Re: How does arm hair know when to stop growing?

Hi Mike

Your question has remained unanswered for awhile, so I jump in (as a 
biochemist) to try to answer your question.

The hairs don't stop growing, they fall off. You wouldn't notice because 
they don't fall off all at once. It's the same thing with the hair on your 
head, only you do cut it now and then. Maybe. :-) People who let their 
hair grow uncut get hair of a certain length. Some down to the middle of 
their back, some down to their buttocks, some a bit further still. This 
difference is genetic. Again, it is not that their hair stops growing, 
it's just that they fall off at this stage.
As for the odd single hair growing longer than usual, it is indeed due to 
a mutation. They are often a bit thicker too. Nothing dangerous though, as 
far as I know.

I hope this answers your question.
Regards,

Erik von Stedingk