Predicting temperature timit values for cold touchable surfaces

During some occupational activities, workers have to handle objects or tools in cold environments. In other circumstances, contact between the hand and the cold surface might be accidental (e.g., when a worker touches a cold surface, a cooler, etc). In both cases, contact between the hands and the cold material can induce discomfort, pain or frostbite, and exposure to cold may negatively influence the dexterity and the manual sensitivity of the subject [1]. Four years ago the EU started to finance a research project to determine the maximum duration of touching and gripping materials in the cold. In four different European labs, six male and six female students participated in the study. All subjects touched 10cm x 10cm blocks of different materials: wood, nylon, steel and aluminium. The 5 materials chosen were representative of a wide range of thermal properties that are relevant to cold surfaces. The subjects were asked to touch with their index finger the materials maintained at temperatures ranging between -40°C and -5°C. Thermocouples were placed on the back of the hand and on the touching surface of the finger. The subjects scored their subjective ratings on scales of pain and numbness with 5 levels (0 to 4; from total absence to intolerable level). From all the experiments, an extensive data set was collected on cooling curves of the fingers touching the mentioned cold materials. From this data set general safety limits could be derived for touching cold surfaces of various materials. Additional modelling allowed extending the duration limits beyond the range of the data. Furthermore, recent developments in the modelling showed that the actual limits are leaning on the safe side, due to the measurement technique that has been used. In conclusion, touching experiments were conducted to determine the maximum allowable tolerable exposure duration at different temperatures and for different materials. This duration varies inversely as a function of the material constants and linearly as a function of the temperature of the material. Modelling this problem allows the development of exposure limits outside the experimental range and to improve the validity of the experimental limits