Provinces and cities across China are slowly lifting their strict coronavirus lockdown measures. Wuhan, where the coronavirus broke out, just lifted its lockdown.

But as people resume their lives, local authorities are trying to prevent further spread by controlling citizens' movements via smartphone software installed in WeChat and Alipay, two hugely popular apps.

After people fill out a quick health survey, the software issues them with a colored health code — green, yellow, or red — which dictates whether they can leave the house and where they can go.

Officials manning various checkpoints across the country are checking people's health apps to see where they are allowed to go. Anyone with a green code is free to travel.

But Western nations are critical of this mass surveillance tool and questioning what else the data is being used for.

Scroll down to see how it works.

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As provinces and cities across China gradually roll back their coronavirus lockdown measures, authorities trying to prevent another outbreak are using smartphone software to monitor citizens' health and dictate where they go.

Officials in Wuhan, where the virus first broke out, are also using this software to limit its citizens' movements, the BBC reported. The city lifted its lockdown at midnight local time on Wednesday.

The software is installed in WeChat and Alipay, the instant-messaging app, and Alipay, the online payment platform operated by Alibaba. Almost everyone with a smartphone in China has one or both of these apps.

In order to travel, people have to fill out a quick health survey. After that, the software issues them with a colored health code — green, yellow, or red — which dictates whether they can leave the house and where they can go.

The initiative was first introduced by officials in the eastern city of Hangzhou, but others have since followed suit. As of February 25, the program was being used in 200 Chinese cities. This number has likely grown.

Scroll down to see how the technology works and why some surveillance experts are concerned.