A popular belief
There's a popular belief that foot blisters are a burn. That rubbing burns the skin.
It sounds feasible ... rubbing two sticks together can generate enough heat to start a fire! But can rubbing overheat the skin so much that it burns?
Rubbing increases skin temperature
Rubbing any two surfaces together causes heat and rubbing the skin is no different. It's easy to assume that the more rubbing there is and the more heat generated, the closer it can get to burning the skin. However, research has found that rubbing causes only mild skin temperature increases, insufficient to burn.
In his experimental blister research in 1955, Naylor compared blister rates at two rubbing speeds - the faster rubbing speed did not cause blisters to form any quicker than the slower rubbing speed. Separately, he compared two materials that differed in thermoconductivity to provide the rubbing (one had the ability to absorb heat away from the skin, keeping it cooler). No difference in blistering rate was found!
"Skin temperature appears to be a minor factor in blister formation. Blisters form somewhat more rapidly when the skin temperature is higher but blisters also occur when the skin temperature is low. In experimental rubbing studies, local heat is produced and skin temperatures have been reported between 41 degrees Celsius and 50 degrees Celsius. However, friction blisters do not resemble second degree thermal burns either clinically or histologically." Knapik et al (1995)
Friction blisters are not a burn
The relevance of heat?
Friction blisters on the feet are more common in warmer temperatures. But blisters can occur in cold temperatures too (Griffin et al, 1969; Akers, 1977; Knapik et al, 1992). Griffin et al (1969) produced experimental blisters on chilled, warmed and “normal” temperature skin and noted:
blisters formed quicker when initial skin temperature was higher
blisters took longer to form on chilled skin (14 degrees) compared to normal (30 degrees) and warmed skin (46 degrees)
when the initial skin temperature was low, the skin temperature at the time of blistering was also lower
Why do people say blisters are caused by heat, moisture and friction?
As the feet get warmer with exercise, the skin sweats. This perspiration is moisture. Moisture increases friction levels (Nacht et al, 1981; Naylor, 1955). And higher friction levels lead to increased incidence of blisters. For more information, read What Causes Blisters.
What causes blisters?
Blisters occur when shear (rubbing under the skin) causes a tear under the surface of the skin. Sulzberger and Akers determined this in 1972. And Comaish confirmed it the following year as he ruled out other potential causes (wear, heat, enzymes, pressure, stretching or ischaemia). He called it epidermal fatigue.
It is in spite of the efforts of the earliest blister researchers (and researchers since) that this myth persists!
Akers, WA and Sulzberger MB. 1972. The Friction Blister. Military Medicine. 137:1-7.
Akers, WA. 1977. Sulzberger on Friction Blistering. International Journal of Dermatology. 16: 369-72.
Comaish, JS. 1973. Epidermal Fatigue as a Cause of Friction Blisters. The Lancet. Jan 13: 81-83.
Hashmi, F, Richards, BS, Forghany, S, Hatton, AL and Nester, CJ. 2013. The Formation of Friction Blisters on the Foot: The Development of a Laboratory-Based Blister Creation Model. Skin Research and Technology. 19: e479-e489.
Knapik, JJ, Reynolds, K, Duplantis, KL and Jones, BH. 1995. Friction blisters – pathophysiology, prevention and treatment. Sports Medicine. 20 (3): 136-147.
Nacht, S, Close, J, Yeung, D and Gans, EH. 1981. Skin friction coefficient: changes induced by skin hydration and emollient application and correlation with perceived skin feel. Journal of the Society of Cosmetic Chemists. 32 (March-April): 55-65.
Naylor, P. 1955. Experimental friction blisters. British Journal of Dermatology. 67: 327 – 42.
Naylor, P. 1955. The skin surface and friction. British Journal of Dermatology. 67: 239 – 48.
Richie, D. 2010. How to manage friction blisters. Podiatry Today. 23 (6): 42-48.