Forest workers are likely to remove head protection in hot and humid conditions because of thermal discomfort. However, a recent Occupational Safety and Health Administration (OSHA) regulation revision requires all workers in logging operations to wear safety helmets, thus creating a compliance problem. To determine which factors contribute to forest workers' thermal discomfort, this study evaluated subjects' physiological and psychophysical responses during tasks approximating the workload of forest workers in a high-temperature environment similar to that found in the southeastern United States during the summer. Environmental conditions in the helmet dome space were also evaluated. Three helmets were used in this study: a standard helmet, a passively ventilated helmet, and an actively ventilated helmet. It was found that none of the tested helmets burdened the body significantly for the physiological variables that were examined. Evaluation of the dome space environmental conditions showed that both the dry-bulb temperature (DBT) and wet-bulb temperature (WBT) varied significantly among the helmets tested. Psychophysical results showed that ventilation contributes to greater helmet comfort, and that weight and fit are important factors in helmet design. Relevance to industry Protective helmets for use in hot and humid environments should be modified to make them more comfortable, encouraging forest workers to wear them and thus comply with OSHA regulations. # 2001 Elsevier Science B.V. All rights reserved.