Heat stress is an increase in human body temperature and metabolism caused by physical exertion and/or a heated environment which can lead to exhaustion, mental confusion, disorientation, dehydration, loss of consciousness, heart attack, stroke and other fatal illnesses.
Heat stress results from internal, metabolic heat buildup (from working in bunker gear, for example) and external stress related to environmental factors, such as personal protective equipment. As the core temperature rises, so does the risk of heat stress. Performing strenuous tasks in the heated environment of a fire scene or in warm or humid weather can also increase the risks of heat stress.
Simple preventative measures can be taken to avoid heat injuries, including drinking fluids frequently throughout the day to stay well-hydrated and wearing a single layer of porous cotton under protective gear to keep the least amount of heat from becoming trapped near the body.
The rate at which people sweat is determined not only by genetics, but by hydration, state of acclimation and aerobic fitness. You can’t sweat if your body doesn’t have enough water. In order to maintain normal body function, fire fighters must replace fluid as soon as possible.
Acclimation is a physiological adaptation that the human body makes with repeated exposures to heat stress during exercise. It increases our rate of sweat production and shortens the time it takes for the sweating response to start and conserves sodium. Regular and sustained aerobic exercise can help with acclimation. Fire fighters who maintain an adequate level of fitness will have reduced cardiovascular strain and lower core temperature for the same level of heat stress. Fit fire fighters also tend to have reduced levels of body fat – and aren’t carrying extra non-functional weight. Therefore, less energy is required by a fit person to do the same job as a less-fit person.
It is important for fire fighters to acclimate themselves to heat and know how to prepare for the summer weather. If sweat cannot evaporate, it doesn’t matter how fit, how acclimated or how hydrated you are -- thermo-regulation will be compromised. In addition, it is essential that fire fighters are aware of the signs and symptoms of heat stress in order to detect it early and take the appropriate measures.
Heat Stress Symptoms
At first sign of symptoms, fire fighters should notify the officer in charge and immediately: institute work/rest cycles; keep cool and avoid radiant heat; drink small amounts of the appropriate fluids; avoid coffee, tea and alcoholic beverages; and use water spray bottles, fans and damp towels.
Some predisposing factors to heat stress include sustained exertion in the heat by unacclimatized workers; lack of physical fitness and/or obesity; recent alcohol intake; dehydration; individual susceptibility; chronic cardiovascular disease; and failure to replace water lost in sweat.
To prevent heat stress, follow these guidelines:
Fire fighters also need rehabilitation to ensure they can safety return to active duty following a work rotation. Measure the heart rate on each emergency responder (this can be measured by the worker himself) at the end of the work period. An effective rehabilitation program must include:
- Provide medical screening of fire fighters.
- Acclimatize for five to seven days by graded work and heat exposure, monitoring workers during sustained work in severe heat.
- Drinking ample water frequently throughout the work day.
- Ensure adequate salt intake with meals and supplement salt intake at meals for unacclimatized fire fighters.
- Provide cool sleeping quarters to allow skin to dry between heat exposures.
Glossary of Terms
- Rest: a “time-out” to help fire fighters stabilize vital signs.
- Rehydration: replacing lost fluids/plasma volume.
- Restoration of core temperature through “active cooling” (warming).
- Medical monitoring and treatment.
- Relief from extreme climatic conditions (heat, cold, wind, rain).
- Refueling of calories and electrolytes.
Heat Index combines air temperature and relative humidity to determine an apparent temperature – or how hot it actually feels. High heat-index days can be health and life threatening, even to the non-exerciser.
Humidity is the amount of moisture in the air. Humidity is of particular concern to those whose primary cooling mechanism is perspiration evaporating. It’s the evaporation of that perspiration that causes some cooling effect, not the process of perspiring itself. In other words, in water-vapor-saturated air (high humidity), there is no evaporation of perspiration, and therefore, our principle cooling mechanism is not functioning for us.
Dew point is the temperature at which a vapor begins to condense. Dew points are sometimes reported or used rather than relative humidity. Beware of dew points above 70.
The table below can be used to estimate the heat index. The heat index is an accurate measure of how hot it really feels when the effects of humidity are added to high temperature. When the heat index is between 90° F and 104° F, sunstroke, heat cramps or heat exhaustion are possible with prolonged exposure and physical activity. When the index is between 105° F and 129° F, sunstroke, heat cramps or heat exhaustion are likely and heatstroke is possible. Heat indices of 130° or higher will result in heatstroke or sunstroke quickly.
The heat index (HI) is an accurate measure of how hot it really feels
when the effects of humidity are added to high temperature.
The heat index is shown below in two tables. The first is a function of temperature/relative humidity (RH). The second is a function of temperature/dew point.