About 9% of the injuries to fire fighters in Finland are burns. The majority of the burns on the shoulders and upper arms were caused by a high condensation rate of steam on the skin. The moisture transport from the skin of the subjects to the outer layers of clothing, as well as the heat transfer through the same dry and wet garment layers were measured by fabric tests. The differences between types of underwear, that is, cotton knit, aramid knit, and wool knit fabrics, were sought by the simulation method. Humidity measurements were performed with Vaisala Humicap sensors. The sensor mechanic and computer control systems of the sensor were developed at the Institute of Occupational Health.
Four male firemen served as subjects at 30°C ambient temperature and at 30% humidity. Each exposure lasted for 70 min (20-min rest, 25-min walk, and 25-min rest). The humidity between the skin and the underwear of the back and on the shoulders and between two-garment layers were recorded. The skin temperature was recorded at nine sites. Core temperature, heart rate, and oxygen consumption were also measured. The same layers of clothing were moistened with different amounts of water, and their thermal behavior was evaluated by exposing the garments to a source of radiant heat, 20 kW/m2, according to the second draft of ISO 6942. The lowest moisture content between skin and underwear on the shoulders was measured when woolen underwear was worn, and the highest content when cotton underwear was worn. The moisture content of the garments was lowest in the aramid underwear. The differences were not significant, however, the moisture in clothing decreased the transmission factor, TF20, the time-to-pain, and second-degree burn time. The decrease was drastic when the moisture content rose from 20% to 30 to 40% in underwear.