PPE and Heat Stress: The Thermal Cost of Protection

Everyone knows clothing can keep you warm. Unless you live in Florida or Hawaii, before heading outside this February you probably threw on a jacket and long pants, maybe also (if you live up north) thermal underwear, hat, and gloves. These garments insulate you, preventing convective and radiative heat loss (or conductive heat loss, if you’re making snowballs or handling cold surfaces). In hot weather, the reverse is true – we generally wear less clothing because removing layers between our skin and the environment promotes evaporative heat loss. No one wears a ski jacket to Daytona beach in the summer! 

Unfortunately, we can’t always wear less clothing whenever it’s hot. In fact, the opposite is often true. Many jobs require a minimum amount of occupational clothing or personal protective equipment (PPE) be worn no matter how hot the environmental temperature. PPE is often a safety requirement - in some professions, PPE can mean the difference between life or death - but in hot environments PPE often elevates the risk of dangerous levels of heat stress.

A classic example of heat-stress inducing PPE is turnout gear worn by firefighters. Designed to protect the firefighter from the extreme heat and toxic environment at a fire scene, turnout gear is, by design, highly insulating. Unfortunately, firefighting is strenuous work, and the insulating properties of turnout gear means heat from the firefighter’s body can’t easily dissipate into the environment. Insulation works in both directions; the turnout gear doesn’t just shield the firefighter from extreme temperatures from an inferno but also traps metabolic heat generated by the firefighter.

Firefighting gear and similarly encapsulating PPE like HAZMAT suits[1] are the sort of PPE that place a wearer at much greater risk of heat stress. Occupations requiring ballistic protection also place individuals at relatively greater risk of heat stress. Personal body armor is typically thick, non-conductive ceramics and tightly woven Kevlar, often weighing many pounds. I’ve sweated under more than 25 pounds of armor at times, not counting helmet, weapon, or other gear. Body armor not only prevents heat exchange but also increases the wearer’s metabolic heat production simply from the physical effort of wearing the additional weight.  

Other occupations, from welders and metal workers (wearing leather aprons, helmets, and gauntlets or gloves) to workers on construction sites (hard hat, long pants and sleeves, fall protection harness) and airport ground, utility repair, and city maintenance crews (various PPE requirements, from safety-toe boots and long pants and to specialty layers including reflective vests, harnesses, and Tyvek suits). In general, the more impermeable the vapor and airflow transmission, the less a wearer’s clothing and PPE allow heat exchange.

Heat Exchange Through PPE

Occupational safety researchers use equations to estimate how much (or little) heat exchange is modified by clothing and PPE. A variable called “Clo” represents how well clothing or PPE thermally insulates convective or radiative heat exchange.  A 1 Clo insulation value keeps the average resting person comfortable (i.e., thermally neutral) at 70^oF conditions[2]  and was originally based on the typical business suit.

The National Institute for Occupational Safety and Health (NIOSH) publishes Clo values for common clothing ensembles. Unsurprisingly, a t-shirt and jeans ensemble has a relatively small Clo value (~0.75). With a Clo less than ‘1’, a resting person wearing jeans and a t-shirt loses heat to the environment at 70^oF. Adding a light jacket and long sleeve shirt increases Clo to 1.0 (thermally neutral). Layering on a heavy  jacket increases Clo to 1.25, so our resting person warms up, retaining heat faster than it is dissipated to the environment.  

Clo values only consider convective, radiative, and conductive (i.e., ‘dry’) heat transfer; in hot environments, us humans primarily dissipate heat through the evaporative cooling power of sweat. Accounting for this requires another variable - the moisture permeability index, abbreviated (i_m). An i_m value of ‘1’ implies no barrier to sweat evaporation while a value of ‘0’ implies complete impermeability and no moisture evaporation.

A quick aside - approaching i_m of ‘1’ is possible only with very light clothing in high wind (or by being naked, but this is an article on occupational clothing!). A ‘0’ value is similarly improbable even in completely impermeable overgarment. Some very small amount of sweat will still evaporate off a wearer’s skin, even if just to condense on the inside of the impermeable fabric; the value 0.08 is often used instead of zero for impermeable oversuits due to the internal evaporation-condensation cycle that effectively transfers some heat across the vapor barrier.

There’s a final step in estimating how clothing and PPE affect heat transfer. Clo and i_m – the insulation and permeability variables – are used together in a ratio: i_m/Clo. The smaller the ratio, the less heat can be dissipated by sweat evaporation. Small ratio values are obtained either by high Clo values (the denominator) or small i_m values (the numerator). The practical effect is that highly insulating clothing – a high Clo value - prevents sweat from evaporating even if made from a breathable material (think a thick cotton sweatshirt), while a thin, non-insulating but impermeable material has the same heat transmission effect (think Tyvek suit).

On the Job

Despite comprising only around 6% of the U.S. workforce, construction workers account for 36% of all occupational-related heat deaths. Unsurprisingly, a summer 2023 study of construction workers in Kansas City and road workers in Pecos, NM, found 43% of Kansas City workers and 29% of the Pecos workers exceeded the recommended safety threshold of core temperatures above 100.4^oF (38^oC). What, if any, effect does their clothing have on their heat stress?

There are a handful of studies attempting to answer that question. One example is research, published by Dr. Bernard and colleagues from the University of South Florida and U.S. Air Force, that quantified how different protective clothing ensembles affect workers' ability to tolerate heat. Using a protocol where temperature was gradually increased until workers could no longer maintain thermal equilibrium, they established 'clothing adjustment factors' - the number of degrees to add to the environmental heat index to account for PPE burden.

Dr. Bernard and team found wearing standard work clothes, like coveralls and basic PPE, added minimal thermal burden to workers from their insulative properties (i.e., Clo value). With no excess thermal insulation burden, standard work clothes adds zero degrees to the temperature index.[3] Instead, thermal stress came from working under the sun and the reduced ventilation from hard hats and vests - factors not captured when only considering insulation.

On the other hand, the study found vapor-barrier suits – completely impermeable protective clothing such as worn during chemical spill response – imposed a massive heat burden. Workers wearing such suits experienced the same heat stress as workers in standard clothing even when environmental conditions were 18°F cooler! Put another way, the vapor-barrier suit added 18^oF of heat stress to the worker.

In between those two extremes, the study found PPE ensembles like Tyvek adds about 2-4.5^oF of thermal stress, while “vapor-permeable water barriers” (i.e., rainproof jackets made out of Gore-Tex or similar material) added about 4.5^oF. Critically, these adjustment factors remain constant regardless of work intensity - whether doing light maintenance or heavy physical labor, the clothing imposes the same relative heat burden. The implication is the thermal burden created by occupational clothing (aside from standard work clothes) is an inherent penalty reducing the safe environmental temperature workers can tolerate regardless of work activity.

Such clothing adjustment factors are corroborated by other studies. A Japanese study using thermal manikins found typical construction clothing increased thermal insulation by 25-40% compared to light indoor clothing. Along with hard hats, which researchers found significantly reduced convective heat loss from the head, construction PPE was estimated to add just over 4.5^oF to a worker’s perceived thermal stress. Another study found heat burden can be even greater – up to 14^oF if working under direct sunlight – due primarily to the hard hat.  

On the Battlefield

With operations dictated by necessity and enemy, not weather or climate, military personnel have always been a population uniquely at risk of heat stroke. Likewise, military specific PPE – from the Spartan’s bronze helmet, breastplate, and greaves to a modern soldier’s integrated head protection system and plate carrier – has been designed for protection, not thermal comfort. Soldiers in combat operations cannot shed their protective gear regardless of environmental conditions, creating the potential for what physiologists call "uncompensable heat stress" – conditions where the body cannot maintain thermal equilibrium even at rest.

To assist the warfighter, researchers at places like the U.S. Army Research Institute of Environmental Medicine (USARIEM) explore the thermal burden imposed by modern body armor. A 2015 study of the flame resistant Army Combat Uniform ensemble found it measured at 1.37 Clo with a moisture permeability index of 0.45. Adding body armor (in this study, the Improved Outer Tactical Vest) increased insulation to 1.57-1.63 Clo and reduced moisture permeability to 0.35-0.42; the armor created a 15-19% increase in thermal insulation and a 22% reduction in evaporative cooling capacity. Heat strain modeling with these thermal values predicts a soldier wearing body armor in hot/humid conditions will reach critical core temperature limits about 24% faster than soldiers without armor.

More generally, U.S. military heat stress recommendations include adjustment factors to heat categories due to PPE. Heat categories, which start at a WBGT thermal index of 78^oF for the lowest and least impactful “white flag” category, extend up to 90^oF for “black flag” category. These thermal thresholds are used to recommend work/rest cycles and hydration to prevent heat injury. Outdoor training, especially physically strenuous training, is often reduced or rescheduled when “black flag” conditions are reached.

Historically, 5^oF was added to the thermal index if body armor was worn during training, although recent research has found the adjustment should actually be higher if armor is worn during moderate and heavy work. The current military heat stress manual recommends adding 10^oF to the environmental thermal index when wearing heavy protective clothing (like chemical protective gear) and performing easy work. This adjustment when performing heavy work is 20^oF, meaning “black flag” heat stress conditions can be experienced due to the thermal burden imposed by PPE even if the thermal index is no warmer than 70^oF!

Qore Performance – Lowering the Thermal Cost of Protection

Understanding how occupational clothing and PPE limit the body's ability to dissipate heat underscores why cooling solutions like those developed by Qore Performance – which work with existing PPE requirements -  provide essential risk mitigation and boost human performance, workplace safety, and operational effectiveness even under the most severe thermal stress.