Back in the mid-1990s, the UK introduced a smoking ban on flights. In America, a Federal law was passed in 2000 that outlawed smoking on all flights by US airlines (previously, it was banned on domestic flights). The vast majority of countries in the world has since followed suit, if they hadn't introduced their own ban already.

Thus, it's been over a decade since the ban was brought into force across many major routes, and yet even new planes still come equipped with ashtrays. The reason? Because people don't follow the rules.

The following is written into the US Code Of Federal Regulation for Airworthiness:

"(g) Regardless of whether smoking is allowed in any other part of the airplane, lavatories must have self-contained, removable ashtrays located conspicuously on or near the entry side of each lavatory door, except that one ashtray may serve more than one lavatory door if the ashtray can be seen readily from the cabin side of each lavatory served".

Again, different territories have different equivalents, but most regulate that a plane must still carry an ashtray.

It's in place because of the continuing problems that airlines have with people who ignore the 'no smoking' signs that are liberally scattered throughout the cabins. As planes have come down in the past due to people stubbing out their cigarette in a bin of paper towels, the thinking is it's better to have something in place on the chance that someone breaks the rules. The alternative—not having ashtrays—risks the safety of the whole plane.

Hence, every plane—even newly manufactured ones—has an ashtray on board.

This Big Question originally appeared on our UK site.