Burned Surface Area
Calculating the Amount of Skin Burned
Overall, burn severity is a measurement of the depth of burning and the size of the burn. Measuring the size of a burn is difficult because every person is different in size, shape and weight. It’s impossible to simply choose what universal size of burn is significant. A square foot of burned surface area is much worse to a person who weighs 130 pounds than it is to someone who weighs 200 pounds.
To account for inequities in size and shape, burned surface area is calculated as a percentage of total body area. Of course, we don’t actually know how many square inches of skin covers any single person, but we do know about how much of our skin it takes to cover our arms and legs, for example.
The Rule of Nines
What is the significance of the amount of body area burned?
In addition to the depth of the burn, the total area of the burn is significant. Burns are measured as a percentage of total body area affected. The “rule of nines” is often used, though this measurement is adjusted for infants and children. This calculation is based upon the fact that the surface area of the following parts of an adult body each correspond to approximately 9% of total (and the total body area of 100% is achieved):• Head = 9%
• Chest (front) = 9%
• Abdomen (front) = 9%
• Upper/mid/low back and buttocks = 18%
• Each arm = 9%
• Each palm = 1%
• Groin = 1%
• Each leg = 18% total (front = 9%, back = 9%)
As an example, if both legs (18% x 2 = 36%), the groin (1%) and the front chest and abdomen were burned, this would involve 55% of the body.
Each of these sections takes about nine percent of the body’s skin to cover it. Added all together, these sections account for 99 percent. The genitals make up the last one percent.
To apply the rule of nines, add up all the areas of the body that are burned deep enough to cause blisters or worse (2nd or 3rd degree burns). For example, the entire left arm and the chest covered in blisters would be 18 percent. Partial areas are approximated. For example, the face is only the front half of the head and would be considered 4.5 percent.
Since kids are shaped so much different than adults, there are adjustments made to the rule of nines, which of course ruins the point of making this tool the rule of nines. So many variations exist that it would be fruitless to go into them here.
The most important thing to remember about the rule of nines is that it is intended to be used in the field to quickly determine if victims need to go to a specialty burn center. Once the victim is in a burn center, more advanced techniques will be used to determine the exact burned surface area.
Degrees of Burns
Identifying Full- and Partial-Thickness Burns
Determining the severity of a burn includes establishing how deep the damage goes into or through the skin. Burn thickness in most burn units is classified by degree — the higher the number, the worse the burn.
This means a superficial burn. The surface of the skin is damaged, but the epidermis (the outermost layer of skin) is still intact, and therefore able to perform its functions (control temperature and protect from infection or injury). When determining severity, care providers ignore first-degree burns.
This means damage that has extended through the epidermis and into the dermis (the second layer of skin). Second-degree burns also are known as partial-thickness burns. In determining the severity of burns, the presence of second degree burns indicates a loss of skin function.
Blisters are the first sign of a second-degree burn. As the epidermis is destroyed, it begins to separate from the dermis. Fluid builds beneath it, causing blisters. Eventually, the blisters will spread into one another until the very thin epidermis falls away, exposing the raw dermis underneath.
Once the epidermis has separated from the raw dermis, the victim begins to lose fluid, heat, and the ability to block infection. The raw nerve cells of the dermis also mean second degree burns are the most painful.
This indicates the burn has destroyed both the epidermis and dermis. The victim has the same trouble with fluid loss, heat loss, and infection that come with second-degree burns. Full-thickness burns also cause nerve death, so the victim may not be able to feel anything in the area of the burn.
There’s no easy way to tell the difference between a deep partial-thickness burn (2nd degree) and a full-thickness burn (3rd degree) when looking at it in the field, so we don’t try. Instead, all burns that are deep enough to separate the epidermis from the dermis are counted when determining severity. In other words, we count all burns that are bad enough to form blisters – or worse – when assessing burn severity.