Figure 1. Estimates of the CO₂, H₂S and vanillin stimuli with respect to pain, smell or pleasantness, rated by means of a visual analog scale displayed at 3.4–6.6 s after stimulus presentation and ranging from “no pain” to “pain experienced at maximum” or “no odour” to “intensive odour” or “unpleasant” to “very pleasant”.

CO₂ was always painful and never smelled. H₂S or vanillin stimuli were never painful but smelled.