A keyless car has a key fob that, when in close proximity to the car, transmits a signal and allows the driver to start the car with a touch of a button. No inserting and turning of a key is required to start the ignition.

The trouble arises when it comes to turning the car off. In a car with a key, a driver has to turn and remove the key to turn off the engine. In a keyless car, the driver hits the off button.

Some drivers, however, according to the New York Times—potentially accustomed to thinking their car is off, if they have the keys with them—are getting out of the car while the engine is still running. This can have dire consequences, as carbon monoxide can fill up a home and silently kill those inside.

Though the government does not track the number of people who have died from leaving their cars on, the Times counted 28 deaths since 2006. The Globe and Mail wrote about the phenomenon in January 2018, and one expert the Canadian news outlet spoke to quoted a similar number.

“We’re aware of 25 deaths involving carbon monoxide in motor vehicles with keyless ignitions which have been left running unintentionally in an enclosed space,” Sean Kane, founder and president of Massachusetts-based consulting firm Safety Research and Strategies Inc., told The Globe and Mail.

Both the Times and The Globe and Mail believe these numbers are on the conservative side.

There aren’t regulations mandating specific safety measures be put into cars to warn drivers that their car is on, or just automatically turn off the engine. Different car manufacturers have different sets of safety systems. Ford, Volkswagen, and Audi shut off the car after a period of time if they detect the driver is not there; for Ford, the car shuts off after 30 minutes if the fob is not near the car. Others like Toyota beep several times, if the car is left on, according to the Times.

Roughly half of the 17 million new cars sold per year in the U.S. are equipped with this keyless car technology, the Times reported.

The National Highway Traffic Safety Administration had devised a set of federal regulations, but faced opposition by the auto industry; the rules are still “under consideration,” the Times notes.

In terms of fixes to this issue, Kane told The Globe and Mail that “engineering out the problem rather than warning about it is the preferred method to mitigate a hazard.”