i think things are getting a little confusing here. being plenum rated has nothing to do with being cl2/3 rated. cl2/3 wire definately gives off toxins. plenum wire is rated to not give off toxic gasses because it is designed to be used in air handling spaces, thus if a fire started, all of your melting wire wont kill everybody breathing the air being distributed by the air handlers. that is why you cant run standard cl2/3 in plenum rated spaces.