With summer rapidly approaching heat stress will again will rise to the top of the list of hazards to be aware of. As everyone should be, heat stress is a very dangerous condition and is particularly prevalent during physical activity in hot and humid working conditions.  During these working conditions, symptoms like heat rashes, heat cramps, heat exhaustion and ultimately heat stroke can manifest themselves.


As a reminder, the body continually generates heat that must be released in order to maintain the proper core temperature. The body’s natural sweating process and the eventual evaporation of the sweat, draws the energy needed to vaporize the moisture from the remaining sweat on the skin. This cooler moisture on the skin helps reduce the overall core temperature. This is called evaporative cooling.

There are four basic mechanisms to cool the body:

Radiation – heat from the skin is absorbed by the surrounding cooler air (normally 64% of the body’s heat release)
Conduction – heat pulled away by direct contact with a cooler object like water (normally 2%)
Convection – moving air (breeze, fan) can encourage evaporation and heat loss (normally 10%)
Evaporation – moisture on the skin’s surface (sweat) evaporates leaving cooler moisture and skin – evaporative cooling (normally 23%)


The fabric and the garment can have an effect on the ability to allow the body to cool itself but it is much less than one might think. The chart below shows how various types and weights of both FR and non-FR fabrics all fall into the heat stress zone with hot humid weather when performing heavy labor. The lighter weight fabrics do a bit better but at elevated temperatures what you wear is not the most critical factor leading to heat stress.

Other factors that affect the level and the speed at which heat stress can occur include:

Age – our ability to produce sweat decreases with age
Gender – men begin sweating at a lower temperature than woman
BMI – more fat insulates the core which increases sweat production
Hydration – not consuming enough water causing dehydration
Activity level – the more active the more heat generated and increased sweat production


There are numerous warning signs as heat stress begins to occur. Knowing the warning signs could allow a worker to take the appropriate steps to get the situation under control. Some of the warning signs are: headache, light-headedness, dizziness, unusual fatigue, irritability, confusion, nausea/vomiting, cramping and diarrhea. These signs manifest due to the body redirecting blood from internal organs and muscles to the skin in an attempt to shed the heat. Plus the sweating action means a loss of fluids and possible dehydration.


There are a number of actions that can be taken to help reduce the effect of heat stress including:

Rest – take regular rest breaks
Hydration – drink lots of water
Shade – take advantage of a shady spot when possible
Type of garment/fabric – wear light weight and light colored garments/fabrics that don’t trap in the heat and promote evaporative cooling
Limit alcohol consumption

 The two most effective ways of reducing heat stress are taking regular rest breaks and drinking plenty of water, particularly above 95°F and at high humidity levels. When it comes to the type of garment and fabric, their function in minimizing heat stress is one of allowing the heat to radiate off the body by not trapping in the heat (weight & weave) and also by helping move sweat off the skin and onto the surface of the fabric (wicking) where it can evaporate. As shown above, the attire worn indicates that single layer garments, in general, play a relatively minor or inconsequential role in causing or reducing heat stress; especially as compared to those more important practices of proper hydration and rest breaks. The information shows that commonly used single layer FR fabrics show little difference in their ability to provide the wearer with more or less heat stress relief in severe heat conditions.

Weight – the weight of the fabric is a factor in that the sheer mass of the fabric serves as insulation and can restrict heat from releasing away from the body. This is why a heavier fabric typically keeps you warmer in cold weather and lighter weight is typically cooler in hot weather.
Weave – the weave plays a role by allowing air flow (air permeability) to assist in cooling (convection) by picking up heat as well as helping to evaporate moisture (evaporation). It can also work in reverse if the air is hotter than the skin. In this case heat is transferred from the air to the skin.
Wickability – the fabric’s ability to absorb moisture from the skin and transfer it to the surface allows for evaporation and for evaporative cooling to take place. This only works well if the fabric easily allows the moisture to evaporate and doesn’t just stay wet


To minimize the potential for heat related problems this summer – plan work activities so workers can take regular rest breaks, drink plenty of water, and get out of the sun. Plus, know the warning signs so you can take the appropriate actions, before the situation gets dangerous.