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However, doctors may need some convincing to accept that some kinds of UV light may be safe to human eyes. “I would like to see more research on longer term exposure before I am convinced,” said Karl Linden, a professor of environmental engineering at the University of Colorado in Boulder, CO, USA. If it can be proven safe at the incidental exposure involved, far UVC light might prove ideal for disinfecting spaces that always have people in them, like a 24-hour market; they could perhaps also be used to provide constant disinfection in hospitals.

No matter what wavelength is used, germicidal light has another problem to overcome: If a surface is in shadow, it will not be disinfected. Shadows abound in a typical hospital room, with multiple surfaces and objects that jut out at odd angles from the floor, walls, and ceiling. In one recently published study, when a standard UVC lamp was placed in the center of the room and operated according to the manufacturer’s instructions, some places like the wardrobe and the sink were partly or completely in shadow, and did not receive the full dose of 40 mJ·cm−2 needed to assure 99.99% disinfection [12].