# 3D Modelling for Solar Erythermal UV Protection Provided by Human Hair
## 1 Introduction  
The skin is the most exposed organ to environmental ultraviolet (UV) radiation (280-400 nm), which is responsible for both beneficial and adverse effects on human health. An adequate exposure to solar radiation represents the main natural source of vitamin D, the hormone that controls calcium levels in the blood and helps maintain healthy bones and muscles. On the contrary, prolonged exposures could give rise to severe and long-term effects on skin, eyes and immune system [1]. UV radiation has a significant influence on the development of cutaneous melanoma, epithelial skin cancer [2] and on the premature skin ageing [3]. Currently, 132,000 melanoma skin cancers and between 2 and 3 million non-melanoma skin cancers occur globally each year, with an average yearly increase of 3%-8% over the last 30 years among white populations in Australia, Europe, Canada and the United States [4; 5; 6]. It is not obvious to calculate the UV radiation exposure received throughout an entire life, since this would mean obtaining information on all the sun-exposure durations and geolocalisations. Moreover, the personal UV exposure depends on the distribution of the three components of the radiation (i.e.,direct, diffuse and reflected) all over the human body and on behavioural factors such as the orientation to the sun, posture, skin complexion, garments and sun-protective habits [7; 8; 9]. Understanding the relationship between UV exposure and skin cancer on different anatomical zones is crucial then to define sun protection practices and messages [10; 11; 12]. Most of the existing measurements consist in photosensitive dosimeters and captors (e.g., [13; 14; 15]), which evaluate the integrated cumulative UV dose received at the specific location where they are affixed during the exposure. Despite their accuracy, dosimetry is expensive, and many dosimeters are needed to measure body-specific exposure, resulting in them being situation-specific and making generalization difficult. Though numerous studies examined the human body exposure to UV radiation [16; 17], to our knowledge, little has been done about the application of human models and tools for predicting and visualizing UV exposure [18], often without taking into consideration sun protections. To address this issue, we developed SimUVEx (Simulating UV Exposure) [19; 20], a modelling tool that combines three-dimensional (3D) techniques and ground-based UV irradiance to evaluate the relative contribution of the components of the UV radiation by different anatomical zones and in various exposure conditions. Typically, exposures of anatomical zones range from 13% to 76% of the exposure to the vertex of the head [21], and horizontal body parts exhibit higher exposure doses [22; 23]. Except for a continued use of a headgear, the head is then one of the most sun-exposed anatomical parts, and the real-time exposure is often underestimated. Moreover, strong dissimilarities in the incidence of head and neck melanoma between men and women have been observed in previous researches [24], bringing to the conclusion that hair might have a protective role in sun exposure. The goal of this paper is to improve our model so that it could estimate how much UV exposure can reach the epidermis of the head, covered by hair, without sun damage risks. The ease and the speed of the model were key points to take into consideration in numerous scenarios, while using a computer with standard requirements. The hairs were considered as independent 3D objects, with different alignments and orientations in relation with the solar position, that is, the solar zenith angle (SZA) that represents the angle between the zenith and the line of sight from that point to the sun. To the best of our knowledge, a very small number of studies have tried to quantify the effectiveness of the hair to protect against UV exposure, by spectrometric techniques [24] or dosimetries [25; 26], but none of them used 3D modelling methods.