A new study from security firm Lookout reveals that over the past year, more than a million American Android users have downloaded adware without knowing it. Perhaps even more worrisome is that some 6.5 percent of free applications available via Google Play contain adware in some shape, form or fashion.

In a blog post on the topic, Lookout noted that adware - described as the most prevalent app-based mobile threat around the world today - can get in the way of user privacy and experience by doing things like capturing personal information such as e-mail, location data and address lists without proper notification and modifying phone settings without consent.

What exactly does and doesn't constitute adware is still grey area to most as there's not really a defined set of rules on the subject. Lookout did, however, come up with some guidelines that they use to differentiate a seemingly innocent ad from adware.

The firm classifies an app as containing adware if it displays advertising that's outside of the normal experience, if it harvests unusual personally identifiable information or if it performs unexpected actions as a response to ad clicks.

Lookout also offers Android users some tips on what types of apps typically have adware. We're told that some 26 percent of free personalization apps in Google Play contain some type of adware. Furthermore, nine percent of free games are laced with adware while social apps are the least likely to have it at just two percent.