The most prevalent method of classifying burns is based upon how deep the burn penetrates the skin and underlying body tissue, also called burn thickness. Most people are familiar with the first three classifications—first, second, and third degree. These burns are measured in terms of how deeply they penetrate the skin and make up an overwhelming majority of burns. Fourth, fifth, and sixth-degree burns penetrate beyond the skin into the muscle and bone underneath. These burns make up only a small portion of the most severe burns.
Burn Depth Factors
There are a variety of factors that determine how deeply a burn will penetrate. Some of the most important factors are the type of burn, the intensity of the burn source, duration of contact between the source and body, and anatomic location of the burn.
Type of Burn
Each of the four types of burns affects the body differently. Thermal burns begin at the outmost layer, the epidermis, and progress inwards through the layers of the body in a systematic fashion. Chemical burns can behave similarly to thermal burns when harmful chemicals come into contact the skin, or they can bypass the skin entirely when chemicals are ingested. Electrical burns bypass the epidermis quickly but rarely affect the deepest tissues. Radiation burns vary depending on the type of radiation; the sun exclusively affects the epidermis and dermis while penetrating radiation, such as X-rays, can bypass the skin easily.
The intensity of a burn source determines the amount of energy that is transmitted into the burn victim over any given period of time. High-intensity burn sources (such as forest fires or power lines) can transmit enough energy to cause mortality in an instant while low-intensity burn sources (such as candles or the sun) may take hours or more to transmit that same amount of energy.
Duration of Contact
Related to intensity is the duration of contact. Longer contact with a burn source allows for more energy to be transferred from the source into the body. Using the examples listed above, three seconds of close proximity to a forest fire exposes the body to more energy than several hours around a candle.
Where the burn occurs on the body will also impact how deep it is. Certain areas of the body have thicker layers of skin, fat, and/or muscle which provide additional protection. For example, the fingers provide little sub-skin protection while the thighs have large muscles beneath the skin. Accordingly, burns to the fingers can reach third or fourth degrees much more quickly than burns to the thighs.
Certain areas of the body are also more sensitive to burns. The throat and esophagus are sensitive and easily burned leading to a large number of inhalation injuries. Orifices like the nostrils and eyes have less protection and serve important functions making burns to those areas more serious regardless of the depth of the burn.
The Levels of Skin
The first measuring stick in classifying burns is the skin. People typically think of skin as the outermost layer of tissue that covers the body. In actuality, the skin is composed of three layers—epidermis, dermis, and hypodermis.
The epidermis is the uppermost and thinnest layer of skin. It is composed of 4-5 sub-layers depending on the section of the body. The outermost sub-layer, the stratum corneum, is comprised of dead cells that form a protective barrier. This is the body’s first line of defense against infections, water loss, mechanical stresses, or other harmful substances (i.e. chemicals, radiation, electricity, or heat). An additional clear layer, stratum lucidum, is found exclusively on the palms and soles; the epidermis is thickest and provides the most protection in these areas.
The final three sub-layers—stratum granulosum, stratum spinosum, and stratum basale—create a transition zone where the living tissue of the dermis transforms into the dead tissue that will eventually make up the new outer layer. This is a continually ongoing process that takes approximately 48 days from beginning to completion. Burns that do not penetrate past the epidermis are first degree burns.
Just below the epidermis is the dermis. The dermis is comprised of living tissues and can be divided into two main regions, the upper stratum papillare and the lower stratum reticulare. The stratum papillare connects the dermis to the epidermis and is composed of connective tissue and blood capillaries—the smallest of the body’s blood vessels. The stratum reticulare is a thicker section that contains the roots of hair, sweat glands, sensory receptors, and more blood vessels. Burns that damage but do not penetrate through the dermis are second-degree burns.
The hypodermis, also called subcutaneous tissue, is the deepest layer of the skin. It is composed primarily of connective tissues—which connect it to the dermis—and subcutaneous fat. Subcutaneous, as opposed to visceral or deep fat, is distributed all around the body. It stores energy that can be put back quickly into circulation through the veins that also run through the hypodermis. While the hypodermis is distributed fairly evenly around the body, it does increase in thickness in areas where fat accumulates— abdomen, buttocks, palms, thighs, and soles. Burns that reach this level destroy nerve endings leading to little or no pain and are classified as third-degree burns.
Muscles, bones, and organs make up the layers beneath the skin. These layers are typically safe until a burn has penetrated through all three layers of the skin. Burns that penetrate this deep are fourth, fifth, or sixth-degree burns which have high mortality rates.
Immediately beneath the skin is the body’s muscular system. Muscles are traditionally categorized into three groups—skeletal (voluntary), smooth (involuntary), and cardiac (heart). Skeletal muscles are the muscles under conscious control and are the most widely distributed muscles in the body. These are also the muscles most likely to be burned if a burn penetrates through the skin. Skeletal muscles are connected to the underlying bone through tendons meaning that this region of the body is composed of skeletal muscle, tendons, and the blood vessels that provide oxygen to the muscle. Burns that penetrate to but not through the muscle are fourth-degree burns.
In the arms and legs, bones make up the most interior layer of the body. Most burns only reach bone after they have penetrated through any protective skin, fat, and/or muscle that surround the bone. This region also contains ligaments which connect bones to each other. Burns that reach to the bone are fifth-degree burns, while burns that char the bones are sixth-degree burns.
In the chest, the ribs and spine form the outer boundary of the thoracic cavity. The thoracic cavity protects the heart and lungs. Burns that reach to the bone in this region are especially dangerous because of the possibility of damage to these vital organs.
A majority of human internal organs reside in the ventral cavity which is comprised of the thoracic, abdominal and pelvic cavities. These areas are protected by thick layers of fat and muscles, both skeletal and smooth. Internal organs serve essential bodily functions that can be compromised by thermal burns to the abdomen or chest, ingested chemicals, high-voltage electrical burns, and high doses of penetrating radiation. Burns that penetrate this deeply could range between fourth and sixth-degree burns.