Confusing conversions

[Herrold@UWYO.EDU (Jim F. Herrold)]

Speaking of conversions...

A quote I like to use in the introduction to my Radiation Safety Class is 
from Dr. Dixy Lee Ray, borrowed from an article in a book I believe is 
called <Rational Readings on Environmental Concerns> (my copy of this book 
has been missing from my office for some time, so I cannot verify this 
title).  She said:

For all those who do not like radioactivity, the Earth is no place to live.

She follows this up with an estimate that we are being constantly bombarded 
by 15,000 radioactive particles per second from our natural background.  To 
put things in perspective, however, I like to add another estimate, which I 
have heard somewhere (could be the same book, but I can't verify that). This 
says that  the chance of one of those radioactive particles causing cancer 
is one in 50 quadrillion.

My questions to RADSAFE are:

1.   Has anyone else heard these estimates, or something similar?

2.   How much is a quadrillion, in this reference? Evidently, according to 
Webster's Dictionary,  it depends on where  you live. In the United States 
it is a one followed by 15 zeros. In the United Kingdom it is followed by 24 
zeros (quite a difference). If it was Dr. Ray who said this, I would assume 
the U.S. number, to be conservative.

Jim Herrold, RSO
University of Wyoming
herrold@uwyo.edu