Travel writer Niall Harbison spotted the devices at stores in Thailand, saying they could scan his temperature and ascertain whether he was wearing a face mask in two seconds. “Doors don’t open if not,” he tweeted. “Three cases in 100+ days here. Removes awkward mask arguments for staff as well.”

Such technology could quickly become pervasive. Similar devices are being sold online for offices, which can use them to restrict entry by employees and visitors. Twitter users said that some schools, nursing homes and workplaces in other countries, including the UK, are using similar systems to control access.

Several observers pointed out the practical shortcomings of face-scanning devices, such as the unreliability of temperature checks, the need to allow for legitimate mask exemptions, and the ability of people to take off their masks or wear them improperly after being granted entry. Another cited the potential for “smashed screens and barrier barging,” while others said scanning systems made by Chinese video-surveillance firm Hikivision have been banned in the US, Indian and Japan, among other countries, because of data security risks.