Can Heat Damage Sunglasses? What a Hot Car Does to Frames and Lenses
Sunglasses Care
Can Heat Damage Sunglasses? What a Hot Car Does to Frames and Lenses
Leaving sunglasses on the dashboard is one of the most common ways a good pair gets ruined. Here is what heat actually does to lenses and frames, how to tell if your pair is damaged, and the simple storage habits that prevent it.
In this article
- Can heat damage sunglasses?
- How hot does a parked car get?
- What does heat do to the lenses?
- What does heat do to the frame?
- How can you tell if your sunglasses have heat damage?
- How should you store sunglasses in the car and in summer?
- Rawbare picks for everyday wear and safe storage
- Frequently asked questions
Can heat damage sunglasses?
Yes. Sustained heat, such as a closed car parked in the sun, can crack the coatings on a lens, damage the film inside polarized lenses and soften or loosen the frame. Short exposure to a warm day is fine. Hours on a dashboard is not.
Sunglasses are designed to be worn in the sun, so most people assume they can handle heat anywhere. The difference is time and temperature. On your face, a pair sits in moving air at body temperature. On a dashboard behind a windscreen, it sits in a closed space that keeps heating up for as long as the car is parked.
How hot does a parked car get?
Much hotter than the air outside, and faster than most people expect. In a study of a dark sedan on sunny days, cabin temperature rose by an average of about 22°C (40°F) within an hour, with most of that rise in the first 30 minutes.
The study, published in Pediatrics by McLaren, Null and Quinn, measured a parked car on 16 clear days with outside temperatures between roughly 22°C and 36°C. The rate of heating was similar whatever the outside temperature was. Even on the coolest test day, the cabin reached about 47°C (117°F). Cracking the windows open made little difference.
Now apply that to an Indian afternoon. When it is already 35°C to 40°C outside, which is normal across much of India from March to June, a similar rise puts the cabin air well past 50°C. Surfaces in direct sun, such as the dashboard and the top of the centre console, run hotter than the air around them.
| Situation | Risk to sunglasses |
|---|---|
| Worn on your face outdoors | Low |
| In a bag, in shade | Low |
| Car glovebox, short parking | Low to moderate |
| Car seat, windows closed, hours | High |
| Dashboard, in direct sun | Highest |
What does heat do to the lenses?
Heat mainly attacks the thin layers on and inside the lens rather than the lens material itself. The three common results are coating crazing, polarized film damage and lens distortion.
- Crazing: a lens is coated with very thin layers, such as anti-reflective and scratch-resistant coatings. The lens and the coating expand at different rates when heated. That stress creates a web of fine cracks called crazing. Optical retailers and lens labs consistently list heat, and a hot car in particular, as the leading cause.
- Polarized film damage: a polarized lens has a filter film sealed inside it. Excess heat can damage this film and the layers bonding it, which can show up as hazy patches, bubbles or uneven tint.
- Distortion: with enough heat, a lens can lose its exact curve or loosen in the frame, so the view through it looks slightly warped.
Crazing cannot be polished away. In some prescription lenses the coating can be stripped by a lab, but for most sunglasses, a crazed or bubbled lens means the pair needs replacing. Read more about what each layer does in our guide to sunglasses lens coatings.
What does heat do to the frame?
Plastic and acetate frames can soften and change shape, metal frames get hot to the touch and their screws can loosen, and nose pads and temple tips can harden or crack over time.
Acetate and many plastic frames soften with heat. That is not a weakness in normal use; it is actually how opticians adjust them. They warm the frame so it can be bent to fit a face. The same thing happens unintentionally on a dashboard, except nobody is holding the frame in the right shape while it cools. The result can be temples that splay outward, a front that twists slightly, or a pair that no longer sits level.
Metal frames do not soften at these temperatures, but they absorb heat and can be uncomfortable or even painful to put on straight away. Repeated heating and cooling can also loosen hinge screws. Silicone nose pads and rubber temple tips age faster in heat and may become sticky, stiff or cracked.
For a breakdown of how each material behaves, see eyewear frame materials explained.
How can you tell if your sunglasses have heat damage?
Look for a fine spider-web pattern on the lens, hazy or bubbled areas, a warped view, or a frame that no longer sits straight on your face.
How should you store sunglasses in the car and in summer?
Keep them in a hard case, out of direct sun, and take them with you when you leave the car for any length of time.
- Never leave them on the dashboard or on the rear shelf, even for a quick stop.
- Use a case. A case shades the pair from direct sun and stops it sliding around and getting scratched.
- Glovebox over seats. If you must leave them in the car, a closed glovebox is shaded and better than a seat or the dashboard. It is not cool, so keep it short.
- Take them with you. Hang them on your shirt or put them in your bag when you park for more than a few minutes.
- Do not cool them suddenly. Running a hot pair under cold water can add to the stress on the coatings. Let them cool in the shade first.
- Keep them away from other heat sources: stove tops, hair dryers, heaters and saunas.
For day-to-day cleaning and storage habits, read how to care for and store sunglasses.
Rawbare picks for everyday wear and safe storage
A good case is the cheapest protection a pair of sunglasses can have. These are in stock on rawbare.com.
Frequently asked questions
Key takeaways
