The human nose can detect over one trillion distinct smells

For decades, the standard figure was that humans could detect around 10,000 smells — a number based on no empirical study whatsoever, just a 1927 textbook estimate. In 2014, Rockefeller University researchers conducted the first systematic study, combining 128 odour molecules into random mixtures and asking participants to distinguish them. They calculated that the olfactory system can theoretically discriminate over 1 trillion distinct odours — more than the number of colours the eye can see or tones the ear can hear. The nose is, by this measure, the most sensitive of all human senses.

Source: Bushdid, C. et al., 'Humans Can Discriminate More than 1 Trillion Olfactory Stimuli', Science (2014)