The reactor accident at Windscale on 10-11 October 1957 resulted in the release of radioactive fission products into the atmosphere over a period of several hours. Measurements were made of the fission products, particularly iodine-131, collected on filters exposed for the study of atmospheric pollution in many places in the United Kingdom and Europe: measurements were also made of the iodine-131 deposited on grass. These showed that the effluent had been widely dispersed, but that activities over England were about an order of magnitude greater than those found over the neighbouring areas of the continent. Meteorological trajectory analysis showed that the main portion of the cloud had remained over England until 15 October, and that the activities over continental Europe were due to the eastern edge of the cloud.
Meteorological data were used together with the air concentration data to deduce that about 22,000 curies of iodine-131 were present in the cloud as it passed over south Lancashire and Yorkshire.