SURVEILLANCE cameras have long spied on public spaces across Britain. Now, they increasingly peep from the shoulders of all sorts of officials roaming streets, trains and public buildings. On July 1st the health department announced that the crews of 500-odd ambulances would be equipped with body-worn cameras, in a pilot study that could be expanded to all ambulances and even hospital wards. The move is intended to cut the rising number of attacks on paramedics.

Britain already stands out for its widespread use of body cameras, especially in the public sector. Most of the country’s police officers now wear them. They are being rolled out to all prison officers and waste-site inspectors, who often encounter threats from fly-tippers. In 2016 more than half of local authorities had issued body cameras to some of their staff, including traffic wardens, dog catchers, health and safety inspectors, bus drivers and rubbish collectors. In the private sector, they have been taken up by nightclub bouncers, football match stewards, building-management firms and car-park attendants, among others.

The cameras’ main job is to record evidence of bad behaviour by members of the public. Hakeem Badru, a security guard at a supermarket in east London, has worn a camera for five years, and finds it indispensable. To prove shoplifting, he must catch the offender with the loot outside the shop, which usually means out of sight of the store’s CCTV cameras.

Body-worn cameras help in other ways, says John Biggs, the mayor of Tower Hamlets, a London borough that has given the gadgets to staff who deal with anti-social behaviour. People think twice about misbehaving when they know their actions may be recorded, he says. And when shown video footage of their offence, they are more likely to admit fault, avoiding a costly and time-consuming dispute in court. Officers also spend less time writing up incidents that are caught on camera.

The firmest evidence on whether body cameras change behaviour concerns the police, who were among the first adopters. Many studies in America and Britain have found that cameras can reduce complaints against officers. Results on whether the gadgets affect the amount of violence meted out by or against the police are varied: in some places these went up; in others down. A study in Essex found that cameras led to more charges in domestic-abuse cases, possibly because they captured context, comments and emotions better than officers’ notes could. But other recent studies in London and Washington, DC, found no effect on charging decisions.

The worries of civil-liberties campaigners are tempered by the fact that the devices can expose wrongdoing by officials, too. Yet Millie Graham Wood of Privacy International says that the intrusions committed by body cameras are not always justified by their benefits. Victims of crime, or patients in hospital, may be less likely to reveal sensitive information on film.

Police protocols say the gadgets should normally be switched on only in certain situations, such as when entering someone’s home by force. But the Surveillance Camera Commissioner’s guidelines—which apply to both public and private sector, but are voluntary—are vague about when they should be on. Last year Big Brother Watch, a charity, found that in some local authorities they were left rolling continuously while officials did their rounds. The British Standards Institute, which sets business standards, has also published guidelines, but it is anybody’s guess how many firms pay attention.

Police guidelines suggest keeping footage for no longer than a month, unless it is needed in an investigation. Local authorities usually follow that rule. Still, Big Brother Watch found that a fifth of them kept footage for much longer, increasing the risk of it being hacked. In 2013-17 more than a quarter of councils lost confidential data to cyber-attacks. Should the public trust them to keep sensitive recordings safe?