In 2017, some people damaged their eyes watching partial eclipses. Eye experts say this is easily avoidable if you take the right safety steps.
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In 2017, people wore special glasses to view a partial eclipse from New York City's 'Top of the Rock' observatory at Rockefeller Center. Drew Angerer/Getty Images hide caption
toggle caption Drew Angerer/Getty Images 
In 2017, people wore special glasses to view a partial eclipse from New York City's 'Top of the Rock' observatory at Rockefeller Center.
Drew Angerer/Getty Images
The eyeglasses store Warby Parker gave out thousands of free eclipse viewers in New York, as did libraries and other organizations around the nation. Spencer Platt/Getty Images hide caption
toggle caption Spencer Platt/Getty Images 
The eyeglasses store Warby Parker gave out thousands of free eclipse viewers in New York, as did libraries and other organizations around the nation.
Spencer Platt/Getty Images
MARY LOUISE KELLY, HOST:
Staring directly at the sun is a bad idea. We know this. But soon, millions of Americans will want to do just that. On April 8, most of North America will see a partial solar eclipse. In parts of 13 states, people will get to see a total eclipse. NPR's Nell Greenfieldboyce reports the last time the U.S. got excited about this kind of event, some people hurt their eyes.
NELL GREENFIELDBOYCE, BYLINE: Back in 2017, more than 150 million people watched as the moon slipped in front of the sun. And in the days after this event, eye doctors were on high alert.
AVNISH DEOBHAKTA: While it may be one of the most beautiful celestial events that you could possibly have, it is dangerous.
GREENFIELDBOYCE: Avnish Deobhakta practices at the New York Eye and Ear Infirmary of Mount Sinai, one of the largest eye hospitals in the nation. He says the eye's retina is designed to be extremely light sensitive, so too much light all at once can cause damage. That's why we have a natural aversion to looking straight at the sun.
DEOBHAKTA: Humans have evolved on planet Earth to sort of not be able to look at the sun. It's so bright that we're not actually capable of looking at it without either tearing or sort of not really feeling comfortable staring at this ball of light.
GREENFIELDBOYCE: But an eclipse makes people curious. They might look up when the sun is partially eclipsed despite warnings that it's never safe to look at any part of the exposed sun without special protective glasses.
DEOBHAKTA: So we had a lot of people come into the emergency room thinking that they had, you know, some kind of damage that was done. Either they felt uncomfortable with their vision, they felt like they couldn't focus or they had some tearing that they were feeling.
GREENFIELDBOYCE: Most of these people were just fine. But one young woman had severe, permanent damage to her retina. Deobhakta says she didn't look at the crescent sun for very long - only around 30 seconds.
DEOBHAKTA: But the problem was that she was handed glasses from someone else that she thought were safe, so she really focused.
GREENFIELDBOYCE: A similar case occurred in California, a few more in Utah. Ralph Chou is an expert on eclipse eye safety with the University of Waterloo in Canada. He says no one knows for sure, but it doesn't look like a large number of eye problems occurred.
RALPH CHOU: We've got less than 100 cases across Canada and the U.S. out of the 2017 eclipse that were noteworthy and reported.
GREENFIELDBOYCE: Chou says, to protect yourself, make sure you get eclipse glasses from a reputable source. The website of the American Astronomical Society has a list of vetted suppliers. And don't sneak peeks.
CHOU: If you look unprotected for a brief moment, that doesn't do any harm. But every time you look after that, it adds to the dosage.
GREENFIELDBOYCE: Eventually, the eye's retina will get overwhelmed.
CHOU: Unfortunately, you don't realize that until far too late.
GREENFIELDBOYCE: The damage will only become apparent hours later, when a spot of extreme fuzziness appears in your field of view. Laura Peticolas is a space physicist at Sonoma State University. She says, don't be so worried about eye safety that you miss the whole show.
LAURA PETICOLAS: People get so concerned to not hurt their eyes - which, of course, is super-important - that they don't take their glasses off when the moon completely covers the sun.
GREENFIELDBOYCE: Depending on your location, this total eclipse phase lasts up to four minutes or so. This is when the sun is a black circle surrounded by a ghostly white ring. It's safe to look at this without eclipse glasses. In fact, if you don't take them off, you won't be able to see it. Just put your glasses back on the instant the sun starts to reemerge. Nell Greenfieldboyce, NPR News.
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