Ivor,
The off-gassing of sodium metabisulphite and sodium metabisulphate can cause severe irritation to eyes, mucous membranes and lungs, but if mixed with an acid the fumes can be toxic.
Another substance to avoid is butyl cellusolve, which was at one time a common ingredient of traffic lane cleaners - hopefully now removed from formulations.
It is strongly suspected as being carcinogenic, with particular association with testicular cancer.