[gilleain]

Not precisely the same:

https://en.wikipedia.org/wiki/Hydrogen_cyanide - 107 ppm (human, 10 min)

https://en.wikipedia.org/wiki/Hydrogen_sulfide - 600 ppm (human, 30 min)

https://en.wikipedia.org/wiki/Carbon_monoxide - 4000 ppm (human, 30 min)

These are "LCLo" values from the databoxes on those pages. More easily comparable numbers may be around somewhere.

[WarmWash]

Hydrogen cyanide has the bonus of being mostly odorless too. Whereas hydrogen sulfide is distinctly bad smelling.

[ted_dunning]

Apparently it only smells bad if you have non-lethal concentration.