That's right.  The smell needs to sublime off into the air.  Both of the
materials used in moth balls are solids that have a low vapor pressure,
which means they sublime directly from solid to vapor.  The process is
always slow.  Good ventilation is important.  It has been fairly well
determined that Napthalene and PDB (paradichlorobenzine) are worse for
humans than moths.  Many "insect folk" say that moth balls don't do any
good at all, and PDB is a carcinogen.  PDB should not be on the market
today, but people have used it extensively in the past.

Timothy Vitale
Paper & Photograph Conservator
Preservation Consultant
408-684-2731