The ESIM2.0 is a portable heat stress monitoring device which calculates an Environmental Stress index as an equivalent to WBGT. Unlike traditional WBGT measuring devices, it’s portable and rugged design mean that it can be used in the field for the determination of safe work rates. The ESIM2.0 uses three parameters; solar intensity, air temperature and relative humidity to calculate an index equivalent to WBGT. ESIM2.0 uses a unique algorithm to calculate the relative weight of each parameter to represent its influence on the human body’s ability to cope with heat. The index is read in conjunction with the tables where the appropriate actions can be taken to implement the correct hydration/work/rest regime.