501,000 self-balancing scooters -- more popularly known as hoverboards -- are being recalled due to fire hazard concerns, said The U.S. Consumer Product Safety. The lithium-ion battery packs in the hoverboards can overheat -- which could result in sparking, smoking, fire, and explosion -- the agency added. ABC reports:The recall involves hoverboards from eight manufacturers/importers that are made with lithium-ion battery packs as well as 4,300 from Overstock.com and 1,300 from a store in Pennsylvania. Amazon.com is not listed in the recall, but in February, the online giant worked with the CPSC to offer refunds to any customer who wanted to return hoverboards purchased on the site. Hoverboards by Swagway make up more than half of those recalled -- 267,000. "We are urging consumers to act quickly," CPSC Chairman Elliot Kaye told ABC News. "We've concluded pretty definitively that these are not safe products the way they were designed."