Editor's Note: (For complete coverage of the Eclipse of the Century go to cnn.com/eclipse. Watch live, in virtual reality, as the eclipse moves coast to coast Monday.)
(CNN) Safety concerns have prompted a recall of solar eclipse glasses given away at Dutch Bros Coffee stands.
"If you received a pair of these glasses, do not use them to view the eclipse," the company said in a statement announcing the voluntary recall.
The moon's shadow will block the sun from view in a total solar eclipse over the United States Monday. Optometrists warn that looking directly at the sun with the naked eye during the eclipse could cause blindness. Glasses meeting an international safety regulation are recommended to protect eyesight.
Dutch Bros said it had purchased its giveaway glasses after receiving certification of ISO compliance from the manufacturer.
"Further investigation has led us to question this certification. Your safety is of the utmost importance to us, so we are issuing this voluntary recall," it said. "Please return these glasses to the Dutch Bros Coffee location where you received them for a free drink of your choice--any drink, any size. We apologize for your inconvenience."
CNN has reached out to the coffee company to see how many pairs of glasses were distributed. The chain has more than 260 locations across seven states. The statement didn't say where the glasses were given out.
The American Astrological Society earlier warned that unsafe eclipse glasses bearing the ISO logo and certification label had been flooding the market and that websites were also displaying false results that claim to show positive test results for glasses they sell.
Glasses certified by the ISO as safe for viewing the eclipse have filters about 100,000 times darker than ordinary sunglasses, according to the society.
"The only way you can be sure your solar viewer is safe is to verify that it comes from a reputable manufacturer or one of their authorized dealers," it said in a release. The organization has a list it keeps updated.
Looking directly at the powerful brightness of the sun can cause damage to the retina, the light-sensitive part of the eye.
"When you look directly at the sun, the intensity of the light and the focus of the light is so great on the retina that it can cook it," said Dr. Christopher Quinn, president of the American Optometric Association. "If the exposure is great enough, that can and will lead to permanent reduction in vision and even blindness."
To test for safety, the only thing you can see through a safe solar filter is the sun itself. If you look through and the sun is too bright, out of focus or surrounded by a murky haze, or if you can see things like ordinary household lights, the glasses aren't safe.