Page 29 - Occupational Health & Safety, May 2019

Best practice dictates taking preventative steps to avoid the oc- currence of heat stress altogether. But in the event heat stress takes hold, early intervention is important. In the early stages, this inter- vention is easily achieved by simple things and without the need for medical professionals. As the illness advances, risk and conse- quence rise, and more complex intervention, often requiring medi- cal professionals, is required. Let’s take a look at the primary causes of heat stress and the role of clothing in general, and flame-resistant or arc-rated clothing in particular.
There are a number of excellent resources readily available, and most don’t require a biology degree to understand. A Google search of heat stress or heat stress + clothing will turn up dozens of hits from OSHA, CDC, and NIOSH, along with many academic papers. While many of these documents have slight differences on some peripheral issues, there are several basics that are common throughout and are considered bedrock heat stress principles.
The primary causes of heat illness are:
■ Poor hydration
■ Lack of rest breaks
■ Lack of shade
■ Overall health/some medications
Did you notice that clothing is not on that list? That’s because single-layer, breathable clothing is NOT a significant factor in causing heat stress. This is true whether the clothing is FR, AR, or non-FR, whether the garment is very light or standard weight, and whether long sleeve or short sleeve. In fact, long sleeves are
actually safer for heat stress because the sun is a radiant heat load and long sleeves shield the wearer. So what kind of clothing does play a significant role in causing heat stress? Non-breathable gar- ments such as rainwear, chemical splash PPE, and other imperme- able barriers, as well as multiple-layer garment systems such as fire service bunker gear, high calorie arc flash suits, and other bulky layered garments.
If it seems hard to believe that single-layer, breathable cloth- ing doesn’t cause heat stress, a closer look at how the human body sheds heat will explain.
We have two ways to cool ourselves: radiation and evaporative cooling. Radiation is simply the movement of heat from a hotter area (the person) to the cooler area (the atmosphere). Of course, as the ambient temperate approaches that of the person, this mecha- nism ceases to be effective. Evaporative cooling (a fancy way to talk about sweating) then begins to kick in. When sweat is evaporated off of your skin, it takes lots of heat with it (latent heat of evapora- tion is basically how air conditioners work). Water is much more efficient at transferring heat, which is why you can stay in a 70-de- gree room all day, but a 70-degree pool will have you shivering in less than an hour. Thus, as long as the clothing being worn allows radiant heat to escape and allows sweat to be carried to the surface and evaporated, the body’s cooling mechanisms have not been in- terrupted and function as they should.
There is no practical difference in these critical areas between non-FR “street” clothing and the vast majority of flame-resistant
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