What is the biological function of nose hair? In general, can we consider underarm hair and pubic hair to be vestigial structures?
From this article I quote
Hair in the nose is one of the body's first lines of defense against harmful environmental pathogens such as germs, fungus, and spores.
Another purpose for nose hair is to provide additional humidity to the inhaled air. As the air passes through the nasal passages, the mucus and hair provide heat and moisture. Humidity is an important factor for the rest of the respiratory system, such as the larynx and lungs.
Hair in the form of tiny cilia also draw solid particles towards the interface between the nose and throat. Harmful debris is generally directed towards the back of the throat and esophagus for swallowing, while the filtered air continues towards the larynx and lungs.
If you look on an evolutionary perspective, the arm pit hair follicles as well as pubic hair have a type of sweat gland associated with them, called an apocrine gland, that begins to function at puberty and is widely associated with scent-communication via chemicals known as pheromones (reference).
Some other functions that are associated with pubic hair are:
- Keeping the genitals warm.
- Drawing attention to the genitalia.
- It also decreases friction during intercourse (reference).
The nose hair is definitely not vestigial. As long as the armpit hair (between the body and arm in certain heavyset individuals) and pubic hair serve to reduce friction and male sweat serves to increase female hormones, I wouldn't call them vestigial (reference).