H05: History of our understanding of ultraviolet A

Albert Sunghwan Hong,¹ Andrew Birnie¹ and Brian Diffey²

¹East Kent Hospitals University NHS Foundation Trust, Canterbury, UK; and ²Newcastle University, Newcastle, UK

The notion of dividing the ultraviolet (UV) spectrum into different wavebands was first put forward at the Second International Congress on Light held in Copenhagen in August 1932, when it was recommended that three spectral regions be defined: UVA (400–315 nm), UVB (315–280 nm) and UVC (280–100 nm). At that time, it was thought that UVA was of little biological consequence, but, as the years progressed, it was increasingly recognized that this belief was unfounded. The first action spectrum for erythema induced by UVA was produced in 1938 by Isolde Hausser, who used her husband’s double-prism monochromator and a high-intensity carbon arc lamp. She showed that UVA erythema appears rapidly and quickly becomes pigmented. By 1939, that immediate pigment darkening phenomenon of UVA can appear without noticeable erythema and that this is due to darkening of pre-existing pigment granules had been published. It was also shown that UVA can produce ‘delayed tanning’ by stimulating melanocytes to produce more melanin. The upheaval of the Second World War closed the big laboratories in Berlin, Copenhagen, Basel and Geneva, in which most of these works originated. It was only really in the 1970s when high-intensity, relatively pure UVA sources were developed that the studies on UVA accelerated. In 1987, Diffey showed that the time course of UVA erythema is biphasic, with an immediate phase and delayed phase. It had already been discovered in 1948 that 8-methoxypsoralen could be used prior to sunlight exposure to treat vitiligo. But studies of psoralen + UVA really took off in the 1970s. In 1973, Voorhees’ study on topical 8-methoxypsoralen and UVA from a small handheld device paved the way for their use in psoriasis. In 1974, Parrish published a landmark study where oral 8-methothxypsoralen and the use of a newly developed UVA source resulted in complete clearance of psoriasis in 21 patients. Prior to studies on UVA and skin cancer in the 1980s, UVA was not considered to be carcinogenic. This had two consequences. Firstly, most of the earlier sunscreens focused on protection against UVB only. Secondly, with the development of UVA fluorescent lamps in the 1970s, sun tanning using UVA lamps flourished. With more understanding of UVA and its role in skin cancer, sunscreens were reformulated to include protection in the UVA region. A metric for expressing the level of UVA protection led to the Boot’s star rating system devised by Diffey in 1992.