When you browse the web, an increasing number of sites and apps are asking for a piece of basic information that you probably hand over without hesitation: your email address. It may seem harmless, but when you enter your email, you’re sharing a lot more than just that.

To advertisers, web publishers and app makers, your email is important not just for contacting you. It acts as a digital breadcrumb for companies to link your activity across sites and apps to serve you relevant ads.

For decades, the digital advertising industry relied on invisible trackers planted inside websites and apps to follow our activities and then serve us targeted ads. There have been sweeping changes to this system in the past few years, including Apple’s release of a software feature in 2021 allowing iPhone users to block apps from tracking them and Google’s decision to prevent websites from using cookies, which follow people’s activities across sites, in its Chrome browser by 2024.

Advertisers, web publishers and app makers now try to track people through other means — and one simple method is by asking for an email address.

Imagine if an employee of a brick-and-mortar store asked for your name before you entered. An email address can be even more revealing, though, because it can be linked to other data, including where you went to school, the make and model of the car you drive, and your ethnicity.

“I can take your email address and find data you may not have even realised you’ve given to a brand,” said Michael Priem, CEO of Modern Impact, an advertising firm in Minneapolis, US. “The amount of data that is out there on us as consumers is shocking.”

Advertising tech is continuing to evolve, so it helps to understand what exactly you’re sharing when you enter an email address.

One technology that is gaining traction is an advertising framework called Unified ID 2.0, or UID 2.0, which was developed by the Trade Desk, an ad-technology company based in Ventura, California, US.

Say, for example, you are shopping on a sneaker website using UID 2.0 when a prompt pops up and asks you to share your email address and agree to receive relevant advertising. Once you enter your email, UID 2.0 transforms it into a token composed of a string of digits and characters. That token travels with your email address when you use it to log in to a sports streaming app on your TV that uses UID 2.0. Advertisers can link the two accounts together based on the token, and they can target you with sneaker advertisements on the sports streaming app because they know you visited the sneaker website.

Since your email address is not revealed to the advertiser, UID 2.0 may be seen as a step up for consumers from traditional cookie-based tracking, which gives advertisers access to your detailed browsing history and personal information.

However, in an analysis, Mozilla, the nonprofit that makes the Firefox web browser, called UID 2.0 a “regression in privacy” because it enabled the type of tracking behaviour that modern web browsers were designed to prevent.

There are various options for limiting the ability of advertising companies to target you based on your email address:

— Create a bunch of email addresses. Each time a site or an app asks for your email, you could create a unique address to log in to it, such as, for example, netflixbrianchen@gmail.com for movie-related apps and services. And if you receive spam mail to a specific account, that will tell you which company is sharing your data with marketers.

— Use email-masking tools. Apple and Mozilla offer tools that automatically create email aliases for logging in to an app or a site; emails sent to the aliases are forwarded to your real email address. Apple’s Hide My Email tool, which is part of its iCloud+ subscription service that costs 99 cents a month, will create aliases, but using it will make it more difficult to log in to the accounts from a non-Apple device. Mozilla’s Firefox Relay generates five email aliases for free.

— When possible, opt out. For sites using the UID 2.0 framework, you can opt out by entering your email address at https://transparentadvertising.org.