The primary causes of heat stress are poor hydration, lack of shade, and lack of rest breaks, not clothing. (Tyndale Company photo)

The Truth About Heat Stress and FRC

It would be a terrible shame to put American workers at risk of catastrophic or fatal burn injury because of an outdated myth about PPE and heat stress.

As we approach warmer weather, questions around comfort and heat stress become significant for people specifying or wearing flame-resistant protective apparel, so it’s important to understand a few basics on the interrelationship, if any, to fabric type and weight. Discomfort and heat stress are not used interchangeably. Comfort is an inherently subjective characteristic that cannot be effectively or predictably measured by lab tests or judged across a desk, and is a nuisance issue. Heat illness, however, is a measureable physiological condition with potential medical consequences, and it is (perhaps surprisingly) unrelated to comfort.

Studies have clearly shown the primary factors correlating to comfort are choice, fit, and brand/style, not fabric. No single fabric property, such as weight, breathability, wickability, or the way the fabric feels, or "hand," correlates to comfort. And combinations of three or more properties are only marginally more predictive. Despite popular perception, comfort is not linked to weight; the significant majority of wear test participants consistently rate many garments as more comfortable despite higher weights.

Combining properties, such as light weight and high breathability, for instance, are meaningless in the absence of other properties (a window screen has excellent numbers in both categories). These only begin to matter when combined with a soft hand and excellent moisture wicking, and even that data ignores the Big Three factors of choice, fit, and brand/style. Comfort differs greatly from person to person and from day to day, and generalizations cannot be made from any single property. For much more detail on comfort and FRC, please refer to the article "Is Comfortable FRC an Urban Myth?" on page 30 of this magazine's March 2019 issue.

Heat Stress
Heat illness is a potentially serious health concern, not a perception issue; it is a series of measurable and increasingly dangerous physiological affects resulting from the overheating of the body and the cascade of effects on organs and systems. This overheating can be caused by hot and/or humid ambient conditions, or by metabolic heat generated by hard physical work, or both. Heat stress has a number of stages, and timely and appropriate intervention can stop or reverse the progression.

Best practice dictates taking preventative steps to avoid the occurrence of heat stress altogether. But in the event heat stress takes hold, early intervention is important. In the early stages, this intervention is easily achieved by simple things and without the need for medical professionals. As the illness advances, risk and consequence rise, and more complex intervention, often requiring medical 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 garments such as rainwear, chemical splash PPE, and other impermeable 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 clothing 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 mechanism 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 evaporation is basically how air conditioners work). Water is much more efficient at transferring heat, which is why you can stay in a 70-degree 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 interrupted 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 and arc-rated garments on the market today, so there is no practical difference between FRC and street clothing when it comes to heat stress. However, as noted above, impermeable garments and multiple-layer garments DO impede radiation and evaporation, and thus can be a contributor to heat stress.

The primary causes of heat stress are poor hydration, lack of shade, and lack of rest breaks, not clothing. Workers who arrive well hydrated and rehydrate during rest breaks of appropriate duration and frequency, taken in the shade, are extremely unlikely to suffer heat illness. In fact, the new OSHA awareness  campaign on heat stress prevention is “Water, Rest, Shade.” The hydration is of course linked to sweating. The rest breaks are to allow the body to shed metabolic heat created by physical work, and the shade is both cooler and eliminates the sun as a radiant heat load. When clothing is addressed, it is far down the list of issues, and NIOSH and OSHA advice is:

Notice it's light color, not light weight, that actually makes a difference. Dark colors can be hotter than light colors by several degrees. A worker can feel comfortable and yet at be at risk for heat stress, or uncomfortable but not at risk, because fabric type and weight (FR or not, short sleeve or long, etc.) are either found to be unrelated, or inconsequential in comparison to the primary factors of hydration, rest breaks, and shade.

The significant majority of single-layer flame-resistant and arc-rated garments commonly sold in the USA today are now virtually indistinguishable from their non-FR counterparts in design, style, fit, weight, moisture management (wicking), breathability, etc. They pose no more risk of heat stress than whatever non-FR garment would otherwise be worn at work. In fact, as a category, single-layer FR garments may pose LESS risk of heat stress because they are long sleeve, while many non-FR choices are short sleeve.

The idea that FRC causes heat stress originated more than 30 years ago when we had essentially two choices in FR fabric; one didn't wick sweat while the other didn't breathe well, and neither would ever be mistaken for standard clothing. This misconception persists today out of lack of information. It would be a terrible shame to put American workers at risk of catastrophic or fatal burn injury because of an outdated myth about PPE and heat stress.

This article originally appeared in the May 2019 issue of Occupational Health & Safety.

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