A dog's sense of smell is more powerful than we can possibly imagine. They can easily detect the presence of one drop of blood in a 55-gallon drum of water.
Tests performed back in the 1950s by Neuhaus, it was concluded that dogs "Having an olfactory acuity which is from 1,000,000 to 100,000,000 times that of man, a dog can detect one mg of butyric acid in 100,000,000 cubic metres of air, the "volume', said Neuhaus, of a whole town."
(Olfaction and Odours by William McCartney).