Tests Show Why Breathing Apparatus Must Be Worn With Proximity Suits
Aluminized proximity suits enable fire fighters to momentarily work in intense radiant heat and high temperatures. The suits provide insulation against heat transfer while the aluminized surface reflects a high proportion of infrared radiation.
The proximity suit is frequently used without supplemental air or cooling. Under such operating conditions reports of heat stress and dizziness are not uncommon.
It was suspected that a proximity suit may not allow adequate dissipation of metabolic heat. It was also believed that the atmosphere in the hood may deteriorate to unfavorable conditions. Excessive heat buildup and insufficient air supply will jeopardize the health and safety of a fire fighter.
To determine the ventilatory characteristics and potential health and safety hazards associated with the use of an aluminized proximity suit, a series of laboratory tests was conducted by the University of California at Irvine with the cooperation of the California Department of Forestry. The aluminized suit had no cooling or supplemental air. In order to predict the environmental limitations of proximity suits, testing was necessary to first make a quantitative determination of metabolic heat buildup, hood oxygen depletion, carbon dioxide accumulation and infiltration of toxic gases.
The fire fighter proximity suit tested consisted of foam and glass fabric material with an aluminized surface. This combination of materials provides high resistance to heat flow and high reflectivity against heat radiation. The suit consisted of a coverall, a hood with a transparent plastic window, a pair of gloves reaching to the elbows, and knee-high rubber boots. All components had a large overlap, which was required to prevent flame intrusion.
Four tests conducted
Four tests were conducted inside a temperature-controlled laboratory room. Ambient air temperature was maintained at 73°F (±3°F) with a relative humidity of 40 percent (±10 percent). A physically fit man, 31 years old, 5 feet 8 inches tall and 145 pounds participated in the tests. During all experiments, the subject maintained a standing position with little lateral movement.
Rectal temperature and skin temperatures on the forehead, back and chest were measured. Air temperature inside the hood also was measured. The oxygen and carbon dioxide concentrations inside the hood were monitored continuously and O2 and CO2 concentrations were recorded on a strip chart.
Under full suit-up conditions, substantial temperature changes were observed during all four tests. The air temperature inside the hood rose from 73°F to 90°F in 30 minutes. Water vapor content also increased significantly as observed by fogging of the face shield 10 minutes into each test. The forehead temperature increased by an average of 5.0° F while the chest and back skin temperatures increased an average of 3.5°F. No significant increase in rectal temperature was observed during any of the experiments. The results are illustrated in figure 1.
Dramatic changes in oxygen and carbon dioxide concentrations were observed as illustrated in figure 2. The oxygen concentration inside the hood compartment decreased from an initial 21 to 16.7 percent (±3 percent) within 4 minutes of enclosure while the carbon dioxide increased from 0 to 3.8 percent (±2 percent) within 5 minutes of enclosure.
The temperature conditions inside the proximity suit increased to levels at which significant discomfort could not be avoided. Profuse sweating always occurred while wearing the suit. A substantial workload imposed upon the fire fighter who must wear this suit will generate metabolic heat which will not be dissipated. This will lead to a substantial increase in body temperature during the exposure and will subsequently limit the performance capability and endurance of the fire fighter.
The measured decrease in oxygen concentration inside the hood must be viewed with concern. Hypoxic conditions may lead to drowsiness, headache, nausea and decreased mental proficiency for judgment. Serious problems in muscular coordination occur when the oxygen level drops below 15 percent. The hypoxic conditions may jeopardize the survival of a fire fighter, especially if he enters oxygen-depleted environments.
Effects of carbon dioxide
Similarly, high CO2 levels inside the hood can cause labored breathing and nausea for concentrations between 4.5 and 5.0 percent. This may become especially hazardous when a fire fighter encounters environments with high ambient CO2 concentrations, thus raising the CO2 inside the hood significantly beyond the ambient conditions. It is therefore important that depletion of oxygen and the accumulat ion of carbon dioxide inside the hood lie considered when assessing the performance limits of a proximity suit.
Furthermore, information about the infiltration characteristics of toxic gases entering the hood compartment is needed. The infiltration of noxious gases into the hood can be predicted from calculations based on oxygen and carbon dioxide data obtained in our laboratory experiments. Using regression analysis, oxygen depletion, CO2 buildup and toxic gas infiltration can be predicted for different fire environments.
Figure 3 illustrates the time associated with predicted oxygen depletion inside the hood for a wide range of oxygen-deficient fire environments. For example, when a fire fighter enters an area where the ambient oxygen concentration is only 19 percent (21 percent is normal) it will take 25 seconds for the hood oxygen level to drop to 18 percent, 50 seconds to drop to 17 percent, and 1 minute and 45 seconds to drop to 16 percent. The danger point of 15 percent will be reached in approximately 3 minutes.
Figure 4 illustrates the time associated with predicted CO2 buildup inside the hood. If the ambient environment contains 2 percent CO2, (the concentration of CO2 in a normal atmosphere is 0.03 percent), then within 30 seconds the hood concentration will reach 3 percent. Within 50 seconds, it will reach 4 percent and within 1 minute and 45 seconds, 5 percent.
Toxic gas infiltration
Figure 5 illustrates the infiltration characteristics of any toxic gas into the hood. If the ambient concentration of a toxic gas, for example, is 0.02 percent, within 15 seconds the concentration inside the hood compartment will be 0.005 percent. In one minute, it will have exceeded 0.010 percent, and in 1 minute and 50 seconds, the concentration will have reached .015 percent.
A major danger at a fire scene is the low level of oxygen in the air due to the combustion of surrounding material. Under certain conditions, the oxygen level may reach zero for short periods of time. Low atmospheric oxygen starves the blood and tissue of oxygen and can thus lead to death. Carbon dioxide is not considered a toxic gas ordinarily. However, as little as 2 percent in the inspired air stimulates respiration while 3 percent doubles the ventilation of the lungs. Also, the presence of carbon monoxide presents a major hazard at a fire environment. CO concentrations of 0.01 to 0.19 percent are not uncommon at typical fire scenes. Therefore, infiltration of CO and other toxic gases into the proximity suit hood must always be expected when it is used in the fire area.
Heat stress during standby
The heat stress observed inside the proximity suit caused discomfort for the subject under the simulated standby conditions even at the ambient 73°F. Poor physical fitness or a heavy workload will generate additional metabolic heat which will lead to heat stress harmful to a fire fighter.
The depletion of oxygen within the hood imposes harmful conditions when the suit is used in oxygen-deficient atmospheres. Furthermore, the infiltration rate of toxic gases into the hood is high, allowing the fire fighter only a short time to accomplish his task.
It must be emphasized that proximity suits are not entry suits. They are not intended for use in a direct flame environment. Nevertheless, this study suggests that proximity suits should always be used in conjunction with selfcontained breathing apparatus where low O2 levels, high CO2 levels or toxic gas concentrations are expected. Undoubtedly, such a requirement will necessitate redesign of some proximity suits. Without self-contained breathing apparatus, a time limit of three to four minutes should be established, and then only if there is no danger of toxic gases being inhaled.
Furthermore, to reduce the metabolic heat accumulation inside a proximity suit, a prefrozen cooling vest worn under the suit can help a fire fighter to keep his body temperature under control. This approach, however, increases the weight burden for the fire fighter, adds to the bulk and would again limit the effectiveness of the fire fighter. The proximity suit weighs in excess of 15 pounds while the 30-minute breathing apparatus weighs between 25 and 30 pounds. A cooling vest would add another 5 to 8 pounds.
Limit to safety gear
Obviously there is a limit to the amount of safety equipment and clothing that even a physically fit fire fighter can wear and still perform his tasks effectively. The limits of physical safety in combination with the fire fighter’s performance capability are being strained when fire suppression strategy calls for the donning of proximity suits.
It is suggested that fire departments review their procedures for proximity suit use in light of the information presented in this report. Although each fire scene presents unique problems, a clear recognition of the limitations inherent in the use of proximity suits will enable fireground commanders to exercise specific safety precautions necessary to the health and well-being of their fire fighters.
