of this report presents the relevant toxicologic and epidemiologic studies for those substances along with selected chemical and physical properties, toxicokinetic and mechanistic data, and published regulatory and guidance levels for inhalation exposures. The subcommittee’s recommendations for exposure guidance levels and the research needed to better define and support those conclusions are provided. The chemical profiles contained in this report are not comprehensive toxicologic profiles. Only those data particularly relevant to the derivation of the EEGLs and CEGLs are discussed. References are provided for recent authoritative reviews of the toxicology for some of the chemicals addressed in this report.
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