Heat rash, also known as prickly heat, miliaria rubra and Lichen tropicus, is a problem of little concern in civil medical practice. While the Pacific fleet was operating in tropical waters, however, this condition consumed a good deal of a medical officer's time aboard these ships. The cause of miliaria in the naval service is excessive heat. The equipment installed aboard men-of-war for purposes of ventilation is placed there at the expense, in terms of tonnage, of military equipment. It is, therefore, not practical to have ideal air conditions in ships' living spaces. With mechanical ventilation of the type currently employed, the compartment air is cooled by dilution with weather air. The air temperature of a space aboard ship is never equal to that “topside.” The dry bulb temperature below decks is always 7 to 10 degrees F. above that of the weather temperature.