Naval Safety Center




Acquisition Safety - Heat Stress

Introduction   |   Discussion   |   Common Shipboard Heat Stress Challenges
Recommendations   |   Conclusion   |   Resources



Flight deck personnel surrounded by steam from the steam-powered catapult

Many shipboard spaces contain environments of high heat and humidity, including engineering spaces, galleys, sculleries, laundries, and weather decks in hot climates especially during flight deck operations, exercises and drills. Sustained high temperatures leading to heat stress conditions can lower work performance and morale and impair mental alertness, increasing the risk of workplace accidents, and ultimately compromising the readiness of the ship. Severe heat stress can lead to heat-related illnesses, disabilities, and even death. Heat stress is defined as any combination of work, airflow, humidity, air temperature, thermal radiation, or internal body condition that strains the body as it tries to regulate its temperature. W hen the strain to regulate body temperature exceeds the body’s capability to adjust, heat stress has become excessive .

Environments of high heat and humidity contribute to heat stress conditions

Controlling heat stress is very important for mission readiness, combat control, and other functions aboard ships and combat vehicles, particularly those tasks that demand mental acuity. Significant heat exposure can increase shipboard manning requirements in specific locations and/or operations by a factor of three as the ability to do physical work in hot environments decreases and the need to make personnel substitutions increases. Designers can reduce this need for increased crew size by reducing heat stress conditions on Navy ships. Planning to eliminate or minimize heat stress conditions aboard ship should be inherent in the design phases of ship system acquisition.



Design and Operational Considerations

The effects of heat exposure are often regarded as a mere transient discomfort that may be low on the list of issues to be addressed by designers. However, significant heat exposures can increase fatigue and decrease comfort, productivity, efficiency, and combat readiness of shipboard and other military personnel. (Control of heat stress is also of great importance in successful operation of combat vehicles). Heat stress conditions can significantly impact mental alertness even before the ability to perform manual tasks is impaired. More significantly, uncontrolled heat exposure can lead to serious medical conditions.

The Navy mandates work-rest regimens, based on work activity, ambient temperature, humidity, and radiant energy to protect personnel who work in hot environments. These limits are published in OPNAVINST 5100.19 series, Chapter B2, Heat Stress Program, and are protective for healthy workers with adequate rest and hydration. Work periods may need to be further shortened for workers wearing protective clothing, using respirators, or taking certain medications.

The risks and prevalence of exposure to temperature extremes, particularly heat stress, are so significant to military operations and readiness that the Department of Defense has historically been a leader in the study of heat stress physiology as well as development of standards for exposure and criteria for medical management of heat illness. [See the Heat Stress Resources section of these Heat Stress Acquisition Safety web pages, for DoD’s and Navy’s acquisition policy and regulations related to heat stress and Human Factors Engineering.]

Effects of Heat Exposure

Direct and Indirect Impacts on Safety and Productivity

Fireman performs a heat index survey in the auxiliary machinery room aboard the mine countermeasure ship USS Dextrous (MCM 13).

Uncontrolled heat stress conditions can lead to heat-related illnesses, disabilities, and even death. A direct impact on work environments with poorly controlled heat stress conditions is reduced work capacity, and in extreme cases, the need for additional manpower to accomplish a given set of tasks. This can have a significant, but often unappreciated, impact on life-cycle costs for manpower and decreased productivity over the life of a system. Heat stress impacts on safety and productivity can be attributed to the following:

•  Reduced Mental Acuity. [Click here for more information]
•  Decrease in Physical Work Capacity. [Click here for more information]
•  Increase in Mishaps ("Accidents"). [Click here for more information]

Potential Medical Effects

A range of physical symptoms of varying severity have been associated with heat stress. As with most medical effects, risk varies with a range of personal and environmental factors. These include individual tolerance, personal acclimatization, fatigue, and prior activities that affect water balance such as consumption of alcohol or caffeinated beverages. Click on one of the following major medical effects of heat stress for more information:

•  Heat Stroke
•  Heat Exhaustion
•  Heat Cramps
•  Heat Rash

See Manual of Preventive Medicine, Ch 3, Ventilation & Thermal Stress Ashore & Afloat, Section II, Design Objectives for more details on the symptoms and treatment of heat stress illnesses.


Common Shipboard Heat Stress Challenges

Heat stress conditions can be caused by ventilation system deficiencies, steam and water leaks, or poor insulation on machinery.

Inspections conducted aboard various ships have identified problems that may cause a heat stress environment. Shipboard heat stress conditions, in general, are caused by the following:

•  Ventilation system deficiencies (see the Ventilation Section of the Acquisition Safety web pages), including poorly maintained ventilation systems and unauthorized alterations to existing ventilation systems
•  Steam and water leaks, and
•  Missing or deteriorated insulation on steam piping, valves, and machinery.

Problems of Inadequate Design Temperature Control that Affect Ships’ Manning Level

Initial acquisition cost, weight, and space constraints result in shipboard ventilation system designs based on the highest temperature limit recommended, rather than optimal comfort levels. I f a physical environment is not designed for appropriate temperature control, creating a heat stress condition for ship personnel, a work-rest cycle has to be implemented.

Heat stress conditions that are high enough to require work-rest rotation will impact manpower aboard ship, which is typically a key performance parameter on new ships. For example, laundry personnel working in an environment of 90 degrees F with 100 percent humidity would be limited to four hours of work, followed by four hours recovery, effectively requiring twice the staffing level to perform the work of a single shift. If the temperature were even higher, more than two workers would be required to do the work of one. On the other hand, if the temperature were lowered to 80 degrees, even with 100 percent humidity, those same workers could perform a full shift. In engineering spaces, temperatures are typically in the range of 92° to 120° F, with 60% to 90% relative humidity. At the higher temperatures, watches in these spaces are limited to about half an hour with an hour’s recovery for each exposure period. This effectively requires three persons to maintain continuous watch levels, instead of one. Out on the flight decks, ambient temperatures combined with radiant solar heat and high humidity levels during the summer can reduce flight watch and mechanics’ shifts to less than half an hour, with corresponding requirements for additional staffing to perform the work.

Annual costs for maintaining an E5 in a shipboard billet are approximately $80,000 per year. Therefore, heat conditions that increase manpower requirements add significantly to the operational and life cycle costs of a ship.

Problems in Laundries, Galleys, and Sculleries

A common ventilation problem aboard ship is controlling heat and humidity produced by cleaning and cooking processes in laundries and galleys. Ineffectively designed and poorly maintained ventilation systems result in exposure to hot, humid air, an inadequate fresh air supply, and/or inadequate exhaust of hot, humid air. For example, when steam is not properly exhausted from dishwashers, high air moisture content is created in sculleries and in adjacent passageways.

Problems in Fire-rooms, Engine Rooms, and Steam Catapult Rooms

Steam fire-rooms and engine rooms generate so much heat that it is neither practical Lack of shade on weather decks is a contributing factor for heat stress conditions.nor feasible to fully control temperatures within the entire space. Excessive moisture is also encountered in these rooms. Steam and water leaks are common sources of increased water vapor.

PrControlling heat and humidity in shipboard galleys presents a design challenge.oblems on Weather Decks and During Flight Deck Operations

Shipboard personnel generally do not have protective cover from sunlight and high temperature exposure on weather decks and during flight deck operations.



Apply Human Systems Integration in Design

Ships Serviceman 3rd steam irons a shirt in the laundry room aboard the aircraft carrier USS Enterprise (CVN 65).

When designing ship ventilation systems, it is imperative to take into consideration the needs of personnel living and working aboard ship as well as the work processes performed in particular spaces. The Naval Sea Systems Command (NAVSEA) has increasingly stressed human systems integration (HSI), addressing manpower, training, occupational safety and health, habitability and survivability, and design of systems and equipment to match the abilities, dimensions, and limitations of the people who will operate and maintain them. (See the Ergonomics/Human Factors Engineering Section of the Naval Safety Center’s Acquisition Safety web pages). According to the Manual of Preventive Medicine, Ch 3, Ventilation & Thermal Stress Ashore & Afloat, the upper thermal design limits within the living compartments, recreation spaces, mess decks (excluding serving lines), sick bay and inpatient wards, operating rooms and intensive care spaces, administrative areas, control, and all operating electronic spaces aboard surface vessels should not be more than 80° F dry bulb (DB) temperature, 68° F wet bulb WT, 55% relative humidity (RH), with 72° F wet bulb globe temperature (WBGT). The new Navy ships, such as DD(X) and LCS, are being designed for interior air conditioned temperatures of 78° F DB, 65° F WB (50% RH). These new interior temperatures fall within industry accepted standard comfort levels as determined by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). A preferred WBGT of 78° F applies to prescribed hot-weather operational conditions in: laundries, galleys, sculleries, passageways not open directly on weather decks, and food serving lines.


Improvement in Ship Thermal Insulation Design, Selection, and Maintenance

Computer Aided Design has been used to assist ship insulation designers. For example, Finite Element Analysis (FEA) modeling is a design tool that can be used to analyze heat flow. Insulation designers can design and test expected heat flow, using thermal FEA models - given input on environmental and other ship conditions - to arrive at proper insulation thickness.

In the design of shipboard spaces, which have radiant heat sources, it is necessary to insulate the radiating surfaces wherever possible. In those situations where metal surfaces cannot be insulated, they should be painted with a low emissivity paint (emissivity less than 0.4). Thermal insulation should have the lowest possible thermal conductivity (k) value. The insulating material should be well fitted together, should be of proper thickness for the source temperature, and should be kept intact and protected by metal sheathing where high traffic and abuse may occur. A reflective aluminized outside surface of thermal insulation pads will reduce radiant heat transfer into the shipboard space. In all cases, thermal insulation should be kept dry to remain effective, which requires that steam and water leaks must be eliminated. Merely re-insulating radiating surfaces, without first correcting steam and water leaks, leads to degradation of insulating properties making frequent replacement of insulation necessary.

Innovative commercial cruise ship kitchen ventilation design with air hood (Graphic created by Halton).

New Design Galley Ventilation

On Navy and commercial cruise ships, a new grease interceptor hood design has a front supply air plenum, which provides cool and fresh air as needed. The supply air is distributed at a low velocity forming an air curtain between the cook and the hot grease. This air curtain helps to reduce the effect of the radiant heat on the cook.

Spot Cooling

Spot cooling is a method of providing cooling to personnel in specific locations within a larger very hot area. The ship's fire and engine rooms are examples of very hot areas that can use spot cooling to provide comfort for Sailors working in those spaces.

Effectively placed spot cooling is a useful means of reducing heat stress conditions. Spot cooling is accomplished by delivering a high velocity blast of outside air via ventilation ducts and adjustable blast terminals to workstations. Due to the high velocity, the incoming air does not rapidly diffuse and mix with the room air and assists in the evaporation of sweat that provides the cooling effect. A "cone" of air is provided to watch-standers, even though the Effective Temperature outside the "cone" of air is very high. The key element in spot cooling is the delivery of an optimal effective air velocity flowing over the worker. This can best be accomplished by positioning the adjustable blast terminal so as to assure a direct, unobstructed air stream three to five feet from the watch-stander's torso. It should be noted that spot cooling works well when the outside air is cooler than the inside air.


Control Booths

Control booths built into shipboard spaces can provide a temperature and noise controlled environment.

Some ships have built in sound and temperature control booths in the propulsion spaces which use air-conditioning units to re-circulate air and provide comfort for personnel on watch. Outside of these control booths, and aboard those ships without control booths, Navy personnel have immediate access to spot cooling.

Replacement of Flash Type Distilling Plants with Reverse Osmosis Units

Distilling plants are used to supply fresh water and boiler feed-water. Distillers use steam, hot water, or electrical energy to boil seawater. The majority of Navy ships have steam-heated distilling plants. As part of the effort to modernize the Aegis cruisers, the Navy is in the process of replacing flash type distilling plants with reverse osmosis units capable of treating potable water. The reverse osmosis units are easier to maintain, more reliable, and do not create high temperatures in work spaces, which reduces heat stress and improves shipboard quality of life.

Protective Covering on Weather Decks

Temporary shelter can be provided for watch-standers on weather decks and in shipyard and port environments where operations are not compromised or other hazards (e.g., tripping) created.

Provide Remote Monitoring for Equipment in Overheated Areas

Sensors can be used to remotely monitor machines and equipment in overheated areas such as engine rooms.  Proper design and placement of such remote monitors can help ship personnel to avoid entering heat stress situations for extended periods to manually monitor equipment.  

Automated Heat Stress Monitoring System

The Automated Heat Stress System measures dry bulb temp., globe temp. and relative humidity, calculates the WB

Navy personnel exposed to high heat and/or highly humid environments are placed in a heat stress prevention program, which identifies safe Physiological Heat Exposure Limits (PHELs). In order to determine PHEL stay times for personnel who work in these hot environments, a Heat Stress Survey must be conducted. Conducting a Heat Stress Survey at each workstation within each "high heat" workspace requires using a portable, hand-held heat stress meter. A complete Heat Stress Survey, which measures the Wet Bulb Globe Temperature (WBGT) and determines the appropriate PHEL stay times in all required shipboard work spaces might take three to five hours depending on the size of the ship. The number of man-hours spent performing Heat Stress Surveys per year has been conservatively estimated at 3,300 for a destroyer and 5,800 for a carrier. An Automated Heat Stress System (AHSS) has been developed which will save hundreds of hours spent on manually conducting HSS aboard ships. The AHSS measures dry bulb (DB) temperature, globe temperature, and relative humidity (RH) and calculates wet bulb temperatures from the known DB and RH. The software program derives a WBGT value, displays the appropriate PHEL stay times, stores the data in a spreadsheet file, and prints the required information on a heat stress form. Completing the Heat Stress Survey, storing the data, and printing out the form can be accomplished within minutes. The AHSS was designed for use on all U.S. Navy ships and can be operated using a desktop computer or interface (see success story Automation of Shipboard Heat Stress Monitoring).


Placement of Heat Stress Measurement Equipment

Monitoring equipment should not touch hot materials.

For accurate readings, place thermometers and other monitoring equipment in the hottest part of the workspace that is not touching hot equipment or materials and is not inside a ventilation duct terminal. AHSS units are placed by the shipyard or contractor according to specifications and must be verified prior to approval for use. AHSS meters should never be moved unless approval is granted in writing by the Naval Health Research Center, which is the program manager for AHSS.

Ice Vests

Ice vests have been effective under certain conditions on aircraft carriers.

The Navy has evaluated ice vests as a new means of maintaining body temperature while Sailors work in hot shipboard environments. Ice vests have been approved for fire fighting by the Navy since 1989. The nine-pound ice vest contains flash frozen gel packs. Three sets are usually worn in each vest. The gel packs are changed every two hours to sustain their cooling capacity. Ice vests have been field tested in the engine room and laundry of USS Constellation and on the flight deck and the hangar bay of USS Abraham Lincoln and have been shown to be effective under certain conditions on aircraft carriers

(see success story Preventing Heat Stress Afloat).



Heat stress conditions have significant impacts on safety and productivity aboard ships. Heat stress can lead to heat-related illnesses, disabilities, and even death. Sustained high temperatures leading to heat stress conditions can lower work performance and morale and impair mental alertness, increasing the risk of workplace accidents, and ultimately compromising the readiness of the ship. Moreover, heat stress directly impacts manpower requirements aboard ship when heat exposures are high enough to require work-rest rotations.

Designing for proper temperature control and utilizing better technology to avoid or reduce heat stress conditions aboard ship will result in Sailors performing their duties in a comfortable and efficient manner. Designs that control or eliminate heat stress conditions will reduce the need for additional manpower (thereby reducing costs) and will improve shipboard safety, productivity, and quality of life.


Resources/Best Practices

•  Acquisition Resources
•  DoD/Navy Instructions and Regulatory Requirements
•  Heat Stress Guidance Documents
•  Heat Stress Standards
•  General Heat Stress References

Acquisition Resources:

DODI 5000.02
Department of Defense Acquisition Regulations, Human Factors Engineering, requires application of human systems integration in design and development of new systems. Salient aspects include requirements to optimize performance through design for users and consideration of work task, manpower, training, and habitability requirements for work and living spaces.

Naval Acquisition Policy requires application of human systems integration in the acquisition strategy (document that describes the programs management approach) and systems engineering process.


DoD/Navy Instructions and Regulatory Requirements:

Navy Safety and Occupational Health (SOH) Program Manual for Forces Afloat - Establishes policy, procedures, and actions for implementing the Navy’s safety and health program. See Chapter B2, Heat Stress.

Shipboard Habitability Program - Establishes policy, procedures, and actions to ensure shipboard facilities and spaces support the needs of shipboard personnel by supporting established habitability criteria for ship design and modernization programs.

Shipboard Heat Stress Program - provides guidance concerning determination of personnel exposure limits under conditions of high heat and humidity aboard ship; establishes procedures for routine surveys of shipboard spaces; and delineates specific reporting and corrective actions to be taken when and where such hazardous conditions are found to exist.


Heat Stress Guidance Documents:

Manual of Naval Preventive Medicine 
   Chapter 3: Prevention of Heat and Cold Stress Injuries, Ashore, Afloat, and Ground Forces of Feb 2009


Heat Stress Standards:

American Bureau of Shipping (ABS) Guidance Notes for the Application of Ergonomics to Marine Systems
Section 12 Crew Habitability - provides a wide range of information and data needed to integrate humans and systems, and thereby improve personnel performance and safety, and reduce human error.

ABS Guide for Crew Habitability on Offshore Installations
Focuses on five categories of habitability criteria that can affect task performance - accommodations design, human whole-body vibration, noise, indoor climate, and lighting.

American National Standards Institute (ASTM F1166-07)
Standard Practice for Human Design Engineering for Marine Systems, Equipment & Facilities - establishes general human engineering design criteria for marine vessels, and systems, subsystems, and equipment contained therein. Provides a useful tool for the designer to incorporate human capabilities into a design.


General Heat Stress References:

Department of Health and Human Services (DHHS) (National Institute for Occupational Safety and Health (NIOSH)) Publication No. 86-112
Working In Hot Environments, Revised 1986 - provides employers and workers with an overview of the health hazards associated with work in hot environments, and alerts them to the precautions which should be taken to prevent injuries and other health problems due to heat stress.

DHHS (NIOSH) Publication No. 86-113
Occupational Exposure to Hot Environments Revised Criteria 1986 - provides criteria for controlling heat stress.

Naval Safety Center Safety Success Stories

•  Automated Heat Stress Monitoring
•  Ice Vest Application

Occupational Safety & Health Administration Technical Manual (Heat Stress)
Covers heat disorders and health effects, investigation guidelines, sampling methods, controls, and personal protective equipment.

      Ship Configurations and Insulation Design/Application
      Gordon H. Hart - article describes new technique in ship insulation design.

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How to Contribute

We need input from the Defense Acquisition community to address each of the ten Acquisition Safety challenges that are the subject of this website. Grow with us as we share information on how to meet the above challenges through the Defense Acquisition Process. Through the exchange of ideas, information resources, and improvements in methodology and design, these challenges can and will be met.

To submit general information or information on Best Practices, or to submit a success story, please send an email to with the subject line "Acquisition Safety."


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