Heat stress during the Black Saturday event in Melbourne, Australia

The Black Saturday bushfire event of February 7 2009 devastated the state of Victoria, Australia resulting in 173 deaths. On this day the maximum temperature in Melbourne (state capital of Victoria, population 4 million people) exceeded 46°C, there were wind gusts of over 100 km hr-1 and the relative humidity dropped below 5%. We investigate the severe meteorological conditions of Black Saturday and the risk of heat stress and dehydration for the residents of Melbourne. This is through the analysis of weather station data, air pollution data, the apparent temperature (AT) and the Comfort Formula human energy budget model.

In Melbourne, the PM10 concentration was at dangerous levels (over 350 μgm-3) due to bushfire smoke on Black Saturday and the AT showed that heat stress conditions were present, albeit underrepresented due to assumptions in the AT formula. Further investigation into dehydration from energy budget analysis revealed that the meteorological conditions required a sweating rate of 1.1 kg h-1 to prevent heat accumulation to the body. If sweating stopped, hyperthermia could occur in 20 minutes. Sensitivity tests on the human energy budget indicate that the dry air and strong winds on Black Saturday aided latent heat release, but the required sweating rate was virtually unattainable for an average person and would have lead to intense dehydration. In the future, for extremely hot, dry and windy events like Black Saturday, we recommend that the AT is not used as a thermal comfort measure as it underestimates the physical stress people experience.