Cause of discomfort
Volunteers first applied glitter eyeliner outside the lash line and then on the inner lid closer to the eye. Within five minutes, 15 percent to 30 percent more eyeliner particles moved into the eye's tear film – the thin coating protecting the eye – when eyeliner was applied to the inner eyelid, compared to outside the lash line.
Particles also moved more quickly into the eye itself when eyeliner was applied inside the lash line, according to study leader Alison Ng, of the Centre for Contact Lens Research at the University of Waterloo in Canada.
That can lead to complications such as irritation and redness, the presence of harmful bacteria, and even eye infections or blurred vision.
The study was published in the journal Eye and Contact Lens Science and Clinical Practice.
Old eye makeup carries bacteria
Previous research has also shown that old eye makeup can harbour bacteria, Ng noted.
"If you thoroughly sharpen your pencil eyeliner before each application and get rid of the stuff that's stuck to the end, you'll have a fresh tip which can help prevent infection," Ng said in a university news release.
Read: Eye pain
"With twist-up eyeliner, cut some off the end before each use," Ng added.
"And always make sure to fully remove eye makeup before bed."
Image: Beautiful woman applying eyeliner from Shutterstock