Smell is the perception of odorants by our brains. Odorants are gaseous chemicals which stimulate sensory cells in our nose called olfactory sensory neurons. Just a few odorant molecules are enough to stimulate a sensory neuron which starts to rapidly fire nerve impulses to the brain. The brain processes the information and identifies the smell. If there is a constant odorant in the room our brain starts to perceive it as decreasing in intensity over time, ie the smell seems to fade.
This is due to a phenomenon called sensory adaptation, which is not yet fully understood. During sensory adaptation our brain adapts, recognises the constant smell is not dangerous and stops identifying it so it is not overloaded with redundant information. Our olfactory sensory neurons also adapt to the repetitive odorant stimuli by reducing their rate of firing. Therefore we perceive the smell to be fading, allowing us to adapt to our environment and perceive new smells.
Agnes Becker, Science Museum