Research

California Studies of Firefighter Clothing

The California Department of Forestry and Fire Protection (CDF) and the University of California, Davis (UCD) agreed to evaluate and improve the protective clothing system for wildland firefighters. The first phase of the project involved three studies: 1) assessment of the radiant heat protection of single- and multiple-layer fabrics, 2) physiological evaluation of current and prototype clothing during work in moderate and hot conditions, and 3) subjective evaluation of standard and prototype garments. Clothing for wildland firefighters must protect from cuts, abrasions, sparks, radiant heat, and occasional flame exposure to the lower legs, without impeding the evaporation of sweat. The traditional view of firefighter protective clothing may have put too much emphasis on thermal protection and underestimated or ignored other important aspects of the garments.

The radiant protection assessment compared several two-layer clothing systems and the one-layer system used by the USDA Forest Service on a thermally instrumented mannequin. When the mannequin was exposed to an average heat flux of 80 kW/m2, producing a flame temperature of 1,800 to 2,000 °F for 4 s, the two-layer systems provided significantly more protection from burns than the one-layer system. The experimental conditions, designed to simulate a burnover, exceed the exposures typically experienced by firefighters (less than 2 kW/m2).

The physiological evaluation compared current CDF and Forest Service clothing and a prototype developed by UCD. Experienced firefighters exercised for 90 min in warm (30 °C, 32-percent relative humidity) and hot (38.5 °C, 23-percent relative humidity) conditions, performing treadmill walking, hose pulling, and raking with a McLeod. Work intensity was set at 47.5 percent and 42 percent of maximum oxygen intake (VO2 max) for warm and hot conditions respectively. Rectal and skin temperatures, total sweat, and subjective responses were lower for the single-layer Forest Service uniform. Results indicate that the two-layer clothing system restricts evaporative cooling and increases the risk of hyperthermia (heat exhaustion and heat stroke). The authors emphasized the importance of fitness, heat acclimatization, and hydration in preventing heat disorders.

The subjective evaluation compared current CDF and Forest Service clothing with the UCD prototype. The CDF clothing was perceived to be heavier, stiffer, and significantly less breathable, comfortable, and flexible than either the UCD prototype or Forest Service clothing. While ratings generally agreed with objective measures, such as air permeability of the fabrics, the slightly heavier UCD prototype was perceived to be lighter than the CDF uniform, perhaps because of the softer material (fire-retardant cotton) and the use of pleats to improve range of motion. Additional field and laboratory testing is planned.

Rucker, M.; Adams, W. 2000. Evaluation of thermal protective properties and heat stress responses to protective clothing for wildland firefighters. University of California, Davis. For information, contact Galen McCray, safety officer, CDF (galen_mccray@fire.ca.gov).

Clothing, Fitness, and Firefighting

The standard on protective clothing and equipment for wildland firefighters (NFPA 1977 and NFPA 1993) addresses outer garments and some accessories. This study compared proposed uniform variations. Four male and four female volunteers performed prolonged (2-h) treadmill tests with four variations of the standard uniform: no T-shirt, a short-sleeved T-shirt, a long-sleeved T-shirt, and a short-sleeved T-shirt plus a shroud for face and neck protection. Test order was determined by a balanced Latin square design. The 2-h tests were conducted with a 3-d rest interval and consisted of a treadmill walk at 5.65 km/h (3.5 mi/h) and 4.5-percent grade, with a 10.9-kg (24-lb) pack. These conditions reflect the energy expenditure of firefighting tasks (7.5 kcal/min).

The tests were conducted at 32.2 °C (90 °F) and 30-percent relative humidity, with an airspeed of 5 km/h (3.1 mi/h), and radiant heat of (0.1 W/cm2) during the first half of each hour. Heart rates (HR), skin and tympanic temperatures, and rates of perceived exertion (RPE) were recorded every 10 min. Weight loss and evaporative loss were determined after each trial. Values for males and females were not significantly different, so the data were pooled for repeated measures ANOVA (analysis of variance). Significant order effects were found for HR and RPE (p < 0.029 and 0.0001, respectively), indicating some acclimatization. Analysis of treatment effects did not reveal significant differences, although HR, RPE, weight, and evaporative loss tended to be greater for subjects wearing the long-sleeved T-shirt. Tympanic and mean body temperatures tended to be higher with the short-sleeved T-shirt and shroud, indicating heat storage. Individual differences in fitness overshadowed the effects of clothing variations; higher levels of fitness were associated with lower working heart rates (r = -0.91). Treatment differences were more pronounced after 2 h than 1 h, and the radiant heat influenced the skin temperatures.

Cordes, K.; Sharkey, B. 1995. Physiological comparison of protective clothing variations. Medicine and Science in Sports and Exercise. May.

Hydration and Blood Glucose

This study examined the differential effects of three hydration methodologies (carbohydrate, glycerol, and placebo) on the mental performance of 10 subjects during 3 h of treadmill walking and simulated line digging in a heated environment. Each subject completed one 3-h exercise trial for each hydration methodology. The paced auditory serial addition task (PASAT) was used to assess mental performance; each subject was given three practice tests before the first trial. The test required subjects to add pairs of single-digit numbers heard on a tape recorder and to respond verbally.

A set of 61 numbers was given at three speeds for each PASAT test, and subjects were given the test three times during each trial. All subjects completed a VO2 peak test. Intensity for each trial was set at 50 percent of the subject's peak VO2. Measures of blood glucose, plasma volume, body weight, RPE, heart rate, core and tympanic temperatures, and urine output were recorded at regular intervals throughout each trial.

A statistically significant difference between final scores in the carbohydrate and placebo trials was found at the speed of one digit every 1.6 s. At a speed of one digit every 1.2 s, scores after 90 min and at the end of 180 min of exercise were significantly higher than baseline scores.

The carbohydrate trial showed significantly higher values than the placebo trial. Females maintained more consistent body weights than males at the end of the exercise trial. Males gained more weight than females during the 90-min prehydration period. The glycerol trial resulted in significantly higher plasmavolume values. Females exhibited a greater ability to maintain plasma volume. Blood glucose values were higher at all data collection points, beginning with 60 min, during the carbohydrate trial. The RPE scores were significantly higher than baseline measures beginning at 90 min of exercise.

The results of this study suggest that mental performance improves after long-duration submaximal exercise in a heated environment and is better maintained with carbohydrate hydration than with glycerol or water. The increase in scores could be attributed to increased attention and arousal of the central nervous system. The improvement with carbohydrate is probably due to the increase in blood glucose, which is needed for cognitive function.

Puchkoff, J.; Curry, L.; Swan, J.; Sharkey, B.; Ruby, B. 1998. The effects of hydration status and blood glucose on mental perform-ance during extended exercise in the heat. Medicine and Science in Sports and Exercise. May.

Hydration Strategies

This study examined gender-specific physiological differences in response to long-term exercise in a heated environment. Nine trained subjects (four males and five females) performed 180 min of treadmill walking at 50-percent VO2 peak followed by a timed performance measure. Exercise was performed in a heat chamber set at 32 °C and 30-percent relative humidity. Hydration strategies included:

• W+W Trial—Water (24.1 mL of water per kilogram of body weight) before exercise and water (5 mL of water per kilogram of body weight per 30 min) during exercise.
  
  
• Gly+W Trial—Glycerol solution (1 g glycerol per kilogram of body weight + 24.1 mL of water per kilogram of body weight) before exercise and water (5 mL of water per kilogram of body weight per 30 min) during exercise.
  
  
• W+CHO Trial—Water (21.4 mL of water per kilogram of body weight) before exercise and a 7-percent carbohy-drate (CHO) solution (5 mL of CHO per kilogram of body weight per 30 min) during exercise.

Measured variables included: heart rate, plasma osmolality, core temperature, tympanic temperature, sweat rate, urine output, body weight, performance time, and plasma volume. Females lost less plasma volume than males. (p < 0.05). Plasma volume was preserved better during the Gly+W trial than during the W+W trial (p < 0.03). However, no significant differences were found between the Gly+W and W+CHO trials when hydration was maintained during exercise-induced heat stress. Females demonstrated a significantly lower sweat rate than males. The difference in weight loss was nearly significant between males (3.5 percent) and females (2.7 percent). These data indicate that females may have more efficient physiological adaptations to an exercise-induced heat stress, including decreased loss of body weight and plasma volume, and a lower sweat rate.

Swan, J.; Ruby, B.; Sharkey, B.; Puchkoff, J. 2000. Gender comparison of long-term exercise and three hydration strategies in a heated environment. Medicine and Science in Sports and Exercise. May.