Skatole is a large nitrogen-containing compound largely responsible for making manure smell like manure. Skatole's detection level is 1.2 ppb, or less than one ounce of skatole in the Superdome. From the previous two examples, you can see that detection and recognition levels are not always directly related. Humans can detect and recognize hydrogen sulfide at very low levels. Skatole, on the other hand, is easily detected, but only becomes recognizable at larger doses.
Ammonia is an extreme example.
Farm odors are never pure samples of one odorant, but rather a mixture of many different odorants. Recently, a sample of air was taken from a hog building in Germany. When chemists analyzed the sample, they measured at least 11 different organic acids, but none of the individual acids were present at detectable levels. Because the individual odorants were below the detection level does not mean they could not be smelled. The brain lumps similar odorants together as a group.