20 x 10$ pounds (9 x 10$ Kg) of NO^ into the stratosphere, compared with a natural flux of 440 to 4400 x 10$ pounds (200 to 2000 Kg), has been calculated, and so is also believed to be of little consequence. Of much greater potential consequence may be emissions of halide compounds which are believed to cause major damage to the ozone layer. If borne out by further study, the required large number of flights of the shuttle's solid fuel booster engines may be unacceptable. Alternative hydrogenoxygen boosters are being considered. c. Microwave Transmission Some atmospheric and ionospheric degradation due to the microwave beams may be marginally important. Possible deleterious impacts on high frequency communications and other uses of the ionosphere have already been noted. The ozone layer's density could be diminished by the interaction of the beam with the ionospheric plasmas. Some atmospheric heating will occur within the beam and this might affect weather circulation patterns. But, as with the preceding concerns, no assurance yet appears to be possible on this. Actual ionospheric heating tests may have to be conducted. Possible biological effects of the beam have received the greatest consideration. Controversy is nevertheless much in evidence. The basic safety standard for humans is not agreed upon: the U.S. standard, which is intended primarily for industrial or occupational type exposure, is 2 2 a power density threshold of 65 nW/in (10 mW/cm ), the U.S.S.R.'s is 2 2 0.065 mW/in (0.01 mW/cm ) for widespread public exposure. The other countries of the world have intermediate exposure standards. The U.S. standard presumes only tissue heating is of consequence; the U.S.S.R. postulates some deleterious effects on the nervous system, as well. Safety can at any rate be established, even at the U.S.S.R. level, if unshielded humans remain outside a radius of about 5-10 miles (10-15 km) from the beam center as shown in Exhibit 57. The potential effects on birds and other biota have not yet been evaluated. However, shielding with a metallic mesh can safeguard all biota except birds. It is thought the latter would avoid the higher intensity region of the beam because of its heating effect.
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