The CPSC says about 9,900 users of the device have reported skin irritation on their wrists.
Another 250 people suffered blistering. One million of the devices are now under recall.
The CPSC told ABC7 News that it's rare for a company to offer a full refund like Fitbit is now doing.
"So in this situation we don't want anyone else to have blistering on their arms. So our message is take advantage of that refund. Get your money back and be safe," CPSC spokesperson said.