[jhayward]

The search space for which molecule or sets of molecules are being recognized is huge. There are at least 120 different receptor types, and the dynamic range of their detection thresholds is large too. So we're looking at perhaps a trillion different combinations in theory.

Plus the detection threshold can be as low as parts per billion, spread across 300 million receptors in a dog.

So just the mathematics of the search space seem like a pretty good reason why it is a hard problem in general.