This article originally appeared on Time.
For the first time in U.S. history, a solar eclipse will travel exclusively across America, enabling millions of people to view the moon block out the sun on Aug. 21. But those who watch this rare celestial event need to take precautions, because staring right at the sun can quickly harm your eyes.
“Looking directly at the sun is unsafe except during the brief total phase of a solar eclipse (“totality”), when the moon entirely blocks the sun’s bright face, which will happen only within the narrow path of totality,” NASA explains on its website. “The only safe way to look directly at the uneclipsed or partially eclipsed sun is through special-purpose solar filters, such as “eclipse glasses.”’
The path of totality, which is about 70 miles wide, is viewable from parts of 14 states, as shown on this solar eclipse map, and only lasts a maximum of two minutes and 40 seconds, according to NASA. Before and after the total solar eclipse, those in its path will see a partial eclipse, in which the moon only partly blocks the sun. The rest of the country will also see a partial eclipse — so essentially, everyone needs to prepare themselves to view the eclipse safely.
Watch Live as the 2017 Total Solar Eclipse Crosses the U.S.
Here’s what you need to know about why a solar eclipse hurts your eyes and how to protect your eyes effectively:
Why Does a Solar Eclipse Damage My Eyes?
According to experts, viewing the sun with your naked eye during the eclipse can burn your retina, damaging the images your brain can view. This phenomenon, known as “eclipse blindness,” can cause temporary or permanent vision impairment, and in worst-case scenarios can lead to legal blindness, which entails significant loss of vision.
“If people look without the proper protection [at the sun], they run the risk of injuring their eyes. And if they get an injury, depending on how often and how long they look at the sun without the protection, they do have a substantial risk of developing a permanent loss of vision,” said Dr. B. Ralph Chou, president of the Royal Astronomical Society of Canada and a former optometry professor. It is not possible to go completely blind from looking at the eclipse, Chou said, because the injury is limited to the central part of your visual field.
There are no immediate symptoms or pain associated with the damage — the retina doesn’t have any pain receptors — so its hard to know at the time if you’ve actually been afflicted with eclipse blindness. If you look at the sun unfiltered, you may immediately notice a dazzle effect, or a glare the way you would from any bright object, but that doesn’t necessarily mean your retina is damaged. According to Chou, symptoms generally begin occurring 12 hours after viewing the eclipse, when people wake up in the morning and notice their vision has been altered.
“They can’t see faces in the mirror, they can’t read the newspaper or the smartphone display, they’re having trouble looking at road signs, and basically they’ve got this center spot in their vision that is intensely blurred,” Chou said.
There are no remedies to effectively mitigate the injury, said Chou, aside from waiting and seeing if the patient regains vision. This does happen, but not until at least three months after the injury.
Has This Happened Before?
Yes. People have hurt their eyes by watching the sun during a solar eclipse unfiltered. However, it is a relatively rare occurrence. Although Chou said there is no definitive data on the number of people afflicted with eclipse blindness, he noted that after a solar eclipse crossed Britain in 1999, ophthalmologists reported 70 instances of eye injuries, and the majority of those people had viewed the eclipse unfiltered. In Canada, 20 cases were reported following the total solar eclipse of 1979. Of the cases reported over the years, Chou said half the people afflicted completely recovered their vision over the course of the following year.
“It’s a fact that for individual practitioners, they are not seeing that many [cases] overall,” Chou said. “It’s only if you start looking at large populations in the hundreds of millions that you start adding up into significant numbers.”
What Can I Do to Protect My Eyes?
To ensure your experience is injury-free, listen to NASA’s advisory and buy eclipse glasses, which block approximately 99.99% of light rays. But also make sure follow NASA’s instructions in using these glasses. When the glasses are on, NASA says, it is imperative that you don’t look at the sun through an unfiltered camera lens, telescope, or binoculars.
Additionally, make sure that the brand of glasses you buy has been verified to meet the international safety standard, something Chou emphasized as critical to injury prevention. The American Astronomical Society has released a list of manufacturers selling these glasses that meet this standard. NASA also suggests you inspect your filter before putting it on, and discard it if it has any scratches or damages.
“If you don’t try to sneak a peek without the filter,” says Chou, “Then you should not run any risk of being hurt.”