YELLOWING or darkening in sunlight is a common occurrence. Ultraviolet light with photon energies in excess of bond energies disrupts molecules and produces coloured and/or reactive groups. Wool yellowing increases below 365 mµ; at 254 mµ greening and free radicals occur1,2. Other materials form free radicals3. Skin reddens only below 320 mµ (ref. 4). Cellulose yellows in middle ultraviolet5. Yellowing is often caused by reactions after irradiation: wool did not yellow in Berkeley sunlight unless liquid water was present during or after exposure1. Lignin did not yellow in near ultraviolet unless oxygen and water vapour were present6.