It has been more than a decade since the National Highway Traffic Safety Administration ordered a full recall of Takata air bags, but millions of vehicles on the road still have the potentially dangerous air bags.

Nissan recently issued a “Do Not Drive” order for 84,000 older vehicles with Takata air bags that have not been replaced.

Centerville, Virginia-based Carfax reports its data shows 6.4 million vehicles in the U.S. that were recalled for air bag replacement but have not had them replaced.

Part of the problem with getting these dangerous air bags off the road is that the vehicles affected are older models, generally 2002 through 2006.

“They are probably on their second, or third, or fourth owner at this point. So it is hard for automakers to track them down. I will say that I know automakers, some in particular, have gone to extraordinary lengths to try to find these owners,” said Patrick Olsen at Carfax.

In the D.C. metro area, Carfax data show 94,000 vehicles with recalled air bags are still on the roads without the replacement work done.

“D.C. is a hot, humid place, particularly in the summer. The more hot and humid a place is, the greater the risk to that air bag. So I think all owners need to take heed and get the work done. It is free to them,” Olsen said.

Extended exposure to heat and humidity causes the air bag inflators to deploy more explosively than expected. That force can launch pieces of metal surrounding the inflator into the vehicle’s cabin, like shrapnel. NHTSA reports 27 people in the U.S. have been killed by the air bags, and at least 400 injured.

Owners of older models can find out free if their vehicle has had the Takata air bag replaced, or if it is still subject to recall replacement at Carfax.com/recall. All it requires is the VIN or vehicle’s registered license plate information.