Until someone collapses on your factory floor with heat stroke, you may not give much thought to the effects of working in heat. High temperatures and humidity challenge the body’s ability to control its internal temperature, and workers involved in strenuous activity and/or wearing hot protective clothing are at serious risk of the sometime fatal condition of heat stroke.

The body reacts to heat by increasing the blood flow to the skin’s surface, and by sweating. This creates a cooling effect by the evaporation of sweat from the skin and the radiation of heat carried to the surface of the body by increased blood flow. But in a humid environment, the moisture on the skin is unable to evaporate into air that is already saturated, and in high ambient temperatures the heat cannot radiate away from the body.

Chain reaction
If the heat produced deep within the body by the work rate cannot escape, core temperature will rise. The body sweats heavily in an attempt to cool itself and starts to dehydrate unless the liquid is replaced; the increased heart rate and blood flow put further strain on the body. Eventually the body’s control mechanism is overwhelmed and starts to fail.

This is the beginning of heat stress. Early symptoms include the inability to concentrate, muscle cramps, heat rash, severe thirst, fainting, headaches, nausea, giddiness and fatigue. It can lead to heat stroke and confusion, convulsions and loss of consciousness; in extreme cases, especially in the case of older workers or those with a pre-existing condition, it can lead to death.

Conduct a risk assessment
Where there is a threat of heat stress, a risk assessment should be carried out. Look at work rate and the working climate, including temperature, humidity, air movement and the proximity of heat sources. Review work clothing and PPE (personal protection equipment) such as respirators, and the worker’s age and medical factors which may affect an individual’s tolerance to heat.

The threat can be managed in a number of ways. Controlling the temperature with fans, air conditioning or barriers to radiated heat from a heat source will change the environment, and alterations to work processes and the use of mechanical aids may reduce the work rate. Regulating breaks or changing work patterns is important, as is reviewing the suitability of PPE in extreme conditions, encouraging workers to drink lots of water and training them on the risks of heat stress.

Measuring heat stress
An index called Wet Bulb Globe Temperature (WBGT) was developed specifically for use in settings where there was a need for a practical method of measuring heat stress; it has been suggested that the system was originated by the United States Marine Corps in the 1950s to predict and reduce heat stress injury amongst recruits under training.

It is an empirical index based on the exchange of heat between man and the environment, and combines parameters including the natural wet bulb temperature (tnw) and the globe temperature (tg), and in some situations the air temperature is also taken into consideration.

It is a complex formula, but simple portable devices measure all relevant parameters in the working environment and calculate the exposure to heat stress. Data can be recorded in real time and in accordance with ISO or OSHA standards, and values of WBGT then used to determine how long an individual can work in the area.

Measurements should be used to see if a problem exists by looking at the established reference values against the measured values. It is an easy and convenient method of measuring a body’s heat exposure without the need for invasive monitoring devices.

Work and rest regimes
A suitable Work Rest Regime (WRR) minimizes the effect of heat stress upon a worker, and different regimes of work and rest are adopted at specific levels of WBGT. The regime depends on the intensity of work and the metabolic rate of the worker. A heat stress survey will define the WRR, and this determines how long the worker can continue and how long a rest period they will need.