Best to leave the paper in the darkroom and do your toning someplace where there is plenty of ventilation. Hydrogen sulfide is a very poisonous gas that has caused the deaths of many sewer workers in the past. The toxicity of H2S is comparable with that of hydrogen cyanide or carbon monoxide. It is particularly dangerous since it first dulls the sense of smell so that people are not aware that the gas is still present in dangerous amounts. Small amounts while they may not reach toxic levels can cause a nasty and persistent headache.