83. Mrs. Butlerasked the Minister of Health, as representing the Minister for Science, what is now believed to be the residence-time of fall-out in the stratosphere; and to what extent the figures of fall-out over Great Britain for the last few months or to the latest convenient date indicate that radioactive material is now virtually absent as a result of the voluntary suspension of hydrogen bomb tests.
§ Mr. Walker-SmithThe time during which radioactive particles remain in the stratosphere following a nuclear explosion depends on the size, location and time of year of the relevant nuclear explosion. The lowest estimates have varied from 15 months for material injected in equatorial regions in recent years to four months for material injected in Arctic regions in autumn. 1958.
Measurements of radioactivity in air over the United Kingdom last autumn showed a very marked fall compared with earlier in the year, in part due to a seasonal decline. Measurements in the first half of 1960 are not yet available.