Photo urticaria caused by exposure to LED emitting 633-nm wavelength during hot yoga exercise.

Photo urticaria (PU) is a rare type of urticaria that develops after exposure to various wavelengths of light. Inducing urticarial wheals using light sources of pertinent wavelengths can help make the definitive diagnosis of PU. The action spectra (AS) in Japanese patients with PU commonly fall within the ultraviolet radiation A and visible light range. Herein, to the best of our knowledge, we present the first case of PU caused by 633-nm wavelength within the visible light spectrum. Our patient worked as a "hot yoga" instructor, where light-emitting diodes (LEDs) on the ceiling were used to irradiate the entire room with 633-nm wavelength of light for "light treatment." She reported itching and wheals on the face and neck during her "hot yoga" sessions. "Hot yoga" has recently gained popularity globally. The "light treatment" is based on the theory that 633-nm wavelength light within the visible light spectrum reportedly prevents the skin from aging. We induced wheals with erythema by irradiating her skin using a 633-nm LED at a dose of 0.008 J/cm2 /s for 1 h. Her condition was diagnosed as PU caused by exposure to 633 nm. Light. Her symptoms have not recurred since she has avoided being exposed to the 633-nm wavelength of LED light.