Details of where and how often you can hear this distinctive call are presented in "Summarised results of a survey of pig abattoirs in England and Wales", a 1993 report by M. Anil and J. McKinstry of Bristol University.
Pigs in Britain are slaughtered by first hanging them up and then applying a technique know as "sticking" (i.e. throat slashing). Before sticking them, however, they are supposed to be stunned to protect the eardrums of slaughterhouse workers. The survey revealed a very different state of affairs.
On visits to 29 abattoirs over the period 1989-92, the researchers identified a range of problem areas conducive to squealing: stunning currents that were too low; improper positioning of electric stunning tongs; a frequent need to stun pigs twice; and the regaining of consciousness by many after being stuck.
Slaughterhouses were divided into two types: those that restrained the pigs in chutes during stunning, and those that did not. The majority were found not to use restraining chutes, and the following data relate to those establishments.
The average current at every abattoir was less than 1.3 amps, despite the fact that the Code of Practice laid down by the Ministry of Agriculture, Fisheries and Food (MAFF) recommends 1.3 amps as the minimum for pigs. The scientists conducting the survey personally felt a lower current than 1.3 amps was sufficient to stun pigs, and recommended at least 0.4 amp. But still, nearly half the abattoirs used average currents even lower than this.
As for the delivery of the stun, for electric tongs to be effective they must span the pig's brain. Yet in fully 36% of the cases observed, the tongs were positioned wrongly. Another problem area was the length of time between stunning and sticking. If a pig's throat is not cut quickly after stunning, it can recover consciousness before death. One of the researchers recommended that the interval between stunning and sticking should be no more than 15 seconds, but found the average interval to be 30.1 seconds.
As a result of one or more of these factors, 15.6% of pigs required stunning twice, while 20.5% of pigs had regained consciousness at the time they were stuck.
The law in Britain requires that animals in slaughterhouses remain stunned until they die. But according to the organisation Compassion in World Farming (CIWF), which reviewed the research results, as many as 1.5 million pigs could be having their throats cut in Britain each year while still conscious.
Since the report was published, MAFF has issued fresh guidelines for slaughterhouses handling pigs, but in the words of the CIWF, "this is no substitute for binding laws."