A common precaution for preparing EtBr-agarose gels is to add EtBr only after the gel solution has cooled somewhat, because EtBr is a dangerous mutagen. Supposedly, adding it to a hot liquid allows EtBr vapors to get into the air and everyone's lungs, where it can act as a carcinogen.
In addition to this are commonly seen special procedures for handling EtBr vials and gels (fresh nitrile gloves, avoid contaminating other work areas, special waste disposal rules).
However, there's also some saying that EtBr isn't all that mutagenic and all this care is over the top.
What's the reality? Just how dangerous is EtBr, especially in amounts used at a typical biological lab? How long does the danger persist? Are even trace amounts hazardous (for instance, touching a "clean" pipette without changing gloves after working with EtBr)?