A thermal burn is tissue damage caused by contact with a heat source, such as flames, hot liquids, steam, or heated objects. When skin is exposed to high enough temperatures for long enough, the proteins inside cells permanently change shape and die, similar to how egg whites turn solid when cooked. The severity depends on how hot the source was and how long the skin was exposed. At 70°C (158°F), a 10-second exposure causes a shallow partial burn, while 98°C (208°F) for the same duration destroys the full thickness of the skin.
How Heat Damages Skin
When heat contacts the skin, it doesn’t cause uniform damage. The injury fans out in three concentric zones, each with a different level of destruction. At the center is the zone of maximum damage, where proteins have solidified and blood vessels are destroyed. This tissue is dead and cannot recover.
Surrounding that center is a ring of compromised but still-living tissue where blood flow is reduced. This middle zone is the most clinically important area because it can go either way. With proper care, adequate fluid intake, and infection prevention, this tissue can survive. But if blood flow drops further, or infection sets in, or swelling gets severe enough, this at-risk tissue dies too, and the burn effectively deepens and widens after the initial injury. The outermost ring has increased blood flow as part of the body’s inflammatory response and will almost always recover on its own.
This is why a burn can look worse on day two or three than it did initially. The middle zone is still deciding whether it will survive.
Burn Depth and Classification
First-Degree (Superficial) Burns
A first-degree burn affects only the outermost layer of skin, the epidermis. The skin turns pink or red, feels painful, and stays dry with no blisters. Sunburns are the classic example. These heal in 3 to 5 days with minimal care and leave no significant scarring.
Second-Degree (Partial-Thickness) Burns
Second-degree burns are split into two subcategories that behave very differently. A superficial partial-thickness burn extends into the upper portion of the dermis. Blisters are common. The wound underneath is uniformly pink or red and turns white briefly when you press on it. These burns are very painful because nerve endings are still intact and exposed. They typically heal in 10 to 15 days with minimal scarring, though some changes in skin color can occur.
A deep partial-thickness burn reaches into the lower dermis. Blisters may still form, but the tissue underneath looks mottled rather than evenly pink, and it responds sluggishly to pressure. Paradoxically, these deeper burns hurt less than superficial ones because more nerve endings have been damaged. Healing takes 3 to 5 weeks, and scarring is unavoidable. New skin can only grow inward from the edges of the wound, which makes the process slow and increases the risk of tight, contracted scars.
Third-Degree (Full-Thickness) Burns
A full-thickness burn destroys the epidermis and dermis entirely and extends into the fatty tissue beneath. The skin looks leathery, stiff, and dry. It may appear white, brown, or charred. It does not blanch when pressed because the blood supply is gone. There is no pain at the burn site itself because the nerves are destroyed, though the surrounding areas with less severe damage will still hurt.
Full-thickness burns cannot heal on their own in any functional way. Without surgical treatment, healing takes more than 8 weeks and produces severe scarring and contractures, where scar tissue pulls surrounding skin tight enough to restrict movement.
Common Causes
The most frequent sources of thermal burns are scalds from hot water, coffee, or cooking oil, which account for a large share of burn injuries, especially in young children. Flame burns from house fires, campfires, or flammable liquids are the other major category. Contact burns happen when skin touches a hot surface like a stove, iron, or exhaust pipe. Steam burns can be deceptively severe because steam carries more heat energy than boiling water at the same temperature and can damage deeper tissue quickly.
The critical variables are always the same: temperature and time. A brief touch on a hot pan may cause a first-degree burn. Holding your hand under a running tap at 70°C for 10 seconds causes a shallow partial-thickness injury. The same duration at near-boiling temperatures causes full-thickness destruction.
How Burn Size Is Estimated
For burns that go beyond a small, localized area, medical teams estimate what percentage of the body’s surface is affected. The standard method divides the adult body into sections, each representing roughly 9% of total surface area: the head is 9%, each arm is 9%, each leg is 18%, the front of the torso is 18%, the back is 18%, and the groin is 1%. For young children, the proportions shift because their heads are proportionally larger (about 20%) and their limbs are smaller.
This percentage matters because burns covering more than about 10% of the body trigger serious systemic effects. The body launches an intense inflammatory response within the first 24 hours, releasing signaling molecules that cause blood vessels throughout the body to become leaky. Fluid shifts out of the bloodstream and into surrounding tissues, leading to massive swelling and a dangerous drop in blood volume. Without aggressive fluid replacement, this can progress to shock. This is why large burns are life-threatening injuries even when the skin damage itself might seem survivable.
First Aid for Thermal Burns
The standard first aid protocol has been consistent since the 1960s because it works. Remove clothing and jewelry from the affected area. Then run cool water (around 15°C, or 59°F) over the burn for at least 20 minutes. This step does more than relieve pain. It stops the burning process in deeper tissue, limits how much the middle zone of damage deteriorates, and reduces swelling. The 20-minute threshold is supported across multiple clinical guidelines, and the cooling is most effective when started within three hours of the injury.
Do not use ice or ice water. While it might seem logical to use the coldest thing available, ice constricts blood vessels and impairs circulation to the already-compromised tissue surrounding the burn center. It can also cause hypothermia, especially in children or when burns cover a large area. After cooling, cover the wound loosely with a clean, non-stick covering and seek medical attention for anything beyond a minor first-degree burn.
How Thermal Burns Heal
Healing time varies dramatically by depth. First-degree burns resolve in 3 to 5 days as the outermost skin layer regenerates from below. Superficial second-degree burns take 10 to 15 days because the structures that produce new skin cells (hair follicles and sweat glands in the upper dermis) are still intact and serve as starting points for regrowth.
Deep second-degree and full-thickness burns are a different situation. Because those regenerative structures are destroyed, new skin can only grow inward from the wound edges. This is inherently slow, and any wound that takes longer than about 3 weeks to close carries a high risk of infection, thick raised scars, and contractures. This is why surgical intervention, typically skin grafting, is the standard approach for deep burns. Grafting replaces the lost skin coverage much faster than the body can do on its own, which reduces complications significantly.
Signs of Infection
Burn wounds are highly vulnerable to infection because the skin’s primary barrier function is gone. Watch for oozing or drainage from the wound, red streaks extending outward from the burn edges, increasing pain after the first day or two (rather than gradually improving pain), and fever. A fever above 103°F (39°C), particularly when accompanied by vomiting, warrants immediate medical evaluation. Infection in a burn wound can progress quickly because the damaged tissue provides an ideal environment for bacterial growth, and the body’s local immune defenses in the burned area are already destroyed.