A method of measuring linear hair growth by the intradermal use of sulfur 35 L-cystine, followed by autoradiography of hairs, has been investigated in experimental animals and humans. It has been shown to produce satisfactory results without danger from radiation as the doses used are very small. The method is superior to other ways of measuring linear hair growth, owing to its accuracy and cosmetic acceptability in human subjects. It is suitable for marking hairs at intervals during a variety of environmental conditions and drug therapy, while the hairs continue to grow undisturbed.
DOWLING D. MUNRO. Hair Growth Measurement Using Intradermal Sulfur 35 Cystine. Arch Dermatol. 1966;93(1):119–122. doi:10.1001/archderm.1966.01600190125028