Abstract: An ultraviolet radiation sensor that may be worn on the skin to indicate exposure of the skin to ultraviolet radiation. The sensor comprises a transparent semi-permeable membrane having an upper and a lower surface, an adhesive capable of releasably adhering the semi-permeable membrane to human skin, and an indicator to visually indicate exposure to ultraviolet radiation. When the sensor is attached to human skin, by way of the adhesive, it allows for the migration of water vapour and gases from the surface of the skin through the sensor and into the atmosphere.