<h4>Purpose</h4>Plaster casts can cause burns. Synthetic casts do not. Composite plaster-synthetic casts have not been thoroughly evaluated. This study analyzed the temperature from plaster casts compared with composite casts in a variety of in vitro conditions that would simulate clinical practice.<h4>Methods</h4>A Pyrex cylinder filled with constant body temperature circulating water simulated a human extremity. Circumferential casts, of either plaster or composite construction (plaster inner layer with outer synthetic layer), were applied to the model. Peak temperatures generated by the exothermic reactions were studied relative to the following variables: dip water temperature (24 °C versus 40 °C), cast thickness (16, 30, and 34 ply), and delayed (5-min) versus immediate application of the synthetic outer layers. Peak temperatures from the all-plaster casts were compared with the composite casts of the same thickness. Finally, the relative cast strength was determined.<h4>Results</h4>Potentially dangerous high temperatures were measured only when 40 °C dip water was used or when thick (30- or 34-ply) casts were made. Cast strength increased with increasing cast thickness. However, the presence of synthetics in the composite casts layers did not increase cast strength in every case.<h4>Conclusion</h4>When applying composite casts, the outer synthetic layers should be applied several minutes after the plaster to minimize temperature rise. Composite casts do not routinely generate peak temperatures higher than plaster casts of similar thickness. Because the skin of children and the elderly is more temperature-sensitive than average adult skin, extra care should be taken to limit the exothermic reaction when casting children and the elderly: clean, room temperature dip water, minimal required cast thickness, avoidance of insulating pillows/blankets while the cast is drying. 20 2 2014 8 2 193 193–201 28 3 2014 © The Author(s) 2014 Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited. Abstract Purpose Plaster casts can cause burns. Synthetic casts do not. Composite plaster–synthetic casts have not been thoroughly evaluated. This study analyzed the temperature from plaster casts compared with composite casts in a variety of in vitro conditions that would simulate clinical practice. Methods A Pyrex cylinder filled with constant body temperature circulating water simulated a human extremity. Circumferential casts, of either plaster or composite construction (plaster inner layer with outer synthetic layer), were applied to the model. Peak temperatures generated by the exothermic reactions were studied relative to the following variables: dip water temperature (24 °C versus 40 °C), cast thickness (16, 30, and 34 ply), and delayed (5-min) versus immediate application of the synthetic outer layers. Peak temperatures from the all-plaster casts were compared with the composite casts of the same thickness. Finally, the relative cast strength was determined. 3 , 4 , 5 ). The 40 °C dip water trials tended to reach peak temperatures approximately 2 min earlier, but the overall shape of the curves is not noticeably different (Figs. 3 , 4 , 5 ). Fig. 3 Sixteen-ply plaster and composites. The results are presented as time versus temperature curves. For every trial performed, the mean temperature was calculated 3 , 4, and 5 are very similar. Figures 3 , 4, and 5 also demonstrate the effect of cast thickness on the peak temperatures generated. Fig. 5 Thirty-four-ply plaster and composites. The results are presented as time versus temperature curves. For every trial performed, the mean temperature was calculated at 1-min intervals by averaging the values from the individual castings in a given trial. Thus, for each trial of castings listed in Table 1 , a single representative time–temperature curve was generated. These curves are grouped in Figs. Based on our data, we concluded the temperature of the dip water was the most important variable in generating heat in composite casts. At no time was a peak temperature greater than 48 °C recorded when 24 °C dip water was used. When applying composite casts, it may be prudent to delay the addition of the outer synthetic layers for several minutes to minimize the peak temperature generated. There appears to be no increased risk of thermal injury with composite casts compared with equally thick plaster casts. Because there is a wide range of susceptibility to thermal injury, it is advisable to take steps to minimize the exothermic reaction at all times. In older children undergoing casting, especially after walking age, plaster casts may break or degrade from scuffing. Adding a layer of synthetic cast improves durability while still allowing the superior moldability of plaster of Paris. One weakness of our study is that our experimental model of a glass tube is not the best currently available model to study the thermal effects of casts. Recent experimental studies by Halanski et al. [ 6 ] have used a different model that more accurately mimics the human skin–cast interface: PVC tubing layered with a carbon fibre heating element. Our model, with its increased thermal conductivity of the glass tube model, may tend to underestimate the risk of burns, as the glass would conduct the heat away from the skin thermocouples. This may explain why very few of our trial results approached the burn threshold. Therefore, we advise all health care professionals applying casts to utilize all available means to minimize the risk of thermal injury when applying plaster or composite casts to children or adults: room temperature, clean, dip water; minimum required thickness of plaster; avoid covering the cast with blankets while it is drying; avoid setting the freshly applied cast on an insulating pillow.