Foundries to Fulfillment Centers: Thermal Physiology and Indoor Heat

What’s the most thermally challenging occupation you can imagine? Perhaps a road crew laying asphalt under the Texas sun, or a roofing team racing through summer to keep pace with the construction schedule at a new Florida subdivision. Maybe more extreme, like a “hot shot” wildland firefighting crew battling a blaze in southern California, or more mundane, like the ball girl who fainted at a match during this year’s Australian Open.

These scenarios have one thing in common: they occur outdoors. We mostly discuss heat stress as a challenge faced outside, where high temperatures are compounded by sun exposure. This narrow view of heat stress overlooks indoor occupations where workers routinely encounter heat stress.

Heat stress can be just as dangerous for indoor workers as it is for those outdoors. In this article we explore indoor occupations where workers are commonly exposed to heat stress, highlighting thermal risks those workers may face.

Basic Indoor Heat Physiology

First, let’s review heat transfer methods from the body: conduction, convection, radiation, and sweat evaporation. Through a combination of these four methods, the human body is constantly exchanging heat with our surrounding environment.

We’ll ignore conduction for now; it requires direct contact with a hot or cold surface, so typically provides negligible thermal transfer in workplace settings (unless you are wearing a frozen ICEPLATE®!). 

For our purposes, radiation and convection are conceptually similar. In both, heat is transferred from the hotter to colder object. They differ because radiative heat transfer flows through, well, radiation. Being energy on the electromagnetic spectrum, it doesn’t require a medium like air or water.

Convection requires a fluid like air (from a physics standpoint, air is a fluid). If the air surrounding you is cooler than skin temperature (~95^oF), heat flows away from your body. If the air is warmer than your skin, thermal energy is added to your body. Since convection requires air to transfer heat, the rate of heat transfer increases if air is constantly flowing across the skin.

Sweat evaporation is the body's most powerful natural cooling tool and also benefits from moving air. As sweat accumulates on the skin surface, the thin boundary layer of air immediately adjacent to the skin becomes saturated with water vapor. Without air movement to displace this layer, sweat evaporation slows, and sweat’s amazing cooling effect is reduced. The same air movement that increases convective heat transfer also blows away this saturated boundary layer, promoting further sweat evaporation.

Since hot air rises (the principle that allows hot air balloons to fly and afternoon thunderstorms to form), some small amount of air movement occurs across the skin even when we are standing still. However, even small increases in air flow across the skin surgace - airflow rates greater than 0.2 m/s, or somewhat slower than a slow walk, start to assist convective and (usually) evaporative heat transfer.

With this in mind, we can immediately see two scenarios where indoor heat stress may be problematic: workplaces with large radiative heat loads generated from hot equipment or hot surfaces, and workplaces where air flow is minimal or non-existent.

Indoor Occupations and Radiant Heat: Hot Equipment

Some indoor occupations are thermally challenging because work just can’t be done without heat. Industrial furnaces and smelting operations must exceed the melting points of metals to refine and shape elements like steel, copper, or aluminum into end use products. Workers in these facilities experience perhaps the most extreme example of indoor heat stress; OSHA even uses the heat stroke death of a foundry worker among its heat safety case studies.

For example, at a Texas aluminum smelter – where over 1,800^oF is required to turn alumina into pure aluminum – researchers analyzed heat strain in 60 workers over four days in July. Most had at least one sign of excessive heat strain. 88% of the unacclimatized workers had body core temperatures exceeding the recommended limit (100.4^oF); one-in-five of the more seasoned workers exceeded the higher core temperature limit (101.3^oF) recommended for acclimatized workers.

Extreme temperatures in metal foundries occur largely because of radiant heat load. Without going too far into the science (I promise), all objects emit energy as electromagnetic radiation. The rate of emission, called radiant flux, is governed by the Stefan-Boltzmann Law. This law states that radiant flux scales with the fourth power of an object's temperature.[1]

In practical terms, this “fourth power” scaling means that as a furnace heats up, the radiant energy emitted increases faster than the temperature rise alone would suggest. At the thousands-of-degrees temperature required to melt metals, massive amounts of radiant energy are released — radiant energy that is transferred as heat to nearby workers. The reflective silver suits used in these industrial settings are designed to keep workers safe by reflecting most of this radiant energy.

Foundries are extreme examples of workplaces with large radiant heat loads (glass manufacturing – such as the plant described in this OSHA heat stress report – require similar thermal conditions). However, any occupation involving hot equipment must contend with radiant heat. Boiler room workers are often noted as a population at elevated risk of heat stress. Paper and pulp and rubber production use heat in the manufacturing process; one tire manufacturing plant (which requires ~350^oF to cure rubber) recorded indoor thermal conditions just above 82^oF – on a brisk, 37^oF December day. Even food processing, if steaming or drying is part of the production process, can expose workers to large amounts of radiative heat stress.

Bakery and industrial kitchen workers also face risk of heat stress exposure due to heat radiating off ovens and hot kitchen surfaces. Here, we’re talking cooking on a large scale – not your home kitchen with a single oven and a few stove burners. In evaluating heat stress by job type on surface ships, the U.S. Navy uses a Physiological Heat Exposure Limits table, with curves calibrated to work rate and thermal environment, to determine safe exposure times. Surprisingly, scullery personnel (those running steam-fed dishwashing equipment) are assigned one of the more restrictive shipboard exposure limits.

 Indoor Occupations and Airflow: Cool A/C and Stifling Heat

Of course, most indoor occupations don’t involve working around hot equipment; many are conducted inside air-conditioned buildings. Workers in sectors where the environment is highly regulated aren’t generally considered at risk of heat illness – I’ve yet to come across a heat safety paper discussing the effects of hot workplace temperatures on bankers, librarians, or office clerks.

However, there are plenty of examples of indoor workplaces where a lack of air conditioning puts workers at risk of heat stress. An obvious example is your own house when the A/C system fails. To keep the rest of us cool, HVAC techs sometimes squeeze into hot, poorly ventilated attics and utility spaces, often at the height of summer – the kind of underappreciated service work we rely on to live comfortably.

This HVAC scenario is what led to the heat stroke death of Matt Nelson, an HVAC company owner, on a 96^oF July day near Prescott, Arizona area. Temperatures in the attic where he was working were reported to be around 150^oF.

Another increasingly common (and litigated) example is warehouse workers. Amazon disputes that a recent death at a New Jersey warehouse was heat related, but employees tell a different story, and the company installed a new industrial air conditioning unit shortly after the incident. At an Amazon fulfillment center in California, workers staged a walkout, citing unsafe working conditions where temperatures often exceeded 95^oF. In a third incident, an OSHA investigation found a third-party contractor delivering Amazon packages in Texas died due to extreme heat conditions in his delivery truck.

While Amazon has recently been a high-profile defendant in heat related worker safety litigation (and, unsurprisingly, the company is quick to point to its heat mitigation programs), other employers face similar challenges. In Memphis, Tennessee, coworkers attribute an employee's death to heat in a non-air conditioned section of a Kroger distribution center. Temperatures that day reached over 100^oF. The local Teamsters union leader had raised concerns a month earlier, specifically requesting extended breaks to accommodate employees cooling off.

Workplace heat illness, including heat-related fatalities, is generally considered underreported, but cases appear in medical research and workplace records if you know where to look. One of the clearest examples is a CDC report covering 2012-2013. Heat illness and fatality cases resulting in employer safety citations during those years includes multiple employees at manufacturing and commercial laundry worksites.

More recently, a 2025 study analyzing injuries reported by large employers (companies >100 employees) highlights work injuries increasing with rising heat, specifically including work that is “predominantly indoors”. Considered by employment type, manufacturing, warehouse, and transportation employees (i.e., typical indoor occupations) were less affected by extreme temperatures than outdoor workers but still experienced an ~30% increase in injuries on the hottest days.

Too Hot Even For Fans

The modern miracle of air conditions allows us to live and work in places we otherwise would find intolerable. In fact, researchers estimate heat related deaths on extremely hot days declined an amazing 80% between the first and second half of the 20^th century, attributed almost exclusively to air conditioning use.

The tradeoff is that air conditioning is energy intensive, and therefore expensive. Many workplaces remain dangerously hot, either because air conditioning use is impractical (e.g., loading docks), partially present (e.g., warehouses where only some sections are chilled), or economically impractical (e.g., areas of the country where extreme heat is relatively infrequent, so industrial units are not installed).

When air conditioning isn’t available, we often use fans. In most situations, fans are a great heat safety measure. For a fraction of the energy required to run A/C, fans promote convective heat transfer and sweat evaporation by blowing away the hot, sweaty boundary layer of air at our skin surface.

Unfortunately, fan effectiveness has a very important limit. Our skin surface temperature is generally maintained around 95^oF. Since convective heat transfer operates by moving thermal energy from hot to cold, if the ambient temperature exceeds 95^oF, air blown by a fan doesn’t draw away heat from the skin surface and instead delivers heat to it! The effect is similar to a convective oven, and the opposite of what we’d want to achieve if trying to stay cool.

Fan use can still be beneficial above 95^oF if it can help promote sweat evaporation. In dry conditions (i.e., low relative humidity levels) and assuming we remain hydrated enough to continuously sweat, fans replace the sweat saturated air at skin surface with drier air, resulting in net heat loss even at temperatures above 95^oF. This doesn’t work in highly humid conditions, since air blown by fans is already approaching moisture saturation and doesn’t do much to assist sweat evaporation from the skin.

Researchers have yet to agree on exactly when temperature and humidity levels combine to switch fan use from beneficial to burdensome. Recommendations vary widely. In the U.S., the CDC recommends indoor fan use only when temperatures are below 90^oF, while the World Health Organization advocates for fan use at much higher temperatures (up to 104^oF). The British Health Security Agency splits the difference, advocating for fans during temperatures under 95^oF.

The Qore Performance Advantage

When indoor temperatures rise and A/C isn’t an option, we need another method to stay cool. Earlier, we dismissed conductive heat exchange because it’s often not practical. Conductive heat exchange requires keeping something cold against your body to directly draw away heat. This is exactly what Qore Performance IcePlate Exo®, IcePlate IMS®, and IcePlate® Drivers Harness are designed to do.

Conductive heat exchange doesn’t rely on radiation or convection to cool you down, so it’s not affected by ambient temperature. All that is required is keeping a colder object – the frozen IcePlate® - against your body. Regardless of the heat, indoors or out, IcePlate® keeps you cool!