3.2 Hypothermia, frostbite, trench foot, chilblains
Four cold injuries appear in OSHA's Cold Stress Guide: hypothermia, frostbite, trench foot and chilblains. Two of them start at temperatures nobody calls cold. All four share one cruel feature — the tissue that is being damaged goes numb, so the body's alarm is disabled exactly where the injury is happening. This chapter covers what each one is, what it looks like from the outside, and where a crew's judgment has to override how the worker says they feel.
1 Hypothermia: the whole-body failure
OSHA's Cold Stress Guide defines it precisely: hypothermia occurs when body heat is lost faster than it can be replaced and the normal body temperature of 98.6 °F drops to less than 95 °F. It is most likely at very cold temperatures, but it can occur even at cool temperatures above 40 °F if a person becomes chilled from rain, sweat or submersion in cold water.
The signs come in two stages, and the transition between them is the moment that matters.
| Stage | What you see |
|---|---|
| Mild | The worker is alert. They may begin to shiver and stomp their feet to generate heat. |
| Moderate to severe | As body temperature keeps falling, shivering stops. The worker may lose coordination and fumble with items in the hand, become confused and disoriented, be unable to walk or stand; pupils dilate, pulse and breathing slow, and loss of consciousness can occur. A person could die if help is not received immediately. |
The other giveaway is the same one that matters in heat: a change in behavior. Fumbling with a familiar tool, slurring, arguing with a simple instruction, wandering — these are neurological signs, and a coworker sees them long before the person notices anything is wrong.
2 Frostbite: freezing of tissue
Frostbite is an injury caused by freezing of the skin and underlying tissues. OSHA's guide notes that the lower the temperature, the more quickly it occurs, that it typically affects the extremities — particularly the feet and hands — and that amputation may be required in severe cases.
Symptoms: numbness in the affected part, tingling or stinging, aching, and bluish or pale, waxy skin. In severe cases, blisters.
Two facts frame it on a jobsite. First, the air temperature has to be below freezing — NWS is explicit that wind chill alone cannot cause frostbite above 32 °F. Second, the parts at risk are the ones the body has already sacrificed to protect the core, and they are numb, which is why frostbite is so often discovered by a coworker or in the truck at the end of the day rather than by the person who has it.
3 Trench foot: the injury that does not need freezing
Trench foot, or immersion foot, is caused by prolonged exposure to wet and cold, and OSHA's guide states it can occur at temperatures as high as 60 °F if the feet are constantly wet. The mechanism is worth quoting because it explains the number: this non-freezing injury occurs because wet feet lose heat 25 times faster than dry feet. To prevent that heat loss the body constricts blood vessels and shuts down circulation in the feet, and skin tissue begins to die from lack of oxygen and nutrients and from the buildup of toxic products.
Symptoms: redness of the skin, numbness, leg cramps, swelling, blisters or ulcers, bleeding under the skin, and in the worst case gangrene — the foot turning dark purple, blue or gray.
This is the cold injury most likely to hit an American jobsite, because it does not require winter. A pipe crew standing in a wet trench in November, a landscaper in soaked boots after a rain, a car wash or food processing worker in permanently damp footwear — all of them are in the conditions the guide describes.
4 Chilblains: the repeated-exposure injury
Chilblains are a painful inflammation of small blood vessels in the skin, caused by repeated exposure of skin to temperatures just above freezing to as high as 60 °F. Symptoms are redness, itching, possible blistering, inflammation, and possible ulceration in severe cases.
Chilblains matter for two reasons even though they rarely put anyone in an ambulance. They are evidence that a worker is being exposed repeatedly without adequate protection — a program signal, not just a skin problem. And they are easy to misread as an allergy or a rash, so they go unreported and the exposure continues.
5 Telling them apart on site
In the field the question is rarely "which of the four is this" — it is "does this person stay or go." The table sorts by that decision.
| Injury | Tell-tale sign | Decision |
|---|---|---|
| Hypothermia, mild | Alert, shivering, stomping feet | Off the job into warmth, dry clothes, warm sweet drinks if alert |
| Hypothermia, moderate to severe | Shivering has stopped; confusion, fumbling, cannot stand | Call 911. Emergency |
| Frostbite | Numb, pale or waxy, bluish; hard or firm tissue; blisters | Medical care; do not rewarm in the field |
| Trench foot | Wet feet all shift; redness, numbness, swelling, blisters | Medical care; dry the feet, avoid walking on them |
| Chilblains | Red, itchy, inflamed skin after repeated cold exposure | Warm slowly, keep clean and covered, fix the exposure |
6 Why cold injuries get reported late
Three forces push cold injuries out of sight, and a program has to push back on all three.
The nerve endings are the casualty. Numbness is a symptom of the injury, so the injured part stops complaining. A worker with numb toes genuinely feels better than one with cold toes.
Cold is treated as a character test. Nobody wants to be the person who called it. That culture problem is the reason OSHA's guide tells employers to allow workers to interrupt their work if they are extremely uncomfortable — a documented right to stop takes the decision out of the realm of toughness.
The damage shows up later. Frostbite blisters and trench foot ulcers can appear hours after the exposure, at home, which puts the injury outside the workday in the worker's mind. Chapter 4.2 covers what to do — and specifically what not to do — when they appear.
- Hypothermia is core temperature below 95 °F and is possible above 40 °F air when someone is wet from rain, sweat or immersion.
- Shivering STOPPING is the sign that hypothermia has gone from mild to severe — it is an emergency, not an improvement.
- Frostbite requires air below freezing: numb, pale or waxy, bluish skin, blisters in severe cases, and amputation is possible.
- Trench foot occurs at temperatures as high as 60 °F when feet stay wet, because wet feet lose heat 25 times faster than dry feet.
- Chilblains come from repeated exposure just above freezing up to 60 °F and are a signal that the program is not protecting someone.
- Numbness disables the warning: the coworker beside you finds cold injuries earlier than you do.
Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.