A cross sectional study to determine the exposure of heat and its biological effects on the workers in a plastic factory located in the Shah Alam Industrial Estate, Selangor, Malaysia. Forty five respondents from the polymer section in the factory were selected as the respondents. Variables measured were the environmental temperature (WBGTin), air velocity, relative humidity, body temperature, average heart and recovery heart rate. QUESTEMP°34 Area Heat Stress Monitor was used to measure the environmental temperature in °C (WBGTin) and relative humidity (%). Velocicheck Model TSI 8830 was used to measure the air velocity in meter per second (m/s) while the OMRON Blood Pressure Monitor Model T3, was used to measure average heart rate and recovery heart rate. Body temperature (°C) was measured by the Instant Ear Thermometer-OMRON Gentle Temperature Model MC509. Interviews using questionnaires were used to determine respondents’ socioeconomic background, previous risk factors on heat exposure and other information related to heat stress. Results showed that the mean environmental temperature for the exposed group was 28.75°C, the mean air velocity was 0.15 m/s and the mean relative humidity was 58.1%. These production workers were exposed occasionally to heat when loading plastic powder into the molds as well as demolding the finished plastic products from the molds. The average time of monitoring was 2 hours for intermittent exposure and 8 hours duration for overall exposure. Maximum demand for work load was measured 1 minute after work activities were stopped at the demolding section. There was a significant difference between body temperature and average heart rate before work, after 2 hours of work and after 8 hours of work ( p < 0.001). The mean recovery heart rate after 1 min was 88.0 ± 12.0 beat per min. (bpm), indicating that there is no excessive physiological demand. Body temperature (36.8 ± 0.40°C) and average heart rate after 8 hours (78 ± 12 bpm) indicated a good body control of heat exposure. Five out of six workplaces monitored had temperatures of greater than 28°C (ACGIH TLV). The workers were exposed to moderate heat stress during the study period, however, body temperature and average heart rate measurement did not reach unacceptable level of physiologic strain.