DENVER — To safely view the epic solar eclipse that will cut a dramatic shadow across the United States on Aug. 21, a special pair of ultra-dark sunglasses will be needed.

Without the glasses, the sun’s rays could cause permanent damage to eyes.

But as demand for the glasses has skyrocketed, scams have entered the market.

The American Astronomical Society said earlier this week that it updated its safety advice “in response to alarming reports” of unsafe “eclipse viewers” popping up online.

The organization said buyers should be skeptical of glasses even if they’re stamped with an ISO seal — which has been used in the past to indicate which glasses comply with standards set by the International Organization for Standardization.

“It now appears that some companies are printing the ISO logo and certification label on fake eclipse glasses and handheld solar viewers made with materials that do not block enough [light],” AAS said.

RELATED: Tips for photographing the Great American solar eclipse

Some vendors are going so far as to put falsified test results on websites to support the bogus claims, AAS said.

To help combat the fraud, the organization posted a list of vendors from which you can snag a pair of glasses that are guaranteed to be safe.

Some of the places AAS recommends to shop online are EclipseGlasses.com, Celestron, Explore Scientific and Meade Instruments.

The organization also has a list of retailers that stock glasses made by verified manufacturers at some (but not all) of their locations. They include 7-Eleven, Best Buy, Lowe’s and Walmart.

Most eclipse viewers are priced at around $2 or $3.

AAS spokesman Rick Fienberg pointed out that products not included on the list aren’t necessarily unsafe — they just haven’t been vetted by AAS experts.

AAS has a tip for testing glasses that have already been purchased.

When you put them on, “you shouldn’t be able to see anything … except the sun itself,” the organizations said.

“If you can see lights of more ordinary brightness through your eclipse glasses or handheld viewer, and you’re not sure the product came from a reputable vendor, it’s no good,” AAS said in a statement.

The group also warns against using some eclipse-viewing home remedies — such as sunglasses or wearing a welding mask.

Ordinary sunglasses are not up to snuff.

“Even very dark ones are not safe for looking directly at the sun; they transmit many thousands of times too much sunlight,” AAS said.

The group recommends only using a welding mask if you’re certain of its shade number, an indication of how dark the mask’s tint is.

To view the eclipse, you’ll need something at a 12, 13 or 14. A shade number of 13 is ideal, AAS said , but those are hard to come by.

Massive crowds are expected at viewing spots across America when the moon passes in front of the sun, blocking its light, the morning of Aug. 21.

The 70-mile-wide swath of land where the full spectacle can be seen stretches from Oregon to South Carolina, with 12 states in its path.

The last time a total solar eclipse was visible from the continental U.S. was about 40 years ago, on Feb. 26, 1979. And it’s been nearly a century since a total solar eclipse was visible coast to coast.