Project Aquarius .6. Heat load from exertion, weather, and fire in men suppressing wildland fires

Firefighters' thermal environment was continuously measured while they attempted to suppress well-developed experimental bushfires of intensities commonly faced by hand-tool crews, and also while they built fireline in the same way without fire, during three summers in Australian eucalypt forests. They worked far enough from the flames to avoid painful intensities of radiant heat (>2 kW m(-2)) on bare skin and usually experienced 1.6 kW m(-2) - little more than the intensity of sunlight Fire had negligible effects on average air temperature, humidity, or wired speed. Exertion accounted for 71% of the total heat load, fire and weather for the remainder. Evaporation was effectively the sole means of heat dissipation, and predicted sweat loss exceeded one litre per hour. Indices of heat stress usually exceeded occupational limits proposed for heat-acclimatized men, although little additional strain resulted. We conclude that firefighters should be fit and acclimatized, wear light and loose clothing that imposes minimal resistance to evaporation, and drink at least 1 litre of water per hour.