Sunglasses left in a hot car: what the heat actually does to them

A hot car damages sunglasses in three ways: it warps plastic frames, it crazes anti-reflective and mirror coatings, and in a layered lens it can start to separate the layers. What survives is the tint and the UV protection built into the lens material. On a summer day a closed car reaches 50 to 60°C inside, and a dark dashboard in direct sun runs far hotter, into the range where frame plastics soften. The glovebox, the door pocket or a case under the seat is where a pair should live in a car. The dashboard is the worst spot in the vehicle.

How hot it gets

Outside air on a summer day might be 30°C. Inside a closed car with the sun on it, the air reaches 50 to 60°C within an hour; the effect is the same one that heats a greenhouse. Surfaces in direct sun run hotter than the air, and a dark dashboard is the hottest surface in the car: measurements above 80°C are common.

That matters because of what plastics do near those temperatures. Many frame plastics begin to soften and take a set between about 60 and 80°C. A frame lying on a dashboard at that temperature, with its own weight pressing on the arms, will come back out of shape.

What heat does

Warps the frame. Arms splay, the front twists, the bridge opens. The pair no longer sits on your face the way it did. An optician can sometimes reheat and reshape a plastic frame, but repeated cycles make the plastic brittle.

Crazes the coatings. Anti-reflective and mirror coatings are thin layers with different expansion rates from the lens beneath. Heat cycles stress the bond; the result is a fine network of cracks or a lifting edge that looks like an oil film. This cannot be fixed.

Delaminates layered lenses. Laminated lenses, including polarized ones where the film is sealed between two layers, rely on an adhesive bond. Extreme or repeated heat can begin to separate the layers at the edge, which shows as a bubble or a cloudy rim. Our lenses are laminated tri acetate cellulose, so this is the one heat risk specific to them.

Yellows and hardens nose pads. Silicone pads absorb sunscreen and go stiff.

What heat does not do

• Fade the tint. Modern lens tints are stable at car temperatures.
• Remove UV protection built into the lens material. That is chemistry, not a coating, and it survives. Our lifespan post covers the difference.
• Affect the polarizing film's function, unless the lens delaminates.

Where to keep them in a car

A case matters as much as the location. It keeps the frame from bearing its own weight on a hot surface and keeps grit off the lens.

If a pair has already warped

Take it to an optician. Most plastic frames can be gently reheated and reshaped once. Do not try a hairdryer at home; uneven heat makes it worse. If the coatings have crazed, the lens is done; there is no repair for a cracked coating, and our scratch post explains why polishing would only remove more of it.

Frequently asked questions