Thirsty Work
Wildland firefighters work hard in the heat, so it's critical that they understand the risk of heat stress and how they can prevent it. Firefighters suffer heat stress when the air temperature, humidity, radiant heat and lack of air movement combine with their hard work and the insulating effects of their protective clothing to raise their body temperature.
Sweating is the body's main line of defense. As sweat evaporates, it cools the body. When water lost by sweating is not replaced, the body's heat controls break down and body temperature climbs dangerously. When our bodies can no longer cope, we experience heat illnesses, including heat exhaustion or heat stroke, a serious disorder that can be fatal.
Unfortunately, thirst is a poor indicator of fluid needs. Individuals normally drink only enough water to replace up to two-thirds of the water they lose as sweat. Even when they have plenty of water available, firefighters working in the heat can lose from 2% to 6% of their weight through dehydration.
The U.S. Forest Service's Missoula Technology and Development Center (MTDC) and University of Montana researchers have conducted field studies of wildland firefighters. In a 2003 study, Ruby and Associates studied how wildland firefighters replaced fluids lost in sweat, urine and respiration. In a 2008 study of voluntary hydration by Cuddy and Associates, wildland firefighters were found to drink 0.5 to 0.8 liters per hour. The highest intakes were late in the shift (hours 6 to 12) when ambient temperatures were highest. The hydration status in these firefighters changed minimally, and no firefighters were dehydrated based on changes in urine chemistry and loss of body weight.
Water tends to not be enough in such extreme conditions, but sports beverages can help. Developed to provide fluid, energy and electrolytes during strenuous activity, one liter of a typical beverage provides fluid; 220 to 320 calories of carbohydrates for energy; and sodium, potassium and other electrolytes. Firefighters who drank sports drinks in addition to water did a better job of maintaining their blood glucose levels and work performance than those who only drank water, especially during the latter portion of the shift (Ruby et al. 2004).
Immune function, the first line of defense against upper respiratory infection, can be measured by salivary immunoglobulin A. Firefighters who drank a sports beverage containing carbohydrate were better able to maintain their immune function than those who only drank water (Harger et al, 2004). Cognitive function and mood also were enhanced when firefighters drank a sports beverage during the work shift.
Sports beverages maintain hydration, replace electrolytes and provide carbohydrate energy to help maintain blood glucose, cognitive function, mood and work performance. However, beware of so-called energy drinks (see sidebar).
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Weight loss is a sign of dehydration. Even a 2% loss of weight (3 pounds for a 150-pound firefighter, representing a little more than 1 liter of fluid) has a measurable effect on endurance. Dehydration increases the likelihood of exhaustion from heat stress. A 6% loss of weight (9 pounds for a 150-pound firefighter, representing about 4 liters of fluid) can result in heat exhaustion. Although firefighters won't have a scale on the fireline, incident commanders should consider having one in camp.
Urine can provide additional evidence of dehydration. Infrequent urination, a low volume of urine and pain when voiding are signs of fluid loss and dehydration. In a 2000 study, Dr. Lawrence Armstrong and colleagues compared urine color to laboratory measures of dehydration. They found that urine color correlated to body weight loss, urine specific gravity and urine osmolality (concentration). When urine is light yellow or wheat-colored, the person is well-hydrated. Dark yellow or orange urine indicates dehydration. A urine color chart keyed to dehydration levels can be viewed at www.humankinetics.com/hkarticles/hk-articles/urine-color-chart.
To remain hydrated, firefighters need to consume fluids before, during and after work:
Before work: Drink one to two cups of juice or water.
During work: Take several fluid breaks every hour, consuming up to one quart of fluid during each hour of hard work. People tend to drink more when lightly flavored beverages are available. Sports beverages will help retain fluids and maintain energy and electrolyte levels. Sodium in the beverage reduces urinary water loss, and carbohydrates help maintain immune function and cognitive performance.
After work: Continue drinking to replace fluid losses. Thirst always underestimates fluid needs, so firefighters should drink more than they think they need. Rehydration is enhanced when fluids contain sodium and potassium, or when foods with sodium and potassium are consumed along with the fluid. During the two hours after a work shift, a person should consume protein to help muscles recover.
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