@Horschtel @stim3on The main engine of the SLS uses Hydro-Lox (liquid hydrogen + liquid oxygen) as a propellant, which produces almost exclusively water vapor during combustion.

The only slightly problematic part are the Aluminium perchlorate boosters, which produce CO2, aluminium oxide and hydrogen chloride (which combines with water into hydrochloridric acid, the same as we have in our stomach, and causes the visual plume).

Because the boosters' combustion happens in the lower atmoshpere, the exhaust particules can fall down back to earth where they get diluted. It's much less of a concern than re-entering satellites, whose combustion creates much more problematic compounds that remain in the higher atmosphere, probably for decades.