This story was originally published on The Conversation. It appears here under a Creative Commons license.

A total solar eclipse will occur across a wide ribbon of North America on April 8. Millions of people along the path of totality in Mexico, the United States, and Canada will witness this spectacular event, while millions more will experience a partial eclipse.

It is imperative to take steps to protect your eyes from solar retinopathy, permanent eye damage caused by looking at directly at the Sun. Any direct viewing should only be done with the correct use of approved solar eclipse glasses that meet an international safety standard known as ISO 12312-2.

Also known as sun blindness, solar retinopathy has been recognized since ancient Greece. It affected astronomers including Sir Isaac Newton, who once used a mirror to look at the Sun and saw “afterimages for months.”

In Turkey in 1976, 58 patients sought treatment for eye damage after an eclipse. While some experienced initial improvements, the damage in others was unchanged 15 years later. In 1999, 45 people presented to the Eye Casualty of Leicester Royal Infirmary after an eclipse seen there. Retinopathy was confirmed in 40 of them. Seven months later, four people could still see “the ghosts of the damage” in their visual field.

And after the solar eclipse of August 2017, 27 patients in Utah presented with concerns about vision. For those people affected by solar retinopathy, the results can be devastating and lifelong.

Looking directly at the sun can cause solar retinopathy, or damage  to the retina's fovea centralis, seen here just left of center.
Looking directly at the sun can cause solar retinopathy, or damage to the retina’s fovea centralis, seen here just left of center. DavidBolnick, CC BY-SA 4.0/Wikimedia

Solar retinopathy is damage to the back of the eye (the fovea centralis in the retina) from exposure to intense light. It is typically caused by sungazing or eclipse viewing but can also result from welding without a shield, looking at laser pointers, and from some surgical and photographic lighting.

A process called “phototoxicity” happens when the energy in the light forms damaging free radicals and reacts with oxygen within the retina. This disrupts the retinal pigment epithelium (a layer of supportive cells beneath the retina) as well as the choriocapillaris (blood vessels) beneath.

Fragmentation of the photoreceptors, nerve cells within the retina that detect light and color, follows and can result in permanent loss of central vision.

Some wavelengths of light that cause solar retinopathy—such as ultraviolet-A radiation and near-infrared wavelengths—are not visible to humans, yet cause solar retinopathy in as little as a few seconds. This exposure doesn’t necessarily hurt at the time.

So eclipse gazing—even with little or no visible light and viewed briefly without pain—can lead to loss of vision. There is no proven treatment for solar retinopathy. Steroid medications have been tried without evidence of success, and may make things worse in some patients. Antioxidant medications are used in some eye diseases, but there are no studies showing a benefit in solar retinopathy. Vision may improve over time without treatment but many patients are left with residual deficits. The mainstay of management is therefore prevention.

When planning for the total solar eclipse, look for safety lenses that meet an international safety standard known as ISO 12312-2.
When planning for the total solar eclipse, look for safety lenses that meet an international safety standard known as ISO 12312-2. LeoPatrizi/Getty Images

Only approved glasses will absorb the appropriate wavelengths of visible, ultraviolet, and infrared light. They must:

  1. be purchased from reputable vendors to ensure they are not counterfeits
  2. display the correct safety certification (ISO 12312-2)
  3. not be scratched, cracked, or show any other signs of damage
  4. fit your face properly so no gaps let light in (check they fit over your usual glasses if you need these to see normally)
  5. be checked by looking at a lamp or light bulb; only light from the Sun should be visible through genuine eclipse glasses. This check doesn’t risk eye damage provided the previous steps have been followed.

Regular sunglasses, polaroid filters, welding shields, X-ray film, neutral density filters, red glass filters, mobile phones, and homemade sun filters are not safe for viewing the Sun or an eclipse.

Symptoms of solar retinopathy to watch out for include blurred vision in one or both eyes within one or two days of exposure. People may also experience blind spots, altered color vision, visual distortion (straight lines appearing kinked or wavy), micropsia (objects appearing smaller than normal), light sensitivity, and headache. There may be no symptoms at all in the first day.

If you have symptoms, abstain from further eclipse viewing. Use dark sunglasses and painkillers (such as acetaminophen) for light sensitivity and headaches. Arrange an urgent appointment with an ophthalmologist or optometrist, or go to an urgent care clinic.

A total solar eclipse may potentially be viewed without eye protection, but only during the brief period while the Moon completely covers the Sun (the period of totality)—and this still has risks. Eclipse glasses should only be removed after totality has commenced, when the Moon has completely covered the Sun and it suddenly becomes dark. Just prior to the Sun reappearing, eclipse glasses must be replaced, to keep observing the remaining partial eclipse.

A solar eclipse is a rare occurrence. People will naturally be curious to observe it. Following the right advice will mean they can do it safely.

Hessom Razavi is an ophthalmologist, and an associate professor at The University of Western Australia.