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 NinesTo approximate the percentage of burned surface area, the body has been divided into eleven sections:
- Right arm
- Left arm
- Upper back
- Lower back
- Right thigh
- Left thigh
- Right leg (below the knee)
- Left leg (below the knee)
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.
Total burned surface area is not the only thing that determines if a burn is critical or not. For more information, read Degrees of Burns.