And now, private companies are being encouraged to go beyond surface recognition and peer into their workers' brains as they deploy new brain scanners that monitor abnormalities in emotions and thought patterns.

When data scientists monitoring brain waves on the other end detect spikes in anger, anxiety or depression. When one is detected, the worker being monitored is asked by management to take the day off, or to move to a less-important role, according to the South China Morning Post.

Jin Jia, associate professor of brain science and cognitive psychology at Ningbo University's business school, said a highly emotional employee in a key post could affect an entire production line, jeopardising his or her own safety as well as that of others.

"When the system issues a warning, the manager asks the worker to take a day off or move to a less critical post. Some jobs require high concentration. There is no room for a mistake," she said.

The SCMP trumpeted the technology's profit-boosting powers, quoting managers at several firms who claimed the technology had saved them hundreds of million - if not billions - of yuan by helping workers avoid mistakes.

The technology is also in use at in Hangzhou at State Grid Zhejiang Electric Power, where it has boosted company profits by about 2 billion yuan (US$315 million) since it was rolled out in 2014, according to Cheng Jingzhou, an official overseeing the company's emotional surveillance programme.

"There is no doubt about its effect," Cheng said.

The company and its roughly 40,000 employees manage the power supply and distribution network to homes and businesses across the province, a task that Cheng said they were able to do to higher standards thanks to the surveillance technology.

But he refused to offer more details about the programme.

Zhao Binjian, a manger of Ningbo Shenyang Logistics, said the company was using the devices mainly to train new employees. The brain sensors were integrated in virtual reality headsets to simulate different scenarios in the work environment.

"It has significantly reduced the number of mistakes made by our workers," Zhao said, because of "improved understanding" between the employees and company.

One researcher said that while employees were initially suspicious of the brainwave-reading helmets, they've managed to adjust to the new technology.