According to BreastCancer.org, hair loss occurs because chemotherapy targets all rapidly dividing cells—healthy cells as well as cancer cells. “Hair follicles, the structures in the skin filled with tiny blood vessels that make hair, are some of the fastest-growing cells in the body. If you’re not in cancer treatment, your hair follicles divide every 23 to 72 hours. But as the chemo does its work against cancer cells, it also destroys hair cells. Within a few weeks of starting chemo, you may lose some or all of your hair,” the organization wrote.