The head is one of the most sun-exposed anatomical zones and its ultraviolet (UV) exposure is often underestimated. The goal of this research is to quantify the solar erythemal UV protection provided by human hair for different orientations and Solar Zenith Angles (SZA). For this purpose, a three-dimensional (3D) numeric simulation tool called SimUVEx was used to forecast the distribution of UV exposure based on geometric information and solar irradiance. The relative UV dose reduction was expressed in terms of predictive protection factor (PPF). Results showed that the effectiveness of the protection depends strongly on the orientation of the head and on the tilt of the hairs. As the SZA increases, the PPF also increases. PPFs provided by hair with a certain degree of order (parallel alignment) were generally higher by a range of about 10%–30% than the ones provided by random arrangements. A significant exposure to the scalp through the hair, even for an average hair density, was found. It confirms that hair is an important natural sunscreen in reducing transmission of UV radiation and must be considered in the prevention messages of skin cancer on the scalp, particularly in people with alopecia.