At which level of heat stress index it is not possible to work comfoably causing threat to health:

HEAT STRESS INDEX(HSI): HSI% Consequence of 8 hour exposure 0 No thermal strain 10-30 Mild-Moderate heat stress, Minimal impairement in work 40-60 Severe heat stress, Threat to health if not fit 70-90 Very severe heat stress, only few can sustain it 100 Maximum heat stress, only young fit acclimatized can sustain it >100 Varying degrees of stress due to hypehermia Ref: Park 25th edition Pgno: 784