The characteristics of a burn depend upon its depth. Superficial burns cause pain lasting two or three days, followed by peeling of the skin over the next few days. Individuals suffering from more severe burns may indicate discomfort or complain of feeling pressure rather than pain. Full-thickness burns may be entirely insensitive to light touch or puncture. While superficial burns are typically red in color, severe burns may be pink, white or black. Burns around the mouth or singed hair inside the nose may indicate that burns to the airways have occurred, but these findings are not definitive. More worrisome signs include: shortness of breath, hoarseness, and stridor or wheezing. Itchiness is common during the healing process, occurring in up to 90% of adults and nearly all children. Numbness or tingling may persist for a prolonged period of time after an electrical injury. Burns may also produce emotional and psychological distress.

A burn is a type of injury to skin, or other tissues, caused by heat, cold, electricity, chemicals, friction, or radiation. Most burns are due to heat from hot liquids, solids, or fire. While rates are similar for males and females the underlying causes often differ. Among women in some areas, risk is related to use of open cooking fires or unsafe cook stoves. Among men, risk is related to the work environments. Alcoholism and smoking are other risk factors. Burns can also occur as a result of self harm or violence between people.

Burns that affect only the superficial skin layers are known as superficial or first-degree burns. They appear red without blisters and pain typically lasts around three days. When the injury extends into some of the underlying skin layer, it is a partial-thickness or second-degree burn. Blisters are frequently present and they are often very painful. Healing can require up to eight weeks and scarring may occur. In a full-thickness or third-degree burn, the injury extends to all layers of the skin. Often there is no pain and the burn area is stiff. Healing typically does not occur on its own. A fourth-degree burn additionally involves injury to deeper tissues, such as muscle, tendons, or bone. The burn is often black and frequently leads to loss of the burned part.

Burns are generally preventable. Treatment depends on the severity of the burn. Superficial burns may be managed with little more than simple pain medication, while major burns may require prolonged treatment in specialized burn centers. Cooling with tap water may help pain and decrease damage; however, prolonged cooling may result in low body temperature. Partial-thickness burns may require cleaning with soap and water, followed by dressings. It is not clear how to manage blisters, but it is probably reasonable to leave them intact if small and drain them if large. Full-thickness burns usually require surgical treatments, such as skin grafting. Extensive burns often require large amounts of intravenous fluid, due to capillary fluid leakage and tissue swelling. The most common complications of burns involve infection. Tetanus toxoid should be given if not up to date.

In 2015, fire and heat resulted in 67 million injuries. This resulted in about 2.9 million hospitalizations and 176,000 deaths. Most deaths due to burns occur in the developing world, particularly in Southeast Asia. While large burns can be fatal, treatments developed since 1960 have improved outcomes, especially in children and young adults. In the United States, approximately 96% of those admitted to a burn center survive their injuries. The long-term outcome is related to the size of burn and the age of the person affected.

A chemical burn occurs when living tissue is exposed to a corrosive substance such as a strong acid or base. Chemical burns follow standard burn classification and may cause extensive tissue damage. The main types of irritant and/or corrosive products are: acids, bases, oxidizers / reducing agents, solvents, and alkylants. Additionally, chemical burns can be caused by some types of chemical weapons, e.g., vesicants such as mustard gas and Lewisite, or urticants such as phosgene oxime.

Chemical burns may:

- need no source of heat,

- occur immediately on contact,

- not be immediately evident or noticeable,

- be extremely painful,

- diffuse into tissue and damage structures under skin without immediately apparent damage to skin surface.

Depending on the duration of exposure aerosol-induced frostbite can vary in depth. Most injuries of this type only affect the epidermis, the outermost layer of skin. However, if contact with the aerosol is prolonged the skin will freeze further and deeper layers of tissue will be affected, causing a more serious burn that reaches the dermis, destroys nerves, and increases the risk of infection and scarring . When the skin thaws, pain and severe discomfort can occur in the affected area. There may be a smell of aerosol products such as deodorant around the affected area, the injury may itch or be painful, the skin may freeze and become hardened, blisters may form on the area, and the flesh can become red and swollen.

Acute radiodermatitis occurs when an "erythema dose" of ionizing radiation is given to the skin, after which visible erythema appears up to 24 hours after. Radiation dermatitis generally manifests within a few weeks after the start of radiotherapy. Acute radiodermatitis, while presenting as red patches, may sometimes also present with desquamation or blistering. Erythema may occur at a dose of 2 Gy radiation or greater.

An aerosol burn is an injury to the skin caused by the pressurized gas within an aerosol spray cooling quickly, with the sudden drop in temperature sufficient to cause frostbite to the applied area. Medical studies have noted an increase of this practice, known as "frosting", in pediatric and teenage patients.

Adiabatic expansion causes the gas (with a low boiling temperature) to rapidly cool on exit from the aerosol applier. According to controlled laboratory experiments, the gas from a typical deodorant spray can reduce skin temperature by up to sixty degrees Celsius.

The form of injury is freezing of the skin, a type of frostbite. It is highly advised for those who suffer from frostbite to seek medical attention.

In rare cases aerosol-induced burns can be severe enough to necessitate skin grafting.

Chronic radiodermatitis occurs with chronic exposure to "sub-erythema" doses of ionizing radiation over a prolonged period, producing varying degrees of damage to the skin and its underlying parts after a variable latent period of several months to several decades. In the distant past this type of radiation reaction occurred most frequently in radiologists and radiographers who were constantly exposed to ionizing radiation, especially before the use of x-ray filters. Restated, chronic radiodermatitis, squamous and

basal cell carcinomas may develop months to years after radiation exposure. Clinically, chronic radiodermatitis presents as atrophic indurated plaques, often whitish or yellowish, with telangiectasia, sometimes with hyperkeratosis.

The exact symptoms of a chemical burn depend on the chemical involved. Symptoms include itching, bleaching or darkening of skin, burning sensations, trouble breathing, coughing blood and/or tissue necrosis. Common sources of chemical burns include sulfuric acid (HSO), hydrochloric acid (HCl), sodium hydroxide (NaOH), lime (CaO), silver nitrate (AgNO), and hydrogen peroxide (HO). Effects depend on the substance; hydrogen peroxide removes a bleached layer of skin, while nitric acid causes a characteristic color change to yellow in the skin, and silver nitrate produces noticeable black stains. Chemical burns may occur through direct contact on body surfaces, including skin and eyes, via inhalation, and/or by ingestion. Lipophilic substances that diffuse efficiently in human tissue, e.g., hydrofluoric acid, sulfur mustard, and dimethyl sulfate, may not react immediately, but instead produce the burns and inflammation hours after the contact. Chemical fabrication, mining, medicine, and related professional fields are examples of occupations where chemical burns may occur. Hydrofluoric acid leaches into the bloodstream and reacts with calcium and magnesium, and the resulting salts can cause cardiac arrest after eating through skin.

People usually complain of intense pain that may seem excessive given the external appearance of the skin. People initially have signs of inflammation, fever and a fast heart rate. With progression of the disease, often within hours, tissue becomes progressively swollen, the skin becomes discolored and develops blisters. Crepitus may be present and there may be a discharge of fluid, said to resemble "dish-water". Diarrhea and vomiting are also common symptoms.

In the early stages, signs of inflammation may not be apparent if the bacteria are deep within the tissue. If they are "not" deep, signs of inflammation, such as redness and swollen or hot skin, develop very quickly. Skin color may progress to violet, and blisters may form, with subsequent necrosis (death) of the subcutaneous tissues.

Furthermore, people with necrotizing fasciitis typically have a fever and appear sick. Mortality rates are as high as 73% if left untreated. Without surgery and medical assistance, such as antibiotics, the infection will rapidly progress and will eventually lead to death.

When it affects the groin it is known as Fournier gangrene.

A Callus is an area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses are most often found on feet because of frequent walking. Calluses are generally not harmful, but may sometimes lead to other problems, such as skin ulceration or infection.

Airbag dermatitis (also known as an "Airbag burn") is caused skin irritation and trauma secondary to the deployment of airbags.

A corn (or "clavus", plural "clavi") is a specially shaped callus of dead skin that usually occurs on thin or glabrous (hairless and smooth) skin surfaces, especially on the dorsal surface of toes or fingers. They can sometimes occur on the thicker palmar or plantar skin surfaces. Corns form when the pressure point against the skin traces an elliptical or semi-elliptical path during the rubbing motion, the center of which is at the point of pressure, gradually widening. If there is constant stimulation of the tissue producing the corns, even after the corn is surgically removed, the skin may continue to grow as a corn.

The hard part at the center of the corn resembles a funnel with a broad raised top and a pointed bottom. Because of their shape, corns intensify the pressure at the tip and can cause deep tissue damage and ulceration. The scientific name for a corn is "heloma" (plural "helomata"). A hard corn is called a "heloma durum", while a soft corn is called a "heloma molle".

The location of the soft corns tends to differ from that of hard corns. Hard corns occur on dry, flat surfaces of skin. Soft corns (frequently found between adjacent toes) stay moist, keeping the surrounding skin soft. The corn's center is not soft however, but indurated.

The specific diagnostic workup and treatments for corns may differ substantially from other forms of calluses.

Necrotizing fasciitis (NF), commonly known as flesh-eating disease, is an infection that results in the death of the body's soft tissue. It is a severe disease of sudden onset that spreads rapidly. Symptoms include red or purple skin in the affected area, severe pain, fever, and vomiting. The most commonly affected areas are the limbs and perineum.

Typically the infection enters the body through a break in the skin such as a cut or burn. Risk factors include poor immune function such as from diabetes or cancer, obesity, alcoholism, intravenous drug use, and peripheral vascular disease. It is not typically spread between people. The disease is classified into four types, depending on the infecting organism. Between 55% and 80% of cases involve more than one type of bacteria. Methicillin-resistant "Staphylococcus aureus" (MRSA) is involved in up to a third of cases. Medical imaging is helpful to confirm the diagnosis.

Prevention is by good wound care and handwashing. It is usually treated with surgery to remove the infected tissue and intravenous antibiotics. Often a combination of antibiotics are used such as penicillin G, clindamycin, vancomycin, and gentamicin. Delays in surgery are associated with a higher risk of death. Despite high quality treatment the risk of death is between 25% and 35%.

Necrotizing fasciitis affects 0.4 to 1 person per 100,000 per year. Both sexes are affected equally. It becomes more common among older people and is very rare in children. Necrotizing fasciitis has been described at least since the time of Hippocrates. The term "necrotising fasciitis" first came into use in 1952.

Millipede burns are a cutaneous condition caused by some millipedes that secrete a toxic liquid that causes a brownish pigmentation or burn when it comes into contact with the skin. Some millipedes produce quinones in their defensive secretions, which have been reported to cause brown staining of the skin.

Eczema is another cause of chronic irritation and affects millions of individuals. Eczema simply means a dry skin which is itchy. The condition usually starts at an early age and continues throughout life. The major complaint of people who suffer from eczema is an itchy dry skin. Sometimes, the itching will be associated with a skin rash. The affected areas are always dry, scaly, reddish and may ooze sometimes. Eczema cannot be cured, but its symptoms can be controlled. One should use moisturizers, use cold compresses and avoid frequent hot showers. There are over the counter corticosteroids creams which can be applied. Sometimes, an anti histamine has to be used to prevent the chronic itching sensations. There are also many individuals who have allergies to a whole host of substances like nuts, hair, dander, plants and fabrics. For these individuals, even the minimal exposure can lead to a full blown skin rash, itching, wheezing and coughing. Unfortunately, other than avoidance, there is no other cure. There are allergy shots which can help desensitize against an allergen but often the results are poor and the treatments are expensive. Most of these individuals with chronic irritation from allergens usually need to take anti histamines or use a bronchodilator to relieve symptoms.

Another common irritation disorder in females is intertrigo. This disorder is associated with chronic irritation under folds of skin. This is typically seen under large breasts, groins and folds of the abdomen in obese individuals. Candida quickly grows in warm moist areas of these folds and presents as a chronic itch. Over time, the skin becomes red and often oozes.

Perspiration is also a chronic type of irritation which can be very annoying. Besides being socially unacceptable, sweat stain the clothes and can present with a foul odor. In some individuals, the warm moist areas often become easily infected. The best way to treat excess sweating is good hygiene, frequent change of clothes and use of deodorants/antiperspirants.

AC almost always affects the lower lip and only rarely the upper lip, probably because the lower lip is more exposed to the sun. In the unusual cases reported where it affects the upper lip, this may be due to upper lip prominence. The commissures (corners of the mouth) are not usually involved.

Affected individuals may experience symptoms such as a dry sensation and cracking of the lips.

It is usually painless and persistent.

The appearance is variable. White lesions indicate hyperkeratosis. Red, erosiive or ulcerative lesions indicate atrophy, loss of epithelium and inflammation. Early, acute lesions may be erythematous (red) and edematous (swollen). With months and years of sun exposure, the lesion becomes chronic and may be grey-white in color and appear dry, scaly and wrinkled.

There is thickening whitish discoloration of the lip at the border of the lip and skin. There is also a loss of the usually sharp border between the red of the lip and the normal skin, known as the vermillion border. The lip may become scaly and indurated as AC progresses.

When palpated, the lip may have a texture similar to rubbing the gloved finger along sandpaper.

AC may occur with skin lesions of actinic keratosis or skin cancer elsewhere, particularly on the head and neck since these are the most sun exposed areas. Rarely it may represent a genetic susceptibility to light damage (e.g. xeroderma pigmentosum or actinic prurigo).

Irritation, in biology and physiology, is a state of inflammation or painful reaction to allergy or cell-lining damage. A stimulus or agent which induces the state of irritation is an irritant. Irritants are typically thought of as chemical agents (for example phenol and capsaicin) but mechanical, thermal (heat), and radiative stimuli (for example ultraviolet light or ionising radiations) can also be irritants. Irritation also has non-clinical usages referring to bothersome physical or psychological pain or discomfort.

Irritation can also be induced by some allergic response due to exposure of some allergens for example contact dermatitis, irritation of mucousal membranes and pruritus. Mucosal membrane is most common site of irritation because it contains secretory glands that release mucous which attracts the allergens due to its sticky nature.

Chronic irritation is a medical term signifying that afflictive health conditions have been present for a while. There are many disorders that can cause chronic irritation, the majority involves the skin, vagina, eyes and lungs.

Curling's ulcer (stress ulcer) or a Curling ulcer is an acute gastric erosion resulting as a complication from severe burns when reduced plasma volume leads to ischemia and cell necrosis (sloughing) of the gastric mucosa. The condition was first described in 1823 and named for a doctor, Thomas Blizard Curling, who observed ten such patients in 1842.

These stress ulcers (actually shallow multiple erosions) were once a common complication of serious burns, presenting in over 10% of cases, and especially common in child burn victims. They result in perforation and hemorrhage more often than other forms of intestinal ulceration and had correspondingly high mortality rates (at least 80%).

A similar condition involving elevated intracranial pressure is known as Cushing's ulcer.

Skin infections and wrestling is the role of skin infections in wrestling. This is an important topic in wrestling since breaks in the skin are easily invaded by bacteria or fungi and wrestling involves constant physical contact that can cause transmission of viral, bacterial, and fungal pathogens. These infections can also be spread through indirect contact, for example, from the skin flora of an infected individual to a wrestling mat, to another wrestler. According to the National Collegiate Athletic Association's (NCAA) Injury Surveillance System, ten percent of all time-loss injuries in wrestling are due to skin infections.

Garlic allergy or allergic contact dermatitis to garlic is a common inflammatory skin condition caused by contact with garlic oil or dust. It mostly affects people who cut and handle fresh garlic, such as chefs, and presents on the tips of the thumb, index and middle fingers of the non-dominant hand (which typically hold garlic bulbs during the cutting). The affected fingertips show an asymmetrical pattern of fissure as well as thickening and shedding of the outer skin layers, which may progress to second- or third-degree burn of injured skin.

Garlic dermatitis is similar to the tulip dermatitis and is induced by a combined mechanical and chemical action. Whereas the former mechanism acts via skin rubbing which progresses into damage, the major cause of the latter is the chemical diallyl disulfide (DADS), together with related compounds allyl propyl disulfide and allicin. These chemicals occur in oils of plants of the genus "Allium", including garlic, onion and leek.

Garlic allergy has been known since at least 1950. It is not limited to hand contact, but can also be induced, with different symptoms, by inhaling garlic dust or ingesting raw garlic, though the latter cases are relatively rare. DADS penetrates through most types of commercial gloves, and thus wearing gloves while handling garlic has proven inefficient against the allergy. Treatment includes avoiding any contact with garlic oil or vapours, as well as medication, such as administering acitretin (25 mg/day, orally) or applying psoralen and ultraviolet light to the affected skin area over a period of 12 weeks (PUVA therapy).

As a result of radiotherapy to the mouth, radiation-induced stomatitis may develop, which can be associated with mucosal erosions and ulceration. If the salivary glands are irradiated, there may also be xerostomia (dry mouth), making the oral mucosa more vulnerable to frictional damage as the lubricating function of saliva is lost, and mucosal atrophy (thinning), which makes a breach of the epithelium more likely. Radiation to the bones of the jaws causes damage to osteocytes and impairs the blood supply. The affected hard tissues become hypovascular (reduced number of blood vessels), hypocellular (reduced number of cells), and hypoxic (low levels of oxygen). Osteoradionecrosis is the term for when such an area of irradiated bone does not heal from this damage. This usually occurs in the mandible, and causes chronic pain and surface ulceration, sometimes resulting in non-healing bone being exposed through a soft tissue defect. Prevention of osteradionecrosis is part of the reason why all teeth of questionable prognosis are removed before the start of a course of radiotherapy.

Thermal burns usually result from placing hot food or beverages in the mouth. This may occur in those who eat or drink before a local anesthetic has worn off. The normal painful sensation is absent and a burn may occur. Microwave ovens sometimes produce food which is cold externally and very hot internally, and this has led to a rise in the frequency of intra-oral thermal burns. Thermal food burns are usually on the palate or posterior buccal mucosa, and appear as zones of erythema and ulceration with necrotic epithelium peripherally. Electrical burns more commonly affect the oral commissure (corner of the mouth). The lesions are usually initially painless, charred and yellow with little bleeding. Swelling then develops and by the fourth day following the burn the area becomes necrotic and the epithelium sloughs off.

Electrical burns in the mouth are usually caused by chewing on live electrical wiring (an act that is relatively common among young children). Saliva acts as a conducting medium and an electrical arc flows between the electrical source and the tissues, causing extreme heat and possible tissue destruction.

Burn scar contracture refers to the tightening of the skin after a second or third degree burn. When skin is burned, the surrounding skin begins to pull together, resulting in a contracture. It needs to be treated as soon as possible because the scar can result in restriction of movement around the injured area.

An eschar (; Greek: "eschara") is a slough or piece of dead tissue that is cast off from the surface of the skin, particularly after a burn injury, but also seen in gangrene, ulcer, fungal infections, necrotizing spider bite wounds, spotted fevers and exposure to cutaneous anthrax. The term "eschar" is not interchangeable with "scab". An eschar contains necrotic tissue, whereas a scab is composed of dried blood and exudate.

Black eschars are most commonly attributed to anthrax, which may be contracted through herd animal exposure, but can also be obtained from "Pasteurella multocida" exposure in cats and rabbits. A newly identified human rickettsial infection, "R. parkeri" rickettsiosis, can be differentiated from Rocky Mountain spotted fever by the presence of an eschar at the site of inoculation.

Eschar is sometimes called a "black wound" because the wound is covered with thick, dry, black necrotic tissue.

Eschar may be allowed to slough off naturally, or it may require surgical removal (debridement) to prevent infection, especially in immunocompromised patients (e.g. if a skin graft is to be conducted).

If eschar is on a limb, it is important to assess peripheral pulses of the affected limb to make sure blood and lymphatic circulation is not compromised. If circulation is compromised, an escharotomy, or surgical incision through the eschar, may be indicated.

While emergency surgery was once the only treatment, combination therapies including enteral feeding with powerful antacids such as H-receptor antagonists or, more recently, proton pump inhibitors such as omeprazole have made Curling's ulcer a rare complication.