The minimisation of energy and operating costs, together with the optimisation of comfort levels must be the aim of any air-conditioning plant. Industrial buildings offer much scope for improvements in the effective operation of air conditioning and ventilation systems. A method of determining the effectiveness of such systems is described-'Thermal Efficacy'. This allows the assessment of total heat removal in hot industrial environments. Summarized results of tests carried out in a synthetic-fibre producing factory are presented. Advantages of using Thermal Efficacy include improved assessment of discomfort levels experienced by workers.