SAN FRANCISCO — A privacy test conducted by Washington Post columnist Geoffrey Fowler has found that popular iPhone apps are using trackers and sharing user data, sometimes including sensitive information like email, phone number, location, and IP address.

In a single week, Fowler encountered 5,400 data trackers, mostly within apps, which according to privacy firm Disconnect would send out 1.5 gigabytes of data in a month.

In most cases, users are not aware that this is happening, or that the trackers are even present.

Trackers can have different purposes. Some analyze user behavior in order to let apps improve performance, create targeted ads, or help identify fraud.

But according to Disconnect Chief Technology Officer Patrick Jackson, there is a lack of transparency. Often, trackers don't anonymize data or limit retention time, and users aren't given clear information on where collected data is going or who it's shared with.

According to Fowler, it's especially disappointing coming from Apple, which claims its software and hardware are "designed to provide advanced security and privacy at every level of the system.

He suggested that Apple require apps to label when they are using third-party trackers, or add privacy controls to give users more visibility.

In the meantime, concerned iOS users can turn off 'Background App Refresh' in settings, or use a VPN to limit the data that apps are able to send to third party companies.