<prompt> Above which level of heat stress index it is not possible to work comfoably -20 - 4040 - 6060 - 8080 - 100  <response>: 0 No thermal stress 10-30 Moderate to mild heat strain 40-60 severe heat strain 70-90 very severe heat strain 100 upper limit of heat tolerance Parks Textbook of Preventive and Social Medicine, 25th edition, Page No. 807 