Asparagus has been eaten and cultivated for at least two millennia but the association between odorous urine and asparagus consumption was not observed until the late 17th century when sulfur-rich fertilisers became common in agriculture. Small-scale studies noted that the "asparagus urine" odour was not produced by all individuals and estimates as to the proportion of the population who are excretors (reporting a noticeable asparagus urine odour after eating asparagus) has ranged from about 40% to as high as 79%. When excretors are exposed to non-excretor urine after asparagus consumption, however, the characteristic asparagus urine odour is usually reported. More recent work has confirmed that a small proportion of individuals do not produce asparagus urine, and amongst those that do, some cannot detect the odour due to a single-nucleotide polymorphism within a cluster of olfactory receptors.