On Thursday the Consumer Product Safety Commission announced a formal recall of some 1 million Samsung Galaxy Note 7 smartphones after reports of the devices overheating and igniting. According to CPSC, Samsung had received 92 reports of the batteries overheating in the U.S., including 26 reports of burns and 55 reports of property damage. That followed a September 2 recall announcement by Samsung—an effort that never fully got off the ground—and restrictions against using and charging the devices during commercial air travel. We asked lithium-ion battery expert K.M. Abraham, research professor in the College of Science and an inventor of the lithium-air battery, to explain why the phones are catching fire, how users should respond, and whether safety issues with mobile devices are more prevalent now than ever before.

Lithium-ion batteries power numerous devices, including smartphones, laptops, and hoverboards. There have been reports of each of those devices igniting. How do the batteries work, and what is causing them to dangerously overheat?

A lithium-ion battery has three primary parts: two “electrode plates”—an “anode plate” made of graphite and a “cathode” made of lithium cobalt oxide or a similar metal oxide —and a very thin, but porous, polyethylene “separator” that keeps the two electrodes apart. The electric current flows between the anode and the cathode via a liquid called the “electrolyte,” which consists of a lithium salt dissolved in an organic solvent. If the two electrode plates, which are normally separated by the polyethylene separator, come in contact with each other during use because of a manufacturing defect, then the two electrodes can spark what’s called a “thermal runaway reaction.” In such a reaction, the electrolyte heats up and the cathode and anode become unstable and react violently with the electrolyte. The temperature may reach the boiling point, causing the battery to eject its hot internal contents, which catch fire or explode when they come in contact with oxygen in the atmosphere.

According to Bloomberg Technology, Samsung reported that a small flaw was responsible for the problem, in particular, a production error that placed pressure on the electrode plates. It’s very possible that this is what happened because the remains of the phones that caught fire resemble those of devices, such as hoverboards, that exploded when their batteries short-circuited, leading to a thermal runway reaction.

What is the actual danger to people who own the Galaxy Note 7, and how should they respond to the warnings, given that the recall is taking so long to take effect?

There is a high probability that the Note 7s from the lot currently in people’s possession are defective and can have dangerous consequences, including personal injury and substantial property damage. Users should return the phones to the store as soon as possible, without hesitation. Until then, they should handle the phones very carefully, being sure to keep them away from their bodies and those of others. They should make no attempt to charge them or use them further.

These types of explosions seem to be happening more frequently. Are they, or are we just hearing about them more? And how can manufacturers ensure that this problem doesn’t happen again?

The lithium-ion battery is an extremely desirable power source for everything from cellphones to electric cars because it is very energy dense. I wrote an article on this for the Electrochemical Society when hoverboards were exploding earlier in the year. But that density makes them extremely susceptible to any defects in materials, engineering, or manufacturing. Manufacturers must take the utmost care in producing them. They should use only the best materials, engineer the batteries properly, and test them thoroughly. In their desire to meet production requirements, manufacturers may miss some of these steps. Such high standards are important for all devices that use the batteries, but because people carry cell phones so close to themselves, and because there are so many of them out there, the consequences of explosions can be particularly devastating. That said, it is worth noting that billions of these batteries are used every day by consumers around world without any problem, attesting to their indispensability when they are produced correctly.