What is the tear film?

The tear film is the thin, clear coating that covers the front of your eye between blinks. It has three layers: mucus closest to the eye, which helps tears cling; a watery middle layer, which makes up most of the film; and a thin oil layer on top, which slows evaporation. Every blink spreads a fresh film, and dry, moving or heated air thins it faster.

Why the eye needs a coating at all

The front of the eye is living tissue exposed to air, which is unusual for the body. Skin has a tough outer layer; the inside of your mouth is constantly bathed in saliva. The eye’s answer is a liquid film that has to be renewed every few seconds. It keeps the surface moist and smooth, washes away dust, and gives light a clear, even surface to pass through, which is part of why vision blurs when the film breaks up.

The three layers, from the eye outward

• Mucus layer: the innermost layer, lying against the surface of the eye. It helps the watery tears spread out evenly instead of beading up, the way a wet surface takes a coat of water more easily than a waxy one.
• Water layer: the thick middle of the film. It carries moisture and keeps the surface wet, and it’s what you notice when your eyes water.
• Oil layer: the outermost, thinnest layer, made by tiny glands along the edges of the eyelids. It sits on top of the water and slows how fast it evaporates into the air.

The layers aren’t sealed apart like the layers of a cake; they blend into one another and work as a single film. But thinking of them separately helps explain why different things make eyes feel dry. A film can run short on water, or have too little oil on top to hold that water in, or fail to spread evenly.

How blinking renews it

Each blink pulls the upper lid down over the eye and back up again. On the way, it sweeps the old film toward the inner corner, where tears drain away, and spreads fresh tears across the surface. Pressure from the lids also helps move oil from the glands at the lid edges onto the top of the film. The result is a new, even coating after every complete blink. How blinking spreads tears across the eye goes through it step by step.

What dries the tear film

• Low humidity, from winter heating, air conditioning, desert air or airplane cabins.
• Moving air, from wind, fans, car vents or riding a bike.
• Heat near the face, from a fireplace, space heater or hair dryer.
• Fewer or incomplete blinks during reading, screen work or driving.
• Long days with eyes open, especially late at night.

Most of these speed up evaporation from the top of the film. That’s why the oil layer gets so much attention: it’s the part that slows water loss into the air. For more on that single layer, see the tear film’s oil layer. For screens in particular, screens and your tear film explains the blinking side.

What happens when the film breaks up

Between blinks, the film thins. If you keep your eyes open long enough, it begins to break apart in spots, leaving small dry areas on the surface. People notice that as stinging, a gritty or sandy feeling, a need to blink hard, or vision that blurs and then clears after a blink. Watery eyes can be part of it too, because the eye may respond to dryness by producing a rush of watery tears that lack the balance of the full film.

Where Quell fits

Quell is designed to reinforce the tear film’s oil layer to help reduce moisture loss. Its Uses read: For use as a protectant against further irritation or to relieve dryness of the eye. The active ingredients are Glycerol (1.0%), Polysorbate 80 (1.0%) and Light Mineral Oil (0.5%), and the formula includes emu oil among its inactives. The directions are Apply 1 drop 3 times per day in each eye. The Drug Facts page has the complete label.

A drop adds to the film from outside; it doesn’t change how your glands make tears. The habits that protect the film, like blinking fully, cutting down on moving air at your face and adding moisture to dry rooms, work alongside it.

When the tear film needs a professional look

Eye doctors have ways to examine each part of the tear film and see which, if any, is falling short. That’s worth doing if dryness is frequent or doesn’t ease with simple changes. The label’s threshold is clear: Stop use and ask a doctor if you feel eye pain or changes in your vision, if irritation or redness continues, or if the condition worsens or persists for 72 hours.