You can't smoke in restaurants, bars or at work anymore. This much we know.

But as anyone who has blearily made their way to the loo on a long flight will attest, sometimes you can't help but wonder: 'why is there still an ashtray there?'

Not only is there an ashtray, it's usually bafflingly accompanied by multiple signs reminding you that you can't smoke in a highly pressurised metal box hurtling through the sky.

Picture: Shane Becker/Flickr

But... why?

Good ol' health and safety, that's why.

While it's definitely illegal to smoke anywhere on a plane, the American Federal Aviation Administration requires all aircraft to have ashtrays on board in the event that if someone did have an illicit cigarette, there is somewhere safe for them to dispose of it.

The rule was implemented following a plane crash in 1973 on a flight from Rio de Janeiro to Paris when a passenger threw their fag end in the rubbish bin in the toilet, starting a fire which filled the cabin with smoke and forced the pilot to make an emergency crash landing in which 123 people died.

The UK's Civil Aviation Authority has no such rule, but since most planes are built to be able to fly in US airspace you'd be hard pressed to find an aircraft missing its useless ashtray.