Solvents really should just be avoided altogether. Like Kleiner said, there are some "inhalants" that are relatively safe like Nitrous Oxide (completely safe) but when you start speaking more broadly and include things like Gasoline, Glue and Paint Thinner, there's not going to be a safe way to ingest them. They are just very dangerous and poisonous by nature. Inhalants really take the cake in a lot of ways in terms of relative danger and damage done. Industrial glue and solvents are a common choice for poor folk in the developing world and to give them credit, they're for the most part unaware of how terribly harmful these things are for them. A lot of them end up mentally handicapped and otherwise just wrecked. Inhalants, for these reasons are not things that we can in any way condone here on BL. In the name of Harm Reduction, they are just too dangerous.