What do i use to heat my garage?

I work in my garage for about 6+ hrs a day. Its getting cold and this will be my last winter in this house. Im trying to figure out how to heat it this winter. I a garage attached to 3 others they are all open at the top to connect to each other. I dont really have a way to use propane cause there is no where for the exhust to go. Im trying to figure out what kind of heater I can use and should use that is safe.