With more temperatures above 35° forecast every summer in Australia, you may be asking yourself, ‘When is it too hot to work?’ A simple answer to that question? There isn’t one.
In 1947 Schickele plotted 157 heat stroke deaths in the military training camps against temperature and humidity and identified the ‘death line’. Air temperatures ranged from 26 to 49◦C and relative humidities of 10 to 100%. She commented that, “Death can occur at surprisingly low temperatures, provided the evaporative power of the air is sufficiently reduced”.
Legally, how hot is too hot?
We know that a healthy acclimatised worker can cope with temperatures well above the body’s normal core temperature of approximately 37°C, so long as they have the ability to sweat, but how hot is too hot to work? Legally, how hot is too hot?
There are no regulations in Australia that set a limit on temperature in a work environment, however it is the employers responsibility to identify hazards and put in control measures to reduce the risk to the hazard, if needed. Worksafe Australia identifies extreme temperatures as an example of a common workplace hazard (Worksafe Australia, Code of Practice “How to manage work health and safety risks” December 2011″). The Code provides a simple four step process to identify, assess, control and manage risks in the workplace.
Temperature alone is misleading
Temperature alone, is not an adequate indicator of heat stress. Moisture in the air, radiation heat sources and the amount of air movement also play an important part in determining the amount of heat our bodies will store. The type of work being undertaken, clothing and other personal factors are also important to understand the overall heat burden on an individual.
Heat stress indices have been developed over the years to help quantify the effect of heat stress or to forewarn of its impending approach. These indices attempt to integrate all the factors affecting the human thermal environment into a single number. There is still much debate as to the best one to use and there are over 50 to choose from.
Three such indices are:
Wet Bulb Globe Temperature (WBGT), originally developed in the 1950’s by the US Military as a means to control heat illness experienced in training camps. This index is basic, simple to use and therefore gets used extensively. Its correlation to actual heat stress in human’s is low, however it can be used as a simple screening tool.
Predicted Heat Strain (PHS), is a relatively complex index as it solves the heat balance equation for each situation. It is supported by the International Standard (ISO 7933, 2004) and correlates well with actual heat stress in humans. It can also be used to determine suitable work/rest regimes.
Thermal Work Limit (TWL), was developed by Brake and Bates initially for underground mining in Australia, but has now been modified for workers above ground. The TWL gives the limiting sustainable metabolic rate that hydrated acclimatised individuals can maintain in a specific thermal environment. It is also based on solving the heat balance equations.
Can I get a heat stress index for my location?
You have 3 options to get a heat stress index for your location.
1. A portable monitor can be used to measure and calculate a heat stress index for your location. These can be costly, can only provide a measurement at one location at a time, require a competent operator who can interpret the data and do not provide any indication of future conditions.
2. If you don’t have a monitor the Bureau of Meteorology provides an estimate of WBGT is from recent observations. These use an estimation of the WBGT from available measurements, are only available for towns with an automatic weather station and again do not provide any indication of future conditions. There are also significant limitations with the WBGT estimation technique which is overly conservative.
3. Heat Manager provides predictions of WBGT, TWL and PHS for any location in Australia out seven days. These can be used as part of the Dynamic Risk Assessment along with specified job profiles to determine the workers at risk and provide recommendations for controls.
The following table compares various heat stress indices for hot dry and windy conditions (typical of southern Australia summer) and hot, humid and calm conditions (typical of tropical conditions). All indices give relatively similar recommendations, except for the WBGT estimate which is overly conservative and, if used, would result in significant restrictions to work. The WBGT calculated correctly give similar recommendations to the PHS and TWL.