The day is over. As you step back into your home, you sigh and relax your shoulders. It's been a long day, and you are tired, and sore. You lay down on your bed, light a scented candle, and throw on some ambient music.
It’s time for a warm compress.
What is a warm compress?
A warm compress is simply a heated towel or other flexible material that you use to warm up an area of your body. A warm compress works on a fairly simple principle: increased blood flow. By placing a warm compress on your body, the area under that compress heats to a temperature that is higher than your body temperature.
Like after exercising, or spending time in the hot sun, your body does not want to be that hot, and makes changes to cool it down. Among other things, your body dilates your blood vessels, to increase blood flow to the area. The blood flowing through carries away extra heat, so more blood flow, more cooling. That increase in blood flow can be useful for many different health complaints.
Common uses for warm compresses are muscle aches, cramps, eye pain, and sinus pressure. In each case, the relief is brought on by the increased blood flow.
There are medical complaints that a warm compress should not be used for as well. While after an injury has somewhat healed, a warm compress may bring relief, applying a warm compress to a fresh injury can significantly increase swelling and pain, and has no benefit. Instead, for such injuries, an ice pack is recommended. And as part of the recovery from muscular or skeletal injuries, both hot and cold are used to improve healing.
A warm compress should not be used for anything other than regular aches and pains; anything beyond and you should consult a medical professional.
A warm compress can come in two general varieties, wet and dry. A wet warm compress is typically a towel or cloth immersed in warm water before application, a dry compress is a towel or cloth that has been heated another way, often in a clothes dryer. While both methods have their advantages and drawbacks, wet compresses are much more common.
In a wet compress, much of the heat is stored in the water soaking the compress; as such wet compresses can carry much more heat than dry compresses. A wet compress will heat an area far more rapidly and deeply than a dry one will, in the same way that cold water cools something faster than cold air does. However, as we will discuss, a wet compress also loses heat more rapidly than a dry one, though this is somewhat offset by the fact that the wet compress carries more heat to start with.
How does a warm compress get cold?
Depending on the surrounding environment, a warm compress gets cold in a couple of different ways. Like any other object, the compress will dissipate heat from its surface and into your body and the air at a normal rate. However, since a warm compress is most often wet, it cools differently thanks to evaporative cooling.
This states that if your wet compress is in an environment that is dry, it will evaporate the hot water faster causing the compress to cool.
Even if the surroundings of the compress are hot, the faster it evaporates the water withing the faster it becomes cool. If the compress is surrounded by cool moist air, it will stay warmer for longer because the hot water will evaporate slowly into the air. This concept can be seen in action any time you sweat. The body cools itself by release warm sweat that can then be evaporated into the air to cool us down.
Any moving air that passes through your compress is actively cooling it. As you walk around or the wind blows, your compress is becoming less and less helpful to you, Limiting the amount of airflow to your compress will keep it warmer for longer.
How to keep your compress hot
For best results a wet compress needs to be used. The water in a wet compress raises the thermal mass of the compress and it will hold heat for longer. Dry compresses lose heat quickly and are far less effective than their wet counterparts.
The best way to keep your compress warm if you do not have access to more hot water is to limit its exposure to the air. Placing another towel over your compress will help trap the heat inside of the compress. With less exposure to the air, the compress will not evaporate its heat quickly. Placing a compress underneath your clothing will have the same effect.
Wrapping your warm compress in plastic wrap or placing it into a Ziploc bag will keep air exposure to a minimum. Although this may make the temperature of the compress feel cooler on your direct skin, the compress will stay much warmer for longer because all the moisture will stay confined to the inside of the plastic container.
If you have access to a sink, the easiest way to keep your compress at a high temperature is to continually wet it with hot water. Soaking your compress in hot water will allow the material to soak up as much thermal mass as possible. When your compress begins to get colder, simply replacing the cooled water in the compress with fresh hot water will keep the compress at a good temperature for its purpose.
Another method you can use to keep your compress warm for longer is to place it in the microwave. Place your damp towel in a microwave safe plastic bag and throw it in the microwave for a minute or two. This will heat the compress to a higher temperature than your running water.
The plastic bag will keep any moisture from the towel from escaping the bag during the heating process. The bag will also keep the compress warmer for longer as it will completely limit the towel’s exposure to the outside. Microwaving the towel inside the bag will cause both to get very warm and they will maintain this heat for a long period of time.
Microwaving your damp towel directly may not be the best option.
As the towel heats quickly in the microwave, the moisture in the compress will evaporate. When you pull it out of the microwave your compress will have lost a lot of its dampness. With this lower thermal mass, the towel will not stay warm for very long. The more moisture that is within your towel when it heats up, the longer it will retain that heat.
A wet compress is far better at retaining heat than a dry compress. Consistently re-wetting your compress will keep it warm. Wet compresses cool rapidly due to how quickly they evaporate when cooling. If you want to keep your wet compress as hot as possible for a long period of time, limit its exposure to the surrounding air.
As always when using heat, be sure to carefully test the temperature of your warm compress before applying it fully and directly to your skin. A compress that is too warm could cause skin irritation and even some minor burns. When your goal is to relax with a warm compress and soothe your tired muscles, the last thing you want is a burn that will cause you more discomfort and pain.