Coverage when it's cold - references?

I've noticed that when I'm carrying a load and moving, I generate a lot of heat. Like, it can be 32 degrees F out (0 degrees C) and I'm in pants, a short sleeve shirt, a hat and gloves to keep from sweating out my clothing.

At what point (environmental temperature) do I need to worry about frostbite, and guard against it by ensuring I have a layer on my exposed skin?

Do you have a reference to support your opinion?

A great question!
This is why I hike in convertible pants/short and strip off after the first hour of hiking. Wind chill makes all the difference though. Even below freezing, I'll still unzip core covering clothes. I rarely ever remove gloves or hat. My fingers/head get cold around 40F and stay that way. Never been frostbit, but got close once on my bicycle with unprotected shoes.

. Pathophysiology of Tissue Freezing - As tissue begins to freeze, ice crystals are formed within the cells. As intracellular fluids freeze, extracellular fluid enters the cell and there is an increase in the levels of extracellular salts due to the water transfer. Cells may rupture due to the increased water and/or from tearing by the ice crystals. Do not rub tissue; it causes cell tearing from the ice crystals. As the ice melts there is an influx of salts into the tissue further damaging the cell membranes. Cell destruction results in tissue death and loss of tissue. Tissue can't freeze if the temperature is above 32 degrees F. It has to be below 28 degrees F because of the salt content in body fluids. Distal areas of the body and areas with a high surface to volume ratio are the most susceptible (e.g ears, nose, fingers and toes - this little rhyme should help remind you what to watch out for in yourself and others).

Surface frostbite generally involves destruction of skin layers resulting in blistering and minor tissue loss. Blisters are formed from the cellular fluid released when cells rupture.

Deep frostbite can involve muscle and bone

http://www.princeton.edu/~oa/safety/hypocold.shtml

I've lost skin layers off my fingertips.

Steadman
I have no reference other than personal experience.

Many people live in extreme cold that also have faces that are exposed to the cold. My Swiss fiancee had permanent spots on her cheeks from the cold. People also function at the poles but I think they wear face coverings.

There are stories of climbers having a glove fall off at extreme cold and not really suffering because the hand was the only part of the body not heavily insulated and it almost glowed with the heat it was disapaiting. I have noticed that the more insulation you wear, the warmer your face is. My military extreme cold mitts have fleece on the back of the hands for warming the face and nose.

Per Lori's post - for your skin to reach 28 degrees it would take either a lot of windchill or a prolonged exposure to temps significantly below 28 degrees.

Following my post - the more of your body that is exposed, the more easily it will frostbite.

I would be more concerned with keeping bare flesh covered [windchill only applies to exposed flesh], and with the wind speed, than a certain temperature. Finally - you don't hear a lot about frostbight at temperatures above zero F.

Jim

Medicine for Mountaineering has a short section on frostbite (Wilkerson, The Mountaineers); a more definitive description is in Hypothermia, Death By Exposure (Forgey, ICS Books). Also, see Wikipedia -the article has a long list of citations.
http://en.wikipedia.org/wiki/Frostbitten

Forgey states that frostbite occurs when skin temp falls to +22-+24 (-5.5C - - 4.4C). Frostnip is a milder form of frostbite. So, based on my reading, outside temp needs to be below freezing for frostbite. There are degrees of frostbite as well and it does not always cause irreversible damage. High altitude and dehydration contribute to the severity.

lori/Tom

Thank you very much; that helps me understand the mechanism of injury a lot better. I am still digesting this...

Steadman

People get around in really cold weather all the time without taking these precautions. If you are not used to real cold or are backpacking, were a Dr is days away and we can assume exposure will be prolonged, here are some guidelines. They come from a Canadian Red Cross/RCMP web site, it is blocked by the firewall of the system I am using so I can’t give you a URL, sorry. The reality is though that in extreme cold (10s to 0F and beyond) you will be bundled as much as you can be, so avoiding hypothermia/frostbite comes down to the quality of your gear and ultimately to access to shelter. In practical terms you are at a greater risk in warmer temps, mid to low 20s. As others pointed out, skin temps well below 25 are needed for freezing to occur. The tricky thing is that you can’t just go by the ambient temperature; wind, wind direction relative to you, and what the wind is blowing all have significant bearing on what happens to exposed skin.
32F to upper 20s: start protecting exposed skin, various balms or zinc oxide work well. You should also remove any exposed piercings at this point (earrings, nose, eyebrow, lip.....) as the metal will efficiently conduct heat away from the site of the piercing (exacerbated by low local blood flow). We saw a lot of tourists in Toronto on very cold days with frozen earlobes (piercings in the upper ear are really bad!!).
Mid 20s to mid teens: cover exposed skin, especially areas with reduced blood flow, cartilaginous tissues, nose & ears.
Teens on down: add double gloves/boots, goggles, thicker balaclava, wind barrier, eventually all your layers and a parka with a good hood (tube opening…).
But really, isn’t super cold why we have cabins, Franklin stoves, fur rugs, mulled wine and significant others……

The majority of hypothermia cases in my neck of the woods happen in summer and spring. People naturally bundle up for snow. They don't think about being cold and wet in summer with a wind blasting them as they cross an alpine pass as a risk, so wear shorts and tank tops, and refuse to layer up.

Lori
while the OP did ask specifically about frost bite; I agree with you that most hypothermia cases happen at more like 35 to 45 degrees with people wearing much too little, and most often in Spring or Fall when runoff can trap people.

Longhair and I were hiking out of the Cascades in a slight snow when we were overtaken by a lone runner wearing nothing but shorts, shoes and a tee shirt. It was 25 miles around the mountain and he said he would be warm enough if he kept running so he couldn't stop to talk to us. The locals here (In Bend) think nothing of this and often carry no water on hikes and carry nothing at all, but wearing a cotton hoody.

Jim