tl;dr: A low blink rate can cause irritated eyes (“eye strain”). Small fonts, poor type contrast, and poor typography all contribute to low blink rates. Eye strain is not permanent.

Several people, after seeing the small fonts on my laptop (2560 x 1600 on a 13″ screen), have told me that I’m “hurting” or “damaging” my eyes my using such small type. I haven’t ever had eye strain symptoms and wasn’t sure whether to be worried.

First, and most importantly, eye strain doesn’t appear to be permanent.

“Despite popular belief, sitting too close to the television, or in this case the computer, won’t cause blindness or other permanent damage,” said Dr. Kimberly Yen, assistant professor of ophthalmology at BCM. “What it can do is cause eye strain symptoms.”

When eyes are focused on one item for a long period of time, whether it’s a computer screen, television or even a book, the blink rate slows. That could lead to eyes feeling dry or uncomfortable. Vision could even begin to blur or double. One common side effect of eye strain is headache.

source: Baylor College of Medicine

A great article at Eye Magazine (a graphic design magazine) breaks down specific stressful conditions:

• reading small text sizes
• reading low-contrast gray text
• reading with a light source behind the reading material to cause glare
• reading from too close a distance, which causes the eyes to point inward towards each other (convergence stress)
• reading from variable focal distances (accommodative stress)
• reading while wearing glasses that simulate an astigmatism (refractive stress)

Small text sizes, low contrast, glare and refractive stress all resulted in increased activity in the orbicularis oculi, while convergence stress and accommodative stress did not, though after reading in these two conditions, readers are more likely to report headaches and pain coming from behind the eye. Stressors such as small text size and glare are reported as irritation on the front of the eye.

Blink rate is a key factor in eye stress. Difficult-to-read text (e.g. small text) causes a lower blink rate.

Light grey text on a white background and small text size both lead to an increased orbicularis oculi activity and decreased blinking. These two conditions are related to text quality, and we would expect to find similar indicators of eye fatigue with poor font quality or condensed letter spacing. To reduce this type of eye strain, we need text of the highest possible quality.

…other studies found that we blink far more often under relaxed conditions than while reading: 22 blinks per minute while relaxed, versus ten blinks per minute while reading a book. A recent study at Pacific University illuminated why blinking is suppressed during reading. Tai and Sheedy found that the eye movement following a blink was far more likely to be a regressive or backward corrective eye movement than one that did not follow a blink.

I’ll be sticking with small fonts for now.  If I experience eye strain though, it’s good to know that cranking up the font size can offer some relief.