The influence of inspired air temperature on tolerance to work in the heat.

In mines rescue operations two main types of self-contained breathing apparatus are used by men working in irrespirable atmospheres. One of these types of apparatus uses compressed oxygen and the other liquid air as the source of oxygensupply for the wearer. Each type of apparatus has its own distinct advantages in use, and these advantages have long been recognized. The present investigation deals only with one aspect and this does not prejudice the merits of other aspects of the apparatus. It is recognized that the temperature of the inspired air may well prove to be a limiting factor for working men. Haldane (1915) refers to the local respiratory effect of high wet-bulb temperatures. He found that when men were exposed to hot environments at a wet-bulb temperature of 450 C. (1130 F.) they were able to breathe "pretty freely" even during hard work. Killick (1932) states that when subjects at rest in cool ambient conditions breathed hot, moist air they did not experience discomfort until the wet-bulb temperature of the inspired air reached 54-5 to 630 C. (130 to 1450 F.), and found breathing tolerable until the wet-bulb temperature reached 59 to 650 C. (138 to 1500 F.). When her subjects were working on an ergometer " at a rate equivalent to walking at 4 to 41j m.p.h." these limits dropped to 51.5 to 54.50 C. (125 to 1300 F.) and 54-5 to 600 C. (130 to 1400 F.) wet-bulb temperatures, respectively. Haldane also remarks that atmospheres which are very hot, but dry, can be breathed without great discomfort. This is confirmed by Killick, who found that, for subjects at rest, dry air of temperatures of up to 1860 C. (3670 F.) could be breathed in comfort, and the corresponding temperature for men working was 104.50 C. (2200 F.). During a recent series of experiments, inspired air temperatures in the two types of breathing apparatus commonly worn in mines rescue work were recorded in hot environments under conditions simulating rescue operations, and their importance physiologically is emphasized in the results.