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In James Cameron’s 2009 blockbuster "Avatar," the forest-loving Na’vi have stunning blue skin. But in real life, can people actually have that skin color?

Yes, it turns out, and a family living in Appalachia had the condition for generations. In their case, blue skin was caused by a rare genetic disease called methemoglobinemia.

Methemoglobinemia is a blood disorder in which an abnormally high amount of methemoglobin — a form of hemoglobin — is produced. Hemoglobin is the molecule in red blood cells that distributes oxygen to the body.

In methemoglobinemia, the hemoglobin is unable to release oxygen effectively to body tissues. People with the disorder have chocolate-colored blood and blue skin. In genetic cases of methemoglobinemia, the recessive gene for the disorder must be passed on by both parents.

The most famous case history of methemoglobinemia comes from six generations of the Fugate family, who lived in isolation in the hills of Kentucky from 1800 to 1960. The family started with an orphan from France named Martin Fugate who married a local Appalachian woman. Both unknowingly carried the recessive gene for the disorder.

Four of their seven children had blue skin. From there, the family members married each other, and the genes were passed down from one inbred generation to the next, making the blue family locally famous. An account of the Fugates from 1982 says their skin was nearly purple.

Despite having the disorder, most of the Fugates lived into their 80s and 90s with no significant health problems. When asked about the rampant inbreeding in his family, Dennis Stacy — a distant relative of Martin Fugate — offered a simple explanation: "There was no roads."

Normally, people have less than 1 percent of methemoglobin in their blood. When those levels rise to greater than 20 percent, heart abnormalities, seizures and even death can occur. But at levels of between 10 and 20 percent — which the Fugate family had — a person can develop blue skin without any other symptoms.

Methemoglobinemia can also be caused by exposure to certain drugs and chemicals such as the anesthetics benzocaine and xylocaine. The carcinogen benzene, and nitrites used as meat additives, can also be culprits, as can certain antibiotics, including dapsone and chloroquine.

Some people get blue skin through another route: silver poisoning, known as argyria. Argyria occurs when people are exposed to silver dust, and the most common symptom is skin that turns bluish-gray. In years past, before the advent of antibiotics, silver nitrate and colloidal silver were used as antiseptics — these, too, caused argyria.