A 40-year-old woman presented to the Radiology Department for a pre-operative chest radiograph. The radiographer noted an area of fogging on one corner of the film (Figure 1a). Suspecting that the cassette was faulty, a repeat radiograph was performed 7 min later. A smaller but persistent area of fogging was noted to be situated over the right nipple (Figure 1b). A further radiograph with yet a different cassette was performed 15 min after the second. This was normal (Figure 1c). What is the cause of the fogging?