Heat Stress Index – Thermal Work Limit (TWL) or Wet-Bulb Globe Temperature (WBGT)?
Understanding the Importance of Heat Stress Indices
Australian summers are one of the harshest is the world, with temperatures soaring up to 50 degrees Celsius in some parts of the country. As we enter the summer, it is important for workplaces to refresh workers about heat stress protocols and the effect heat stress has on workers who spend most of their time outdoors. Workers in industries such as building and construction, mining and manufacturing undergo a large physiological strain during the hot summer months.
Heat stress occurs when the body is unable to cool itself naturally to maintain a healthy body temperature. Sweating is the body’s natural response to controlling and maintaining body temperature however, in environments with high temperature and humidity, sweating may not always be sufficient in preventing the body from overheating.
Portable heat stress monitors such as the TWL-1S Heat Stress Monitor and the QUESTemp Heat Stress Monitor Series are commonly used as part of workplace safety programs to monitor heat stress parameters including heat stress indices to prevent heat stress and fatigue in workers. In addition to careful monitoring, staying hydrated, taking frequent breaks in the shade or cooler indoor area can also assist in preventing heat related injuries and fatalities.
Thermal Work Limit (TWL) vs Wet-Bulb Globe Temperature (WBGT) Index
Thermal Work Limit and Wet-Bulb Globe Temperature (WBGT) are indices used to measure the level of heat stress experienced by workers in thermally stressful environments. Monitoring for WBGT requires a dry bulb to measure air temperature along with a thermometer encased in a wet cotton sleeve and a thermometer in a block globe that accounts for radiant heat. WBGT is the apparent temperature used to estimate the effect of temperature, humidity, wind speed and visible infrared radiation on humans. It is important to note however that, WBGT does not consider factors such as clothing and PPE worn by workers on site as well as their workload and therefore affecting the accuracy of the readings.
Thermal Work Limit, or TWL, is a simple index developed to address the inadequacies of WBGT and is used to predict safe work-rest schedules. TWL is calculated using five environmental parameters – dry bulb, wet bulb, and globe temperatures (all used by WBGT), with the addition of wind speed and atmospheric pressure. TWL also accommodates for clothing factors to arrive at a prediction of a safe maximum continuously sustainable metabolic rate (W m2) for the working conditions. A high TWL value would indicate that the thermal conditions impose no limits on the work while TWL at very low levels, the TWL indicates that no useful work rate may be sustained.
With many notable studies, TWL has been widely accepted by industry professionals as the more accurate and reliable predictor of heat stress in outdoor environments. TWL has been shown to be a more realistic and valid index of heat stress and provided management with workable strategies for managing heat stress (Miller and Bates, 2007 1).
How to Measure Heat Index
Whether you are measuring heat stress at a construction site, underground mine or a manufacturing plant, Air-Met Scientific offers a range of heat stress monitors designed for quick and easy evaluation of potential heat stress environments including the Scarlet Tech TWL-1S Heat Stress Monitor has been designed for easy heat risk assessment and features built-in WBGT and TWL algorithm with an easy to understand backlit display showing different colours according to the risk levels.
References
1. VERONICA S. MILLER, GRAHAM P. BATES, The Thermal Work Limit Is a Simple Reliable Heat Index for the Protection of Workers in Thermally Stressful Environments, The Annals of Occupational Hygiene, Volume 51, Issue 6, August 2007, Pages 553–561, https://doi.org/10.1093/annhyg/mem035
