“Olfactory cilia” is a fancy way of saying “nose hairs,” but is important to distinguish between the macroscopic nose hairs near the opening of the nostrils, and the microscopic hairs in the olfactory epithelium, the part of the nose which traps smells and communicates them to the brain. The microscopic olfactory cilia play a very important role in the perception of smell, and they perform several other functions for the nose as well.
Properly speaking, the visible nose hair is just hair, not cilia. Cilia are specialized biological structures which closely resemble hair, but on a much smaller scale. The nose hair near the front of the nose helps to trap particulate matter, preventing harmful materials from entering the nasal passages and defending the body from potential sources of infection. Because of this important function, many doctors do not recommend trimming nose hair, no matter how aesthetically displeasing it may be.
The olfactory cilia inside the nose line the mucus membranes of the nose, and unlike most other cilia in the body, they are non-motile, remaining stationary in the nose rather than wiggling around in the mucus like the cilia which line the trachea and intestines do. As smells enter the nose, they dissolve in the mucus and come into contact with the olfactory cilia. The cilia in turn transmit the smell to the olfactory nerve, which passes the information on to the brain. This process can be lightning-fast, as anyone who has ever walked past a sewage treatment plant can tell you.
Many people are aware that dogs and some other animals have a much better sense of smell than humans. This is because the interiors of their noses have a much higher surface area, providing more of a space for smells to come into contact with the cilia and therefore creating a larger filter for incoming smells. Because humans have shortened noses and flat faces, rather than elongated snouts, they don't have the room for the extensive sensory membranes common to many animals. Intriguingly, many domesticated animals have shorter snouts than their wild relatives, suggesting that sense of smell may be one of the first senses to decrease with domestication.
There are cases in which olfactory cilia can be damaged or absent, impeding sense of smell and creating a condition called anosmia. While anosmia may sound like a minor inconvenience in humans, it can actually be quite dangerous, as the sense of smell is used to determine when food is going bad, whether gas leaks are present in an area, and to check for other signs of potential danger.