Does the dust in my house really include my own skin?
You are indeed correct; people are losing bits of skin all the time (and not just when a sunburn peels). Taking off some of the dead skin cells on the surface of your skin is what some people do on purpose and refer to as "exfoliation." This is meant to reveal a more radiant, glowing layer of healthy skin below.
The outermost layer of your skin is called the "epidermis" and is made of some 25 to 30 layers of fairly tough dead skin cells that protect the layers below. New skin cells grow up from underneath, and gradually the topmost layers are sloughed off, either of their own accord or via friction.
Skin cells only last about three or four weeks, and the rate at which you lose them is pretty dramatic: About 30,000-40,000 skin cells fall off you per minute, and a total of 8.8 pounds of dead, dried-up skin cells fall off you per year. About 75 percent of household dust is made of dead skin cells.
If tattoos were made by putting ink onto or even into the epidermis, they would soon fall off. Tattoo ink, however, is injected into lower layers of skin cells that are pretty stable - this is what makes them permanent.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 