Long-term trends in cloud type and amount are largely unknown due to the relatively short data record of comprehensive cloud observations, and the high variability of clouds on inter-annual and longer time scales. Some evidence exists showing that, at least over the time span of satellite-based ozone measurements, changes in cloud cover have been much less important than stratospheric ozone reductions in causing surface UV changes.
19) Are the risks of ultraviolet (UV) exposure at the beach less on a cloudy day?
Not necessarily. The effect of clouds on UV radiation is as varied as the clouds themselves. Fully overcast skies lead to reductions in surface UV irradiance. On average, scattered or broken clouds also cause reductions, but short-term or localised UV levels can be larger than for cloud-free skies if direct sunlight is also present. Clouds tend to randomise the directions of the incoming radiation (because of scattering) so that a hat may provide less protection on a cloudy day relative to a clear day.
Furthermore, people often change their behaviour on cloudy days. If they spend more time out in the open, or forego the use of sunscreen, they may end up with a very bad sunburn. In general, less UV radiation is received per hour under an overcast sky than under a clear sky, but extending one’s stay at the beach may easily compensate for this effect. A completely cloud-covered sky may still transmit substantial amounts of UV-B radiation. In principle, any amount of UV-B radiation exposure contributes to the skin cancer risk.
Not always. Sunscreens applied to human skin limit the penetration of UV radiation into the skin, and thus sunburn can be prevented. Sunscreens were primarily developed for this purpose. The effectiveness of sunscreens in protecting against skin cancer and immune suppressions is under debate. Any effectiveness in these respects may well be lost if the sunscreen is used to stay out in the sunlight longer than would be done without the sunscreen. It should also be kept in mind that there are other ways to protect the skin. These include staying out of the sunlight during the hours when the UV-B is maximal around solar noon, seeking the shade, wearing clothes, and especially hats.
21) Will getting a suntan help prevent skin cancer?
No. There is no evidence that getting a suntan will help prevent skin cancer. The UV exposure needed to acquire the tan adds to the skin cancer risk. The fact that one is able to tan well does, however, signify that the personal risk is lower (by a factor of 2 to 3) than for people who do not tan. Naturally dark-skinned people have a built-in protection of their skin against sunlight.
22) Is tanning with UV lamps safer than with sunlight?
No. The risks are approximately equal. For some time it was hoped that UV lamps could be made safer by making more use of long-wavelength (UV-A) radiation. That type of radiation is much less carcinogenic than the shorter-wavelength UV-B radiation, but one needs more UV-A than UV-B for acquiring a tan.
Yes. Several attempts have been made to investigate the economic impacts of the problem of a depleted ozone layer. Such attempts meet with many problems. There are good reasons for concern for effects on humans, animals, plants and materials, but most of these cannot be estimated in quantitative terms. Calculating the economic impact of such effects is uncertain. Moreover, economic terms are applicable only to some of the effects, such as the cost of medical treatments, and the loss of production in fisheries and agriculture, and damage to materials; but what is the cost equivalent of suffering, of a person becoming blind or dying, or the loss of a rare plant or animal species?
In spite of all these difficulties, attempts have been made. The most comprehensive example is a study initiated by Environment Canada for the 10th anniversary of the Montreal Protocol on Substances that Deplete the Ozone Layer. In this study, ‘Global Costs and Benefits of the Montreal Protocol’ (1997), the costs were calculated for all measures taken internationally to protect the ozone layer, such as replacement of technologies using ozone-depleting substances. The benefits are the total value of the damaging effects avoided in this way. The total costs of the measures taken to protect the ozone layer were calculated to be 235 billion US (1997) dollars. The effects avoided world-wide, though far less quantifiable, were estimated to be almost twice that amount. This latter estimate included only reduced damage to fisheries, agriculture and materials. The cataracts and skin cancers, as well as the potential associated fatalities avoided, were listed as additional benefits, and not expressed in economic terms.