In case you haven't heard, it's not safe to look directly at the sun.

To safely view the epic solar eclipse that will cut a dramatic shadow across the United States on August 21, you'll need a special pair of ultra-dark sunglasses. Without glasses, the sun's rays could cause permanent damage to your eyes.

But as demand for these 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 says 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: What you need to know about the eclipse

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

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

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.

Related: Where to see the solar eclipse

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

If you've already purchased a pair of specs, AAS has a tip for testing them out.

When you put them on, "you shouldn't be able to see anything...except the sun itself," the organizations says.

"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 says.

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 says, but those are hard to come by.

Related: See the path of the solar eclipse

Massive crowds are expected at viewing spots across America when the moon passes in front of the sun, blocking its light, the morning of August 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 February 26, 1979. And it's been nearly a century since a total solar eclipse was visible coast to coast.