We all know working in heat isn't fun, but exposure to prolonged or excessive heat can lead to impaired cognition, heat stress, permanent organ damage and even death.

But how is heat measured in the occupational environment?

There are seven factors that define human thermal environments. The first four are external variables that effect our physiological response to heat. The last three change an individual’s ability to respond to a hot environment. They are:

1. air temperature
2. humidity
3. radiant temperature
4. air movement
5. physical activity (including work postures)
6. clothing
7. individual factors (i.e. acclimatisation, presence of medical conditions, pregnancy, fitness level etc).

All factors need to be taken into account when designing work that may occur in extreme conditions.

But let's say you do need to measure risk of heat in the workplace. 

Heat stress can be measured using various indices and physiological parameters to assess the level of thermal discomfort and potential health risks. Methods include:

1. Wet Bulb Globe Temperature (WBGT)

WBGT is a composite temperature that considers air temperature, humidity, air movement and radiant heat. WBGT is measured using a specialized instrument, such as the QUESTemp° that combines readings from a wet bulb thermometer, a dry bulb thermometer, and a black globe thermometer.

2. Heat Index

The heat index, also known as the "apparent temperature," combines air temperature and humidity to provide an estimate of how hot it feels. It is often used in weather forecasts to communicate the perceived temperature, but it may not consider other factors such as radiant heat and air movement.

3. Predicted Heat Strain (PHS) Models

PHS models use physiological parameters, clothing data, and environmental conditions to predict the physiological strain on the human body. These models consider factors like metabolic rate, clothing insulation, and evaporative heat loss to estimate the risk of heat-related illnesses.

4. Heart Rate Monitoring

Monitoring heart rate can provide valuable information about the physiological response to heat stress. An elevated heart rate may indicate increased stress on the cardiovascular system. Continuous heart rate monitoring using wearable devices or chest straps can offer real-time insights.

5. Core Body Temperature

Measurement of core body temperature, typically taken using oral, rectal or ingestible temperature sensors, provides a direct indicator of the body's response to heat stress. It's essential to note that measuring core temperature is invasive and often reserved for specific occupational or research settings.

6. Perceived Temperature Scales

Scales such as the Universal Thermal Climate Index (UTCI) and the Thermal Work Limit (TWL) consider both environmental factors and the subjective perception of individuals. These scales aim to provide a more comprehensive understanding of the impact of thermal conditions on human comfort.

It’s important to recognise that the above methods measure heat (or perceived heat), however risk associated with clinical heat-related disorders is highly dependant on the:

• tasks
• working environment, and
• individuals performing the work.

To manage risk associated with heat, a broad-based, participatory risk management program must be adopted, allowing for the application of multiple controls to ensure safe work, because prescriptive 'temperature limits' simply do not work.

Regular monitoring, proper training, and the implementation of preventive measures such as removal of heat sources, air conditioning and work scheduling to avoid the heat of the day are also crucial to managing and mitigating the risks associated with heat stress. 

Resources

• Working in Hot Conditions WRAC
• Working in Hot Conditions Risk Assessment Form
• Working in Hot Conditions Procedure