Physics 101

These are not scientific definitions but are included for the intent of allowing one to better understand potential phase change cooling vests.

Heat always travels to cold. Understanding this basic principle is important to being able to understand basic body cooling.

Convective cooling is when heat is exchanged via air. Common examples include refrigerator cooling and air conditioned room cooling.

Conductive cooling is when heat is exchanged via physical contact with a cooling substance. Common examples include water immersion (jumping into a cold swimming pool) and ice pack therapy for medical purposes.

Evaporative cooling is when heat is exchanged via energy generated by the change of phase of a liquid to a gas. Common examples include body sweat evaporating during physical exertion and misting fans.

The most energy effective cooling process is conductive heat transfer. To help illustrate this point, imagine being very hot and needing to cool down quick, you have the choice of:

1. Walking into an air-conditioned room with a temperature of 65° F. (Convective)
2. Standing in front of a misting fan. (Evaporative)
3. Jumping into a swimming pool at 65° F. (Conductive)

The choice is obvious; your body would sharply lower it’s core temperature if you stayed in the pool too long. Also consider the fact that you can put your hand into the freezer and keep it there for quite a long time even though the air temperature is around 20° F. Now see how long you can keep your hand in a bucket of ice water with a temperature around 38° F. Easy to understand and illustrate.

Evaporative cooling is the least effective of these due to the fact that the energy exchange is very low. In the above scenario, you would choose the misting fan as the last option.

ΔT (pronounced delta t) is quite simply the difference in temperature of 2 relateditems. Perfect example is the ΔT of an outside temperature of 100° F and an air-conditioned room of 65° F would be termed as “the ΔT of 35° F”.

How The Body Cools

It is important to understand how the body naturally cools so that you can help people understand how artificial body cooling fits in. For the sake of argument, we will quote general and not precise temperatures.

The human body is arguably the most efficient heat pump known to man. Our core temperature maintains an approximate 98° F temperature consistently. This core temperature is maintained and largely created within the cardiovascular cavity which resides mostly inside the chest cavity. The blood circulates in and out of the cardio-cavity transferring blood to the outer capillaries exchanging heat via conduction to the skin. The skin transfers this heat to the outside air via convection. In higher heat environments and/or high levels of activity causing more heat to be generated internally, the body will produce sweat to evaporate the added heat away from the body. As you can see, the human body incorporates all 3 heat exchange processes mentioned above with great precision.

Focusing on the heat transfer from the body to the ambient environment, one must consider the fact that human skin temperature is actually around 91° F. This creates a ΔT of about 7° F from the core to the skin which is enough to keep a continual “trickle” cooling of the body as long as the heat is transferred from the skin to the environment. For this to occur optimally, the effective air temperature must be around 75° F offering a ΔT of about 16° F. One way to visualize this factor is to consider what temperature you are “comfortable” at when in an air-conditioned room. We are most comfortable when our bodies are giving off heat freely to the environment without the addition of “auxiliary” evaporative cooling (sweat). If you decrease the ΔT between skin temp and the environment by raising the environment temp level, the body must resort to sweating to maintain core temp levels. Most people start to sweat in an environment a little over 80° F because there is no longer enough ΔT to extract heat from the body effectively.

Vasoconstriction is a condition that must be considered when artificially cooling the body. Vasoconstriction is a body’s physiological response to extreme cold and is dangerous when the body is trying to get rid of heat. It has been scientifically proven that applying conductive or convective cooling at temperatures below 55° F to the human body will cause the capillaries closest to the skin to constrict, cutting off blood flow to that area effectively insulating the body from the perceived cold climate. This is extremely dangerous and has been known to elevate a person’s core temp even though they are wearing an artificial cooling system. Ice vests cooling systems are most commonly associated with this dangerous condition.

Acclimation is a condition in which the human body adapts to a certain temperature level. The body accomplishes this acclimation by increasing sweat and by opening capillaries closest to the skin earlier than would normally be expected. There is an extreme amount of controversy regarding acclimation within the safety community when it comes to workers. It is best to keep the explanation as simple as possible. To illustrate true acclimation, consider roofers. Even though they are exposed to the harsh environments during the summer, they have acclimated mostly because they do not wear restrictive safety apparel. It is impossible for a worker donning restrictive PPE to ever acclimate as that microclimate environment can exceed 140° F within an hour. No physiological response performed by the body can overcome that extreme environment. Acclimation is also extremely subjective to the worker and even then, consideration must be given to that worker’s day to day health conditions.

Role Of PPE In Natural Body Cooling

So, for the human body to cool effectively on it’s own, the ambient environment shouldn’t be above 80° F and a large percentage of the body’s skin should be exposed to the air without being exposed to an external heat source like the sun. This probably describes most “white collar” jobs. For everyone else, artificial cooling should be considered. The most effective way to control body temperature is always to control the applied environment. Workers performing tasks in an air-conditioned environment will almost never need any further artificial cooling assistance provided the worker’s bodies are not “cut-off” from the controlled environment via personal protective apparel.

The wearing of PPE like tyvek coveralls, fire retardant coveralls, Level A / Level B and any other personal protective apparel which prohibits air flow over skin creates what is known as a “microclimate” environment. A microclimate environment is the effective air temp exposed to the human body and confined to the close area around the body encapsulated by some type of barrier. In a level A suit, it will not matter much what the external ambient temperature is, what matters more is the work load and the amount of heat the body is creating performing required tasks. Since there is no way to transfer this heat to the outside environment, the heat continues to build within the confines of the protective apparel causing a complete break-down of the body’s natural heat transfer capabilities. The only way to address this level of heat build-up within a microclimate environment is to either limit time within that environment or incorporate artificial cooling assistance.

Artificial Body Cooling Devices

Outside of environment control, there are several technologies utilized to assist the body in it’s effort to stay cool. We will briefly cover these technologies with a simple pros/cons listing.

Fluid-Chilled System:

Works by circulating chilled water from an external reservoir through a series of tubes sewn into an underwear style garment or vest. Requires power source for pump, reservoir of ice and water and operator switching / thermostat control. Most commonly recognized system like this is what the Space Shuttle astronauts wear.

Pros:

• Extremely effective at removing heat from the body
• Extended duration cooling
• Custom fit garments offer small addition to overall profile of wearer
• Lightweight (less than 2 pounds) if pump and reservoir are remote

Cons:

• Very Expensive (most expensive body cooling technology)
• Custom fit garments not interchangeable to other wearers
• Pump system must be powered via electrical plug or battery
• Battery life relatively short
• Limited mobility due to umbilically connected to remote reservoir
• More moving parts to maintain

Air-Chilled System:

Works by circulating pre-chilled (vortex) air from an external compressor through a vent channeled garment attached via an air line. Also known as vortex cooling. Requires remote compressor, air line, and vortex device usually worn on user’s hip at attachment to air line location.

Pros:

• Very Effective at removing heat from the body
• Creates a very comfortable microclimate environment inside suit
• Long duration cooling
• Garment is lightweight

Cons:

• Very Expensive
• Mobility is limited and encumbered by the air line
• Requires electricity to operate
• More moving parts to maintain

Ice or Gel Pack Vests:

Usually consisting of a torso garment with pockets sewn onto the inside, next to the body, which hold ice or gel packs that must be frozen in a freezer. Body heat is absorbed by the ice packs.

Pros:

• Generally inexpensive
• Portable; no umbilical device needed
• Rechargeable

Cons:

• Grave risk of Vasoconstriction
• Undergarment usually required protecting from frostbite
• Bulkier to wear than umbilical systems
• Requires freezer to chill packs (standard charge time 5 hours)
• Heavy, typical 2 hour cooling duration units weigh 8 pounds plus
• Limited duration cooling

Phase Change Vests:

Phase change material (PCM) vests consist of a torso garment similar to ice vests but the internal pockets are designed to carry PCM packs instead of ice or gel packs. Body heat is absorbed by thermally stable packs at temperatures substantially higher than ice or gel packs. The most common and effective temperature for phase change vests has proven to be 65° F, well above the vasoconstriction level.

Pros:

• Inexpensive
• Comfortable temperature that can be worn directly against the skin all day
• Portable; no umbilical device needed
• Rechargeable in ice water (20 min.) or in a refrigerator or freezer (1 hour)
• Dry technology
• Lightweight, usually half the weight of similar duration ice/gel vests

Cons:

• Bulkier to wear than umbilical systems
• Limited duration cooling

Evaporation Vests:

Generally a torso garment containing a water absorption material. The garment is soaked in water and then donned. The crystals within the garment swell up and begin to evaporate at a higher than normal level. The process simulates the body’s natural evaporative cooling system as it evaporates the water held within the garment to the atmosphere. This is not a conductive heat transfer but rather a convective heat transfer as the evaporating water actually cools the air between the garment and the wearer. Effectiveness is extremely dependent on ambient humidity levels and does not work under PPE.

Pros:

• Most inexpensive
• Relatively lightweight
• Portable, no umbilical required

Cons:

• Requires the movement of dry air across vest to be effective
• Will not work under protective apparel
• Won’t work in high humidity
• Tends to be damp against the body
• Can cause skin irritation, bacterial growth, mold and odor
• Can not determine a specific value for heat removal