As firefighters battled a five-alarm fire at a supermarket in the Bronx earlier this month, New York City officials gathered beside what they said was the cause of the fire: the blackened shell of what was once a sit-on electric scooter.

Officials said that a faulty lithium-ion battery in the scooter had suddenly burst into flame, as captured on surveillance video. The resulting fire was so intense, they said, that it enveloped the building in a matter of minutes.

"There is extraordinary damage. This entire building behind me is completely destroyed. The roof is caved in. There is nothing left. And it is all because of this one single bike," said Laura Kavanaugh, the city's fire commissioner.

Last week's blaze joined the more than 200 fires in New York City last year caused by batteries from e-bikes, electric scooters and similar devices. Lithium-ion battery explosions are now the third leading cause of fires in the city, the fire department says.

As the popularity of so-called micromobility devices has soared across the U.S., so too have risen the number of fires associated with the lithium-ion batteries that power them.

Some lawmakers and federal regulators have taken note. Late last year, the U.S. Consumer Product Safety Commission announced it had received reports of more than 200 incidents since the start of 2021 in which micromobility devices caught fire or overheated — incidents that led to the deaths of 19 people.

"Destructive and deadly fires from lithium-ion batteries in e-bikes have reached a crisis level. The tragic loss of life from battery fires is heartbreaking and preventable," said Commissioner Richard Trumka in December.

Read on for more about why these fires are happening and how to keep yourself safe:

Why are batteries in e-bikes and scooters vulnerable to catching fire?

Lithium-ion batteries power many rechargeable devices that are part of our modern lives: cell phones, laptops, vapes, cordless power tools and electric vehicles of all kinds, from cars to scooters to e-bikes to hoverboards.

They're small, lightweight and powerful — but they're also prone to overheating and catching fire, said Michael Pecht, a professor of engineering at the University of Maryland. "Ever since lithium-ion batteries started to be prevalent in products, we've seen fires," he said.

At issue is the high density of the batteries, which is a double-edged sword, said Pecht, who also serves as director of the Center for Advanced Life Cycle Engineering, a university research center that consults with companies on reliability and safety issues, including batteries.

"They can provide a lot of power to our cell phones and to our computers for a relatively long period of time in a very small volume," he said. "But because we have so much energy packed in that small volume, if there is a problem, then they're very flammable."

Defects or contamination in the manufacturing process can eventually lead to short circuiting or other failures.

In 2006, Dell, Apple and other major laptop makers urged millions of customers to return laptop batteries after Sony discovered a flaw in their battery manufacturing process. Chevy, Hyundai and Chrysler have all been forced to issue recalls over battery fires in electric vehicles. The Federal Aviation Administration reported more than 60 incidents last year in which lithium-ion batteries — mostly battery packs, vapes or cell phones — overheated, began smoking or caught fire on airplanes.

Why do there seem to be more e-bike- and scooter-related fires now?

In short, there are more fires because there are so many more e-bikes and scooters these days.

Their small size and low cost relative to gas-powered vehicles have made micromobility devices an attractive transportation and recreation option for millions of Americans. That's especially true for those living in urban areas where parking and traffic are challenges for drivers. Electric bikes and scooters have also been embraced by delivery drivers.

The burst in popularity is so recent that there isn't yet much solid data about how many e-bikes, scooters and other devices are sold each year.

But what information we do have shows that their numbers are growing rapidly. The Light Electric Vehicle Association, an industry group, estimates that about 880,000 e-bikes were imported to the U.S. in 2021. That's about double the number imported in 2020, and three times the total from 2019.

More devices means more fires, experts say, especially since the industry is relatively new and unregulated, and there are a lot of different companies and products on the market.

What's being done about it?

There's not currently much regulation of e-bikes and scooters.

Regulation could go in several directions. One would be to require devices be certified under the safety standards recommended by Underwriter Laboratories, a group that has produced safety certifications for electric products for over a century.

Earlier this month, the New York City Council passed a package of local bills that would require all e-bikes and other electric mobility devices sold, rented or leased in the city to be certified under the appropriate UL safety standards.

The legislation also bans the sale of uncertified or used batteries. Retailers found to be in violation of the laws can be fined up to $1,000 per violation.

At the national level, the Consumer Product Safety Commission issued a letter in December calling on more than 2,000 manufacturers, importers and retailers to voluntarily adhere to UL safety standards for e-bikes and other micromobility devices.

Following the guidelines "significantly reduces the risk of injuries and deaths from micromobility device fires," wrote Robert Kaye, the agency's director of compliance and field operations. "Consumers face an unreasonable risk of fire and risk serious injury or death if their micromobility devices do not meet the level of safety provided by the relevant UL standards."

Additionally, the agency has vowed to pursue penalties against companies who fail to inform the CPSC of safety hazards.

Recommendations to keep yourself safe

The main recommendation that comes from both the CPSC and the FDNY is to be present while you're charging your device, and to not charge it while you're sleeping. Unplug the device once it is fully charged.

The CPSC also recommends that you only use the charger that came included with your device and to follow the manufacturer's instructions for proper charging.

Fire officials add that you should charge your device away from flammable materials like furniture and pillows, and that you shouldn't charge or store your device in a location that blocks your access to an exit.

When you're buying an e-bike or other micromobility device, try to find what battery comes stocked with it, Pecht said. Does the maker of the device state where the battery is sourced from? Is the battery made by a reputable manufacturer? Experts also suggest that consumers look for batteries that have a UL certification.

Be warned that some online sellers may falsely claim to have UL certification. Others may sell "re-wrapped" batteries, meaning counterfeit batteries produced to appear as though they're made by reputable manufacturers.

If your battery starts to fail, it may be safest to buy a new one. "Don't repair anything yourself, and buy from a company where you know that they're using brand-name batteries," Pecht said. It may work best to buy a new battery from the same company that produced your bike or scooter.

To dispose of an old battery, bring it to a battery recycling center or other e-waste facility. Don't throw away lithium-ion batteries in conventional trash.