One million US Android users have downloaded apps that could be classified as adware in the last year, security firm Lookout Mobile Security has calculated as it started flagging named ad networks breaking its new blocking criteria.

The firm advertised its adware definitions six weeks ago as part of a marketing campaign that nevertheless raises some little-discussed issues for the industry as a whole.

The first of these is simply what marks an app out as non-legitimate adware that should raise suspicion? Every mobile security firm makes its own assessment when scanning users' mobile devices but not every firm is as up front about the factors it takes into consideration.

Lookout has identified three, in rough order of seriousness; displays advertising outside of the app experience, harvests personally identifiable data, and performs unexpected actions when being used.

Any one of those will now attract a negative rating by the firm although the most serious adware would likely embrace two or more of those characteristics.

And there is plenty of adware on both Google's Play and third-party app stores, the firm reckons.

The global chance of a new Lookout user already having adware was 1.6 percent with some national variation; the US was 0.9 percent, the UK at 1 percent, Germany at 1.2 percent and India at 3.8 percent, the firm said.

Lookout said that as many as 6.5 percent of free apps on Google's Play contained adware (based on a scan of 200,000), a surprisingly high figure but still well below that prevalence on third-party stores.

The rise of adware was down to ad networks used to support free apps, with the worst offenders being LeadBolt, Moolah Media, RevMob, SellARing, and SendDroid.

“While the majority of mobile ads are A-OK, as the industry grows, it needs to protect user privacy and excellent user experience,” wrote Lookout's Jeremy Linden in a blog announcing the firm's policy.

“Beginning today, we’ll be flagging ad networks that we have newly classified as adware in Lookout Mobile Security. We’re also releasing our classification of adware to help guide the industry,” he said.