Six key factors of significance in determining possible heat stress situations include the ambient air temperature, ambient vapor pressure, air motion, mean radiant temperature, metabolic heat production, and clothing. The insulation of chemical protective clothing is largely a function of its thickness, looseness of fit and number of layers, while its permeability is a direct characteristic of the nature of the chemical protection sought. Chemical protective clothing has been divided into five area classifications: head, face and eye; hand and arm; footwear; partial torso; and complete torso protection. Environmental heat stress indices include direct indices, and rational and empirical indices. Many studies have investigated a relationship between productivity of a work force and environmental heat stress. One of the most massive such studies on the effects of mild heat (and cold) stress on performance suggests that the interaction between environmental discomfort and performance is far from clear, with some tasks decremented and others enhanced in the various subpopulations studied. Additional information includes guidelines for heat stress in protective clothing; the effects of age and gender on heat stress; body responses to heat stress as exposure limits; predictive modeling and heat stress guidelines; and convergence as the best physiological guide to heat limits.