Heat Stress – A Threat to Health

Until someone collapses on the quarry or factory floor, little thought may be given 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 sometimes fatal condition heat stroke.

The body reacts to heat by increasing 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. 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. Eventually, the body’s control mechanism is overwhelmed and starts to fail.

Early symptoms of heat stress 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 it can lead to death.

Where there is a threat, a risk assessment should be carried out to look at the work rate and the working environment, including temperature, humidity, air movement and the proximity of heat sources. It should review work clothing and PPE (personal protective equipment) such as respirators, and the worker’s age and state of health should also be considered.

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 reducing work intensity with the use of mechanical aids will also help. Regulating breaks or changing work patterns is also important, as is reviewing the suitability of PPE, encouraging workers to drink lots of water and training them in the risks of heat stress.

An index called wet-bulb globe temperature (WBGT) was developed by the US Marine Corps to reduce heat stress injury among recruits under training and is used to quantify the risk. A complex formula using the WBGT and other readings can determine the likelihood of heat stress, but now a simple, portable device called the Microtherm WBGT from Casella can measure all relevant parameters in the working environment and calculate the exposure to heat stress, and determine how long an individual can work – and rest.

A suitable work–rest regime (WRR) minimizes the effect of heat stress on a worker, and different regimes of work and rest can be adopted according to conditions. The regime will be dependant upon the intensity of the work and the metabolic rate of the worker; a heat stress survey will define the WRR and determine how long the worker can continue and the pattern of breaks they will need to adopt.