Why Do Eyes in Contact Lenses Feel Drier in Winter?

Come in from the cold wind and your face is the first thing to relax. You take off your coat and sit in a heated room, and an hour or two later it is your lens-wearing eyes that slowly start to feel dry. The same lenses, worn for the same hours, seemed fine in summer. Why the change in winter?

Winter indoor air is cold, low-moisture air that has been warmed, so its relative humidity tends to fall, and the thin tear layer on a lens appears to thin more easily in such air. It is not so much that the lens has suddenly changed as that the air around it has.

How eyes dry out over the day is covered in our article on afternoon dryness, and aircraft cabins in our article on long flights. This time we look only at what happens to indoor air as the seasons change and how that air affects the tears on a lens.

Why does warming cold air lower humidity?

Humidity guidance from South Dakota State University in the United States explains that cold winter outdoor air holds relatively little moisture, and that when this air passes through a home heating system its relative humidity becomes very low. The material dates from 1979, but the principle it describes is still the same.

Relative humidity is the proportion of water vapour actually present, out of what air at its current temperature can hold. Warming air raises the amount it can hold, while the actual amount stays the same unless more vapour is added. Only the denominator grows, so the proportion falls. That is why a room does not become moist even on a day when snow is falling outside.

How much humidity difference can eyes feel?

TFOS, an organization that studies tears and the eye's surface, published an environmental conditions report in 2023 that includes a study of office workers. Scores for eye dryness, irritation and itching were lower when they worked at an indoor humidity of 30–40% than at the natural humidity of 20–30%.

The same report also describes an experiment that fixed the temperature at 21°C and lowered humidity from 40% to 5%. Healthy participants' symptoms worsened, and test results such as how long tears held together also got worse.

A humidity of 5% is an extreme condition created for research, so the result does not transfer directly to an ordinary winter room. It does, however, show that when the air changes, tears really do change too.

With a lens in, tears divide into front and back

The tear film is a very thin layer of liquid covering the surface of the eye. According to the 2013 TFOS report on contact lens discomfort, wearing a lens divides this film into a layer in front of the lens and a layer behind it. The front layer, which meets the air, has a thinner oil layer than a normal tear film, holds less tear volume and evaporates faster.

The oil layer acts like a cover that slows the loss of water. In effect, that cover meets dry air in a thinned state. You could say the eye has less margin from the start than an eye without a lens.

The report brings together several experiments, most of which defined “low humidity” as below 30%, and concludes that evidence has built up clearly linking these conditions to lens discomfort.

Creating winter air in a laboratory

An experiment published in 2004 tested 11 men who wore soft lenses every day, in a room with controlled temperature and humidity. There were four conditions: 5°C and 10%, 15°C and 20%, 25°C and 40%, and 35°C and 50%.

As temperature and humidity fell, the tear layer on the lens grew thinner, the time until it broke up became shorter and dryness scores rose. By contrast, the measure of how much tear fluid had pooled in the eye did not change. It is closer to tears struggling to hold on over the lens than to tears running short.

In eyes with the lenses removed, tear break-up time did not change much. Dryness scores rose even without lenses. However, with so few participants, and with temperature and humidity changed together, it is impossible to separate which of the two was the cause. The TFOS report also wrote that there appear to be no well-controlled studies that fix humidity and change temperature alone.

Is water leaving the lens?

“My lens has dried out” is a natural thing to say, but the actual process may be a little different. The TFOS report describes research in which even extremely low humidity and temperature did not affect the overall water loss of hydrogel lenses, and it names surface evaporation and thinning of the tear layer as possible causes.

There are differences between materials. Another chapter of the same report says that in laboratory studies, lenses with lower water content lost less water than those with higher water content. It adds that surface evaporation happens in certain areas and may show up as only a small change in the lens's total water content.

Studies have not been consistent on the relationship between water loss from lenses and discomfort either. Winter dryness is hard to explain by reduced water in the lens alone.

Is it really more common in winter?

In a study analysing five years of eye clinic records from US veterans, the proportion of dry eye diagnoses was 18.7% in winter, 18.5% in spring and 15.3% in summer. Winter was high, but spring was almost the same.

The authors gave lower indoor humidity from heating as one reason for the higher winter figure, while stating clearly that this was a hypothesis. The study did not measure indoor humidity directly, its subjects were US veterans and it did not look at lens wearers separately.

The 2013 TFOS report likewise wrote that increased lens discomfort in dry climates such as deserts or polar regions is well known clinically, but that no study has properly confirmed it under controlled conditions outdoors.

What you can look at in a winter room

The 2013 TFOS report also names wind and air flow as factors that increase evaporation from the eye's surface. If a fan heater or car heater blows straight at your face, it may be worth turning it away. The authors of the veterans study listed indoor humidifiers as one environmental measure to consider. If you use a humidifier, look after the water tank as the product instructions say.

These measures do not give the same result for every pair of eyes. If dryness continues, or you have pain, severe redness or a change in vision, it is best to remove your lenses and check with an ophthalmologist or optometrist.

Next time your eyes feel dry in a heated room, glance at the room's hygrometer before you look at your lens box. If you do not have one, try holding out a hand to feel where the warm air is heading. Part of the reason the same lenses feel different from season to season lies in the air in front of your eyes.