SALT LAKE CITY — Next to being diagnosed with cancer or heart disease, most Americans fear going blind.

"It is so scary because being blind is like somebody turned off the lights on you," said Colleen Malouf, CEO of Utah's Friends for Sight program, a nonprofit organization that provides free vision screenings. A lot of people don't know they're losing their sight until they have already experienced a 40 to 60 percent loss in their ability to see, she said.

Blindness can be caused by a number of factors, including accidents and injury to the eyes, cataracts or degenerative and other medical maladies, such as glaucoma. However, blindness from glaucoma is easily detected and entirely preventable — if a person has their eyes tested.

"Since there are often no symptoms of glaucoma, one of the greatest things we can do as eye care specialists is to make people aware of this disease," Malouf said. Glaucoma is often called the "sneak thief" of sight.

Approximately 120,000 Americans are blind because of glaucoma, accounting for up to 12 percent of all cases of blindness in the United States. It is estimated that more than 4 million Americans have glaucoma but fewer than half know they have it, according to Prevent Blindness America, an organization that promotes eye health.

And while the test for glaucoma includes an intrusive puff of air into a person's eye, it is quick and relatively painless, Malouf said.

The Friends for Sight office, located at 661 S. 200 East in Salt Lake City, screens about 10,000 adults every year for glaucoma. Screening is free and when problems are detected, Malouf said people are referred to a regular eye doctor for further treatment. She recommends every adult be screened at least every two to three years, even if their sight is not noticeably obstructed.

Through the free screenings, Malouf said about 2 to 3 percent end up needing medication for glaucoma. Doctors can prescribe eye drops that help to open the angles in the eye, relieving the pressure from coagulating fluid that causes pressure on the fragile optic nerve and leads to blindness.

Glaucoma is most common in those with darker skin but is also hereditary and can be present in people with diabetes, high blood pressure and those who have used steroids long term. It is also more common in adults and typically, doctors screen patients after age 35.

Children also need regular eye tests, Malouf said, as "their eyes change as quickly as they grow." Behavior or performance problems in school can often be the result of problems with sight and most school districts require vision tests before a child enters kindergarten and every two years following.

"If people didn't wait until they had vision problems to be tested," Malouf said, the growing blindness trend could be changed for the better.

"Sight is one of our most valuable senses," Malouf said. "Without it, you can't drive, you can't read, you can't see the faces of those you love and you can't appreciate your physical surroundings. Life can be unpleasant if you can't see."