But most people might not realise that TikTok holds data about them even if they have never used the social media platform.

Tracking pixels are nothing new. For years, companies that run advertising networks – including Google, Meta and hundreds of others – have used them to eavesdrop on what people do across the web. They're an invisible image the size of one pixel of your screen that loads in the background of a website, full of data-harvesting tech. They're everywhere, and they're constantly watching you.

Here's how it works. TikTok, for example, encourages companies to put pixels on their websites to help the social media giant harvest more data. Let's say I have an online shoe store. If I use a pixel, it lets TikTok collect lots of data about my customers in order to show them targeted ads. Plus, it helps TikTok figure out whether people who see those shoe ads end up making a purchase. That way, I know the ads I paid for are working, and maybe I'll pay for more. (Like most news organisations, the BBC uses analytics tools and shares data with advertising partners in accordance with our privacy policy. The BBC does not use TikTok tracking pixels on its website or place advertising pixels on third-party sites.)

When it's shoe store data, the information might be innocuous. But I've reported on TikTok's data collection for years and pixels can collect extremely personal information. 

For example, last week I visited the website for a cancer support group. According to Disconnect, when I clicked a button on a form that said I was a cancer patient or a survivor, the website sent TikTok my email address along with those details. A women's health company sent TikTok data when I looked at fertility tests. A mental health organisation pinged TikTok when I indicated I'm looking for a crisis counsellor. Websites that use pixels send data about every single visitor, so it doesn't matter if you don't have a TikTok account.