Why do glasses fog up?

Any glasses wearer is familiar with this ordeal, as foggy lenses are a common complaint. Prescription eyeglasses, athletic sunglasses, and snowboard goggles end up with condensation on the lenses at inopportune times. Why does this happen, and how do you stop glasses from fogging up in winter?

Glasses Fog Up Because of Condensation

Condensation occurs on eyeglass lenses when water vapor—from your sweat, breath, and ambient humidity—lands on a cold surface, cools, and then changes into tiny drops of liquid, forming a film that you see as fog.

Your lenses will be relatively cool compared to your breath, especially when the outside air is cold. When vapor hits a surface that has less thermal energy, some of its energy goes into that surface. Condensation occurs when the gas, or vapor, doesn't have enough energy to maintain itself in a gas state, so it transitions back to liquid. Particles in a liquid have less internal energy than gas, but more internal energy than particles in a solid.

Matter can exist in three phases: solid, liquid, or gas. The change from vapor to liquid is an example of a phase transition. Other examples of phase changes are melting (changing from a solid to a liquid), freezing (changing from a liquid to a solid), and evaporation (changing from a liquid to a gas.) Changes in external conditions such as temperature or pressure can cause matter to shift from one form to another.

Stopping Your Glasses from Fogging Up

Choose Fog-Free Materials Designed to Keep Your Lenses Fog-Proof

Glass lenses are more likely to fog, but lens materials like polycarbonate and plastic may resist fogging. Anti-reflective treatments are another good option, and not only reduce glare, but often include a hydrophobic coating that resists water, smudging, and fogging.

An anti-fog treatment is especially important for snow sports. Most sports sunglasses and ski or snowboard goggles are designed to allow venting and prevent fogging, even when you're breaking a sweat. You can keep your goggles from fogging up when you're wearing a helmet by positioning them where the helmet won't block the vents. And if you remove the goggles when you're riding the lift, place them on the helmet itself instead of on your forehead, where your sweat is likely to create condensation.

Anti-fog technology is common in sports lenses and can make eyewear safer by preventing vision-obscuring fog on the lenses. Wiley X and Liberty Sport are known for their technologically advanced anti-fog lenses. Prescription glasses that rest further from the face will also allow air to circulate around the lenses to minimize fogging. Contact lenses are another option for avoiding foggy lenses, especially in winter.

Clean Your Eyeglasses

Dirt, smudges, and scratches give condensation more to cling to. Keep your lenses clean, avoid leaving fingerprints on your lenses, and handle glasses gently to prevent scratching. A simple way to clean your glasses is to use a gentle dish soap, which has properties that can reduce fogging.

Because the surfactants in dish soap reduce surface tension, a thin film of water coats the lens instead of a mist. Though there's still moisture on the lens, it allows light to pass through rather than being scattered in multiple directions.

A lens cleaner or reusable or single-use anti-fog wipe made specifically for preventing foggy lenses is safest for your lenses. Similar to dish soap, it contains surfactants. Other treatments not made for glasses can harm anti-glare or photochromic coatings, and can void any warranties your lenses carry.

Get Your Frames Adjusted

If your glasses keep fogging up, letting more airflow around the lenses can reduce the problem. Adjusting the nose pads or arms slightly can help keep your glasses where they belong. Frames that sit too close to your face may need adjusting by your optometrist, but there's only so much they can do. If fogging is a common occurrence, your glasses frame size may be off. Consider a new frame with more space between the lenses and your face.

Foggy Lens Hacks to Avoid

We've found a variety of ill-advised remedies for foggy glasses. Your glasses require careful handling, and just because you read it on the internet doesn't make it a good idea. Here are some foggy lens home remedies to avoid:

Rubbing your lenses with a potato is not going to do it. The starch may have kept your grandfather's windshield from fogging, but there are better options out there. Keep the taters on your plate and not on your lenses.
Toothpaste should not go on your glasses lenses because many preparations contain tiny, gritty bits that are great for cleaning your teeth but can scratch your lenses. And never use toothpaste to try to buff scratches out of your glasses.
Hot water can damage your lenses, lens treatments, and frames. Use lukewarm water instead.
Household cleaners like vinegar, ammonia, glass, or surface sprays can damage lens treatments. Don't use them on eyeglasses.
Spitting on your lenses is disgusting. SCUBA divers do it, but please don't. Use a lens cleaner instead.

Warm air hitting cold eyeglasses is a problem, and the science of foggy lenses knows no season. Humid summers combined with your car's air conditioning offer similar troubles, and sports in any season can produce fogged up lenses. Though treatments made for eyeglasses are your best option, wearers have found creative ways to prevent foggy lenses in winter. These tips can help provide fog-free lenses and clearer vision any time of year. Unless your lenses include an anti-fog treatment, fog prevention remedies are only temporary, and you must reapply them regularly for the best results.