The sky looks blue because the blue rays from sunlight are highly scattered by the atmosphere. UV is scattered even more than blue light, and this can lead to an increase in a person's exposure. Clouds generally reduce UV intensity, but light or thin clouds have little effect and under certain conditions may even enhance the UV intensity. Thus, even though haze or cloud cover can cause one to feel cooler, the UV exposure can still be high. The total daily exposure to UV depends on the intensity of the UV and on the exposure time.
Exposures vary considerably according to the type of treatment.
Fluorescent lamps are common in the workplace and are often used in the home. These lamps emit small amounts of UV and typically contribute only a few percent to a person's annual UV exposure. For example, a sunscreen with SPF 4 means that the UV exposure received after spending a given time in the sun is one-quarter that received in the absence of any protection. An even thickness should be applied liberally to clean, dry skin and allowed to dry for 15 minutes or so before going outside. Sunscreens applied too thinly or too infrequently will not provide adequate protection.