With early studies showing that firefighters are three times as likely as the general population to develop cancer, Greenfield Fire Chief Jon Cohn decided it was time to try something. That something is a sauna where it is hoped that firefighters can sweat out the toxins carried by smoke that get into their bodies through their skin. The sauna arrived at the firehouse last week to be used after firefighters return from fires. Cohn pointed to informal evidence that the sauna might work.