为什么我的头发会这么干？

不要脱发

严格地说所有的头发都是干性。一般死掉的头发就会有皮脂腺分泌液，并且附着在头发的毛囊上。事实上，当人们说头发很干，真正的意思是头发看上去很脏、粗糙。这是因为角质层层外的头发纤维被破坏成片状。比较健康的新的头发看上去会更加有光泽，摸上去更光滑。随着年龄的增长，头发纤维暴露在各种污染的下的时间增加，并且通过频繁的洗涤，使角质层失去其光泽的外观，角质层鳞片远离头发骨干。当轻轻拍打头发的时候，角质层鳞片（俗称头屑）就会向各个方向飘落，这使得头发看起来很“干”。
头发的角质层强度和弹性一般和人种有很多关系。比如，亚洲人的头发相对比较厚和粗糙，而白种人一般拥有较细的头发和平滑的角质层。非洲裔美国人的头发相比亚洲人或白种人来说，含有更多低硫蛋白。这使地头发更强劲，能够迅速适应各种恶劣的天气和环境。这些结论都有助于研究治疗头发过于干燥的药物。

不要脱发

Why is my hair really dry and dull?
Strictly speaking all hair is dry. It's dead and any oil is on the outside of the hair as supplied by the oil secreting sebaceous glands attached to the hair follicle. When people say hair looks dry what they really mean is the hair looks dull and feels rough. This is because the outer cuticle layer of the hair fiber is broken and flaked. Healthy new cuticle looks shiny and the scales of the cuticle adhere closely to the hair shaft making it look smooth. As the hair fiber ages and gets weathered by exposure to pollution and through washing so the cuticle layer loses its shiny appearance and the cuticle scales flake away from the hair shaft. As the light hits this uneven surface it gets reflected in all directions and this makes the hair fiber look dull and "dry".
Some people have hair that is more robust and with a smoother and shinier cuticle than others. For example, Asians produce thick and relatively coarse hair whereas Caucasians have finer hair with a smoother cuticle. African American hair has a greater amount of low sulfur protein compared to high sulfur protein than observed in Asian or Caucasian individuals. This makes hair less robust and so this type of hair weathers and deteriorates more quickly. All these things can contribute to dull and dry looking hair.