Twenty-two asthmatic young adult volunteers, predetermined to be reactive to sulfur dioxide (SO2) with exercise at normal room temperature, were studied to document short-term effects of SO2 exposure under hot conditions, both humid and dry. For comparison, similar exposures were conducted at mild temperatures. All subjects were exposed in an environmental control chamber to all possible combinations of 2 atmospheric conditions (purified air and 0.6 ppm SO2), 2 temperatures (near 21 and 38 degrees C), and 2 levels of relative humidity (near 20 and 80%). Exposures involved 5 min of heavy exercise (target ventilation rate, 50 L/min) plus brief warm-up and cool-down periods. Body plethysmographic measurements and symptom questionnaire interviews were administered before and at the end of each exposure. Response was expressed in terms of change in airway size or change in intensity of symptoms during exposure. Atmospheric condition showed the most marked and significant overall effect on physiologic responses; temperature and humidity effects were also significant. High temperature and high humidity tended to mitigate the bronchoconstriction produced by 0.6 ppm SO2 exposure: group mean specific airway resistance approximately tripled at 21 degrees C and low humidity, but increased by less than 40% at 38 degrees C and high humidity. Temperature and humidity affected symptoms less consistently than physiologic responses, but in general, symptom responses paralleled physiologic responses.