Manishearth asked me to chime in, so here we go:
I think toxicology and pharmacology questions, in addition to reagent safety questions, are perfectly well suited for this site. Not to show disrespect to biologists, but I feel that many chemists here would probably be willing/able to cast much more light on the mechanisms of toxicity through chemical structure and reactivity, whereas I get the feeling that biology takes more of a large-scale systems approach to these explanations.
On the big Venn/Euler diagram of science, there's a tremendous amount of overlap between chemistry and biology (and chemistry and physics) so I guess my take is that if you ask a biochemistry question on a chemistry site, expect an answer in terms of atoms and bonds and conformers, whilst if you ask the same biochemistry question on the biology site, expect an answer in terms of metabolite concentrations and anatomy and even epidemiology. Both answers will have a different focus and both will be worthwhile.
To address the OP's question, the bad smell in question is almost certainly hydrogen sulfide (aka rotten egg gas), which actually has high acute toxicity, but which the nose is exquisitely sensitive to (section 1.2.1 gives a breakdown of detectability and toxicity thresholds). The fun* thing about hydrogen sulfide is that it actually anaesthetises the nose at high (i.e. toxic) concentrations so that you can no longer smell it.
*not actually fun.