Wikiknowitall:
"Nose-picking, according to conventional wisdom, may carry a number of medical risks, thus some doctors recommend against it and encourage the use of a tissue instead, although others claim the finger is more sensitive, and is more indicated. Risks may include nasal infections, occasional nosebleeds (in 25% of those who pick their nose, considering that most of the population picks their nose occasionally) and in rare cases perforation of the nasal septum or self-induced ethmoidectomy. Nose picking, however, should not affect the sense of smell, as the nasal cavity where the olfactory nerves are located is too high up to reach. Also, due to the special nature of the blood supply to the nose and surrounding area, it is possible for retrograde infections from the nasal area to spread to the brain, although this scenario is unlikely to arise from normal rhinotillexis. For this reason, the area from the corners of the mouth to the bridge of the nose, including the nose and maxilla, is known to doctors as the "danger triangle of the face."

Nose-picking can become medical issue if it becomes a body-focused repetitive behavior or obsessive compulsive disorder, at which point the accepted medical term is rhinotillexomania (etymology: Greek, rhino "nose" + tillexis "habit of picking" + mania). Most cases do not meet this pathological threshold."

Sing a song, c'mon get happy! Pick your nose, pick your bum - now you've got a chewing gum...

"If it hurts... you're not doing it right!"