amount of solar ultraviolet which does get through. To defend themselves
against this low level of ultraviolet, evolved external shielding
(feathers, fur, cuticular waxes on fruit), internal shielding (melanin
pigment in human skin, flavenoids in plant tissue), avoidance strategies
(plankton migration to greater depths in the daytime, shade-seeking by
desert iguanas) and, in almost all organisms but placental mammals,
elaborate mechanisms to repair photochemical damage.

It is possible, however, that a major increase in solar ultraviolet might
overwhelm the defenses of some and perhaps many terrestrial life forms.
Both direct and indirect damage would then occur among the bacteria,
insects, plants, and other links in the ecosystems on which human
well-being depends. This disruption, particularly if it occurred in the
aftermath of a major war involving many other dislocations, could pose a
serious additional threat to the recovery of postwar society. The National
Academy of Sciences report concludes that in 20 years the ecological
systems would have essentially recovered from the increase in ultraviolet
radiation--though not necessarily from radioactivity or other damage in
areas close to the war zone. However, a delayed effect of the increase in
ultraviolet radiation would be an estimated 3 to 30 percent increase in
skin cancer for 40 years in the Northern Hemisphere's mid-latitudes.



SOME CONCLUSIONS


We have considered the problems of large-scale nuclear war from the
standpoint of the countries not under direct attack, and the difficulties
they might encounter in postwar recovery. It is true that most of the