INTRODUCTION
Research has shown that scalp and hair health are connected, but the link is not yet fully understood or characterized. Hair growth begins within the hair follicle, about 4 mm deep in the skin.1 The cells proliferate in the bulb of the follicle and produce a nascent hair fiber that continues to grow to the scalp surface.1 As the hair extends to the skin surface, the hair fiber undergoes keratinization.1 At the same time, the skin microenvironment surrounding each follicle propagates signals to the hair fiber, thereby impacting the quality and character of the resultant hair.1 The hair follicle undergoes cycles of growth (anagen), apoptosis-mediated regression (catagen), and relative quiescence (telogen).1,2 During its life cycle, the hair follicle is highly sensitive to surrounding growth factors, cytokines, neuropeptides, and hormones, often produced by the hair follicle itself.2 Thus, scalp and hair follicle health is critical to hair care. Factors affecting scalp health may go into four categories: hair follicle health, inflammatory conditions, environmental/exogenous factors, and scalp microbiome.2 While overall scalp health has yet to be understood, inflammatory scalp conditions such as psoriasis, dandruff/seborrheic dermatitis (D/SD), atopic dermatitis (AD), as well as fungal scalp conditions such as pityriasis versicolor (PV), and tinea capitis have been extensively studied.1 It has long been known that inflammatory scalp conditions negatively impact hair growth; however, the characterization of overall scalp health and its impact on hair growth has been less evident in the literature. There is little information about scalp care in scalp-related inflammatory conditions. Furthermore, exogenous agents such as chemotherapy, smoking, ultraviolet radiation, and chemical insults from hair products and pollutants, amongst others, can negatively impact hair growth.3 Contact dermatitis from fragrances, hair colorants, or treatments containing formaldehyde and formaldehyde releasers may also result in an irritated scalp and inflammation, leading to hair shedding.3 Exogenous factors such as psychological and physical stressors have also been reported to impact the hair growth cycle, likely through modu-
