Human hair consists of three layers. The outside layer, known as the cuticle, is made of scales of the protein keratin. Much like the shingle coverings of a roof, the keratin scales protect the inner portions of the hair. The next layer also contains keratin, but is made of protein fibers more tightly knit together. Finally, the inner core--known as the medulla--is composed of round cells.
The sebaceous glands produce oil on the scalp. Hormones control the amount of oil a person secretes--which explains why some people may have oilier hair than others.
When oil is applied to the hair (or coated via the sebaceous glands), two things take place. The first is that the oil is able to seep into areas where the keratin scales may have flaked off or do not fully cover the hair, leaving open areas exposed. Once again, consider the hair strand as a roof. Over time, damage from heat styling, excessive brushing, pollutants or other occurrences can strip the roof's shingles away. The result is that oil is able to flow under the panels and be absorbed by the hair.
After this oil seeps into the hair, the remaining oil will cling to the tiny scales outside the hair's cuticle.
The hair's ability to absorb oil means more than just that a person can have a bad hair day. Hair's ability to absorb oil has been used to sop up oil following several oil spills, including a 50,000-gallon fuel leak off the Philippines' Guimaras island. Using hair mats created from discarded hair clippings from salons and animal hair, rescuers applied the mats to the oil spill, which in turn absorbed the oil.
When it comes to everyday application, hair's ability to absorb oil or to become coated with oil means most people must wash their hair every day or every other day.