The Thermal Work Limit (TWL) heat stress index, which has been validated for gulf conditions. TWL gives a measure of the maximum safe work rate for the environmental conditions present at a worksite. If TWL is too low then even low rates of work cannot safely be carried out continuously and extra rest breaks and other precautions are needed to ensure worker safety.
TWL, measured in watts per square meter (W/m2), is the maximum rate at which heat can be lost to the environment in the conditions. TWL is calculated from environmental parameters assuming that workers are well hydrated and acclimatised to the conditions and are self paced.
Determination of TWL
To determine TWL the following must be measured:
- Dry Bulb Temperature (ambient air temperature) (in degrees °C)
- Wet Bulb Temperature (determined by the humidity/evaporation) (In degrees °C)
- Globe Temperature (determined by the radiant heat) (In degrees °C)
- Wind speed in metres per second
Instruments are available that carry out all of these measurements and internally compute the TWL.
Alternative instruments can be used to take the measurements which can then be entered into the online TWL calculator .
The Thermal Work Limit (TWL) is a measure of the maximum sustainable metabolic rate under the environmental conditions being experienced. The thermal environment can be classified on the basis of TWL into the following working zones.
Work in the Cautionary (medium risk) Zone?
This is where the TWL is between 115 and 140 W/m2. Anything more than light work may lead to heat accumulation and illness if work without rest is continued for prolonged durations.
- All possible measures must be taken to increase TWL to above 140 W/m2 e.g. ventilation, shade, shielding, cooling PPE etc;
- Work may still be carried out but work-rest cycling and increased fluid intake may be required;
- No unacclimatised employees may work in this zone;
- Rotate workers if possible;
- No lone or isolated workers to work in this zone;
Heat Stress Monitoring
How to monitor.
The work environment should be monitored or assessed using specialised instruments to calculate the Thermal Work Limit to know what precautions to take for worker protection in that zone.
Environmental monitoring should:
- Be representative of an area where a group of workers are exposed (the working zone);
- Be taken at regular intervals (or constantly) during the work shift especially if environmental conditions change;
- Be taken using an accurate instrument that can calculate TWL or using the online TWL calculator on the safety in heat website (if other instruments are used);
- Allow for adequate time for an instrument to settle and adjust to the environment before measurements are recorded (approximately 10-15 minutes if coming from an air-conditioned environment ).