AILSA CHANG, HOST:

The biggest auto recall in history is not over yet. In fact, it's not even close to being done. We're talking about the nationwide recall of Takata air bags. It began in 2014 after a handful of air bags exploded and sprayed shrapnel at drivers and passengers. Fifteen people have been killed in the U.S. by these air bag explosions, and more than 180 have been injured worldwide. David Friedman was acting administrator of the National Highway Traffic Safety Administration when the recall began. He's now with Consumers Union. That's the policy arm of Consumer Reports. And he joins me now.

Hi, David.

DAVID FRIEDMAN: Hi. Thank you for having me.

CHANG: OK. Just this month, Takata has announced it's recalling an additional 3.3 million air bags. Why are there still so many more faulty air bags out there? What is going on?

FRIEDMAN: Well, it is stunning, isn't it? I mean, they're already up to 50 million air bags recalled, and we already know it's going to head north to maybe 65 or 70 million air bags.

CHANG: Wow.

FRIEDMAN: And the reason why it's cycling through like this is different air bags are at greater risk. So if you have an older air bag or an air bag that's been in a region of the country with high humidity and significant changes in temperature, your car is much more likely to have an air bag explode, with shrapnel ripping through that airbag potentially killing or injuring people.

CHANG: So they're prioritizing these recalls according to where you live, depending on the climate.

FRIEDMAN: Absolutely. Because the recall is so big, there have been challenges getting the parts made quickly enough. Right now, the plan is to continue announcing new recalls through 2020.

CHANG: So what exactly is wrong with these air bags that's causing them to deploy like this?

FRIEDMAN: So the fundamental problem with these air bags is the inflator. That's a chemical inside the air bags that goes through a small explosion in order to really rapidly expand that air bag so it can save your life in a crash. The problem is, the chemical that they use - it's ammonium nitrate - when it's exposed to high humidity and significant temperature changes, it breaks down. And when it breaks down, there's more surface area, and when you have more surface area and an explosive material, it explodes faster and more violently.

CHANG: I mean, you know, some people have gotten these recalled notices. They've been told that the parts are just on back order, so it's not necessarily their fault that the cars are not repaired. Is this still an issue?

FRIEDMAN: Well, the National Highway Traffic Safety Administration has said that there aren't any parts problems anymore, but I was looking at their records recently, and you've got General Motors, Ford and Mazda now all saying they're starting to have trouble getting parts, and they want delays in their recalls. That scares me. The other problem is, let's face it. It's not trivial to have to deal with bringing your car in for a recall. And most car companies are not doing enough to reach out to consumers to get them to come in and get their vehicles repaired.

CHANG: That's still a problem - publicizing to people that they need to bring their cars in.

FRIEDMAN: Publicizing it to people - and in fact, at least one company is going above and beyond. Honda, for example, is literally going door to door, even fixing some people's air bags in their driveway. Now, there's a good reason for Honda to be doing that. They have some of the riskiest vehicles out there. Some of the Honda vehicles have a risk as high as 50 percent of rupturing if you're in a crash...

CHANG: Oh, my God.

FRIEDMAN: ...Where the airbag deploys. That is frightening.

CHANG: David Friedman is director of cars and product policy analysis for Consumers Union. Thank you very much for coming in today.

FRIEDMAN: Thank you so much.

CHANG: And you can find out whether your car has been recalled. Just write down your VIN number, and go to the National Highway Traffic Safety Administration's webpage. That's nhtsa.gov/recalls.