A thermal injury is any damage to the skin caused by contact with a hot object, as in exposure to a flame, or scalding with a hot liquid; by exposure to a source of radiation, such as a sunburn or contact with a radioactive material; through contact with an electrical charge; or by exposure to a corrosive chemical, such as lye or strong acid. Frostbite is sometimes classified as a thermal injury, although it does not result from contact with a hot or corrosive material because its initial symptoms are similar to those of thermal injury.
The most common type of thermal injury is one in which the skin is heated to an abnormally high temperature. Studies indicate that human skin normally is not damaged at temperatures less than 111°F (44°C) if exposure does not exceed six hours. At temperatures ranging from 111–124°F (44–51°C), the rate of skin damage doubles with every one degree Celsius increase in temperature. Above 185°F (70°C), serious damage occurs in less than one second. Individuals respond somewhat differently to thermal injuries depending on certain characteristics of the skin, including moisture content of the skin, degree of pigmentation, presence of insulating materials, such as hair and skin oil, and efficiency of blood circulation in the affected area.
Skin cells damaged by thermal injury may experience one of three fates. Those most directly affected by the injury die almost immediately. The region in which this damage occurs is called the zone of coagulation because cells die when the materials of which they are made clump together and lose their ability to function normally. Cells that are somewhat more distant from the source of heat may also be damaged, but not so severely as to result in their death. With prompt and proper treatment, they may survive and continue functioning as normal cells. The region covered by cells of this type is known as the zone of stasis. Cells in the general region of a thermal injury, but at still greater distances from its main source, are likely to recover within a matter of about seven days provided that infection does not develop or shock does not occur. This area of cells is known as the zone of hyperemia, a term that means an increase of blood supply because cells in this region produce a more abundant supply of blood in response to the thermal injury.
Injuries caused by chemicals, radiation, electrical current, and very low temperatures all produce similar cellular and biochemical responses at the site of injury although the precise mechanism of cell destruction varies. For example, exposure to any chemical substance that has the potential for coagulating cellular contents can produce a burn-like response in the skin. Substances in this category include strong acids and bases, whose low or high pH, respectively, can damage and destroy cells, as well as a number of organic compounds that can also cause coagulation of cell contents. Electrical charges and radiation attack biochemical molecules directly, destroying them and interrupting the normal metabolic functions of a cell. For example, ultraviolet radiation in sunlight causes blood vessels in the skin to dilate, allowing a larger volume of blood to flow through the area, resulting in the characteristic pink or red color associated with sunburn. Extended exposure to sunlight increases the risk of damage to cells in the affected region, which initiates the host of immune responses that are also associated with other types of thermal injury.
Another group of thermal injuries include those that are not directly related to exposure to a source of heat, chemicals, radiation, or electrical current, but that develop as the result of exposure to extended or abnormal periods of heat and humidity without benefit of rehydration. The simplest of such injuries are heat cramps, characterized by severe muscle pain and cramping, usually after extended periods of exercise in high heat. A somewhat more serious condition resulting from the same cause is heat exhaustion, characterized by dizziness, nausea, and general body weakness resulting from a loss of liquids and electrolytes. The most serious consequence of heat exhaustion is that it may lead to heat stroke (also known as sunstroke), a potentially life-threatening condition in which the body is no longer able to cool itself adequately with symptoms such as a temperature of more than 104°F, nausea, vomiting, headache, rapid heartbeat, fatigue, confusion, lethargy, stupor, and, eventually, coma and death.
Probably the most complete set of data on burn injuries in the United States is produced by the American Burn Association (ABA). In the ABA's report for 2009, the most common type of thermal injury for which treatment was sought in medical facilities was contact with fire or flames, accounting for 40% of all cases for which an etiology was known. The second most common source of burns was scalding, accounting for 30% of such cases. Other sources of thermal injury were contact with a hot object (9%), electrical injuries (4%), and chemical burns (3%). Other and nonspecified etiology accounted for the remaining portion of thermal injuries. Seventy-one percent of burn victims in 2009 were males and 29%, females. Burns occurred most commonly among males in the age groups 20–30 (16.5%), 31–40 (15.1%), and 41–50 (15.4%), and among females in the age groups 5–16 (11%), 21–30 (11.3%), and 41–50 (12.3%). Whites accounted for the largest fraction of burn victims, 60.0% in this study, followed by African Americans (16.5%), Hispanics (12%), and Asian Americans (2%). These data are somewhat incomplete because the overall distribution of burn victims by gender, age group, ethnicity, and other characteristics was not reported.
Additional findings of the ABA study included:
Thermal injuries are caused by exposure to or contact with hot objects, such as irons, stoves, curling irons, steam, chemicals, electrical current, and radiation. Extended exposure to sunlight can also cause certain types of thermal injuries, such as sunburn, heat cramps, heat exhaustion, and heat stroke.
The symptoms of a burn differ according to the severity of the injury. For the least damaging burns, the only symptoms may be a reddening and swelling of the skin and some discomfort or pain. In more serious burns, redness, swelling, and pain are likely to increase, and blisters may begin to form. In the most severe burn cases, the skin may turn a whitish or black, charred, color with numbness and a leathery or shiny appearance. Other systemic symptoms may also be observed with more serious burns, the most dangerous of which is shock. Symptoms of shock include pale, clammy skin; general bodily weakness; and bluish lips and fingernails. Since the early stages of shock may be nearly asymptomatic, caregivers must be alert for this complication in burn victims.
Thermal injuries are usually easily diagnosed because patients can describe the circumstances that led to their injury: they spilled scalding water on their arm, placed their hand on a hot stove, touched a live wire, spilled a chemical on their lap, or played too many sets of tennis on a hot day. Visual inspection of the injured site provides important direct information as to the injury that has occurred. No blood or imaging tests are typically required to confirm a thermal injury.
Most thermal injuries can be classified according to the degree of damage to the skin. The skin consists of three major layers, the outer layer (epidermis), middle layer (dermis), and inner layer (subcutaneous layer). Burns that damage, but do not penetrate, the epidermis are known as first degree burns. First degree burns may be painful and unsightly, but they are not life-threatening and usually resolve with first aid treatment within a week. Burns that destroy a portion of the epidermis and penetrate to the dermis are called second degree burns. They are characterized not only by swollen, red skin, but also by the appearance of blisters formed from liquid released by damaged cells in the dermis. Burns that penetrate to the subcutaneous layer are called third degree burns. Skin that has been damaged may turn white or black and develop a leathery feel, although there may be little or no pain. The most serious burns are those that penetrate all three layers of the skin and reach into muscle and nerve tissue. These fourth degree burns may produce skin that looks very much as if it has been set on fire, with a black, charred appearance.
Burn treatment depends to a considerable extent on the classification scheme. The vast majority of first degree burns resolve on their own with little or no special care. The use of skin moisturizers, such as aloe vera cream, or an antibiotic or pain-suppressant cream may reduce the pain associated with the burn and increase the rate at which it heals. Pain killers such as acetaminophen can also be used to reduce pain and discomfort. Second degree burns require more aggressive treatment that includes cooling the burned area with a cold compress and covering the burned area to reduce the risk of infection. Use of an analgesic to reduce pain and discomfort is also warranted. Third degree burns are serious medical emergencies that require professional treatment. The only first aid involved is getting the patient to a medical care facility as quickly as possible. Care may then involve a number of procedures, including a tetanus shot (since burned skin is especially at risk for a tetanus infection), IV hydration (to replace water lost as a result of the burn), IV administration of antibiotics (to reduce the risk of infection), debridement and excision (to remove dead and damaged skin), and possible skin grafting (to replace lost and damaged skin). Fourth degree burns are life-threatening conditions that require more aggressive treatment, which may include amputation of a damaged limb.
Prognosis for a thermal injury depends to a large extent on three factors: the severity of the burn, the area of the body covered by the burn, and the age of the victim. A common measure for determining a patient's prognosis from burn is the Baux score, which is equal to the age of the patient added to the percent of the patient's body surface area covered by the burn. Traditionally, a burn victim with a Baux score of more than 100 was thought to be at high risk for death from the burn injury, while a patient with a Baux score over 130 was thought to be beyond the means of medical science for survival. In recent years, a number of adjustments in the Baux score have been suggested. These changes reflect the fact that the care available for a burn victim has improved dramatically in the last few decades, making possible the chance of survival for many patients who, in earlier years, would probably have died from their burns.
First degree burns resolve with little or no treatment in a matter of a few days or few weeks, depending on the severity of the burn. A second degree burn often requires between one to three weeks for full recovery, usually after debridement or excision has been used. Third degree burns often require a skin graft, which may require three weeks or more to heal.
The healing process often depends to a considerable degree on the speed and expertise with which the patient is treated. ABA has listed a number of instances in which a burn victim should be transported as soon as possible to a specialized burn center, including:
According to some studies, as many as 85% of all cases of thermal injury could have been prevented. A number of organizations have recommended a variety of steps that individuals and families can take to reduce the risk of thermal injury in the home. These include:
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American Burn Association, 311 S Wacker Dr., Ste. 4150, Chicago, IL, 60606, USA, (312) 642-9260, Fax: 1 (312) 642-9130, firstname.lastname@example.org, http://www.ameriburn.org .
Burn Prevention Network, 236 N. 17th St., Allentown, PA, 18104, (610) 969-3930, Fax: (610) 969-3940, http://www.burnprevention.org .
David E. Newton, A.B., M.A. Ed.D.