Lethal?

Potassium Cyanide for wet plate is pretty serious stuff. Too much DiHydrogen Monoxide kills many every year - probably more in one year than all the photo chemistry combined for the past 100 years.

On a more serious note... and since someone else brought up pesticides. Zyclon B, used by the Germans in WWII in the extermination camps, was a pesticide.

Leaving the darkroom per se.... a lot of people died working with aerosol adhesives which contained chlorinated propellants. They had such a
bad reputation among the picture framing industry that using them was commonly referred to as suicide. The spray contact cements of today
might not be quite as lethal, but will certainly rot your brain cells out fast enough - unless the people that use these are just so stupid to begin
with that nobody can tell the difference! - same goes for "street artists" who inhale spray paint fumes all day - but their work generally looks
brainless anyway, so it fits.

One of the methods the body uses for detoxifying certain chemicals is a process called conjugation. The toxin is combined with another chemical such as glutathione in the liver to produce a less toxic and easily excreted compound. In order to do this the body needs a "handle" on the toxin molecule, a site where conjugation can take place. This can be as simple as a carboxyi (-COOH) group. A classic example where conjugation cannot take place since there is no handle, is carbon tetrachloride CCl4. To a lesser extent the simple hydrocarbons such as trichloroethane CH3CCl3 all have this problem.