The BBC highlights a study that shows humans can actually sense danger with smell, assuming prior experience/exposure:

The 12 volunteers were exposed to two “grassy” odours, and none of them could accurately tell the difference between them.

After they were shocked while smelling one of them, they developed the ability to discriminate between the two.

Do they mean marijuana? Was this really a study to get people with addictions to steer clear? Imagine the impact to the cocaine market if convicted addicts were conditioned so the smell would turn them away.

Ethical questions abound, but it also makes me think about the discussions I often have with risk managers. They do not smell danger while the technical security engineers do, and thus a discussion erupts about who is right and what is real. In the end, the primary issue I see with the study above is that it shows pre-conditioning response as opposed to the ability to actually smell danger itself.