The human nose is an underestimated tool. Humans are often thought to be 1___ smellers compared with animals, but this is largely because, unlike animals, we stand upright. This means that our noses are  2 to detecting those smells which float through the air,  3  the majority of smells which stick to surfaces. In fact though, we are extremely sensitive to smells, even if we do not generally realize it. Our noses are capable of  4 human smells even when these are  5  to far below one part in one million.