Leaving eyeglasses or sunglasses in a parked vehicle is a common habit, but it is not recommended for the long-term health of your eyewear. The interior of a car can become a hostile environment for modern frames and lenses due to extreme temperature fluctuations. Understanding how heat and cold affect these materials makes it clear why removing your glasses from the vehicle is the safest choice to maintain their integrity and performance.
The Primary Risk: Damage from Excessive Heat
A parked car quickly transforms into a miniature greenhouse, trapping solar radiation and causing the internal temperature to skyrocket. On an 80-degree Fahrenheit day, the interior can reach 109 degrees in just 20 minutes. On hotter days, temperatures can soar to 120–150°F (50–65°C) or higher, and surfaces like the dashboard can become even hotter, sometimes reaching 180–200°F (82–93°C).
This intense heat poses a threat to plastic-based frames, causing the material to soften, warp, and deform. A warped frame compromises the fit of the glasses and can shift the optical center of the lenses, potentially leading to discomfort or eye strain. While metal frames are more resilient, the heat can still compromise delicate adjustments and loosen the small screws holding the components together.
The most common heat-related damage occurs to lens coatings, such as anti-reflective (AR) or scratch-resistant layers. These coatings are composed of multiple microscopic layers that expand and contract at different rates than the underlying lens material when exposed to heat. This unequal thermal expansion creates internal stress, resulting in tiny cracks known as “crazing,” which appears as a web or permanent smudging on the lens surface. The heat can also cause plastic lens materials, particularly polycarbonate, to warp and distort their optical properties, impairing focus and visual clarity.
How Extreme Cold Affects Eyewear
While heat damage is more dramatic, prolonged exposure to frigid temperatures also presents risks to eyewear materials. Plastics used in frames, such as acetate, become more brittle when temperatures drop below freezing. This makes them susceptible to cracking or snapping upon impact or stress. This loss of flexibility increases the likelihood of a break, especially at stress points like hinge connections.
Moving glasses rapidly from a freezing car into a warm environment can induce thermal shock. This shock places stress on the materials, potentially leading to fine cracks in the lenses, especially glass ones. The abrupt temperature change also causes immediate, heavy condensation or fogging. While not direct damage, this temporarily obscures vision and introduces moisture that could affect coatings over time.
Specialized photochromic lenses, which darken when exposed to UV light, are influenced by cold temperatures. The chemical reaction that causes them to darken is more efficient in cold conditions, resulting in a darker tint. However, the cold slows the reverse reaction, causing the lenses to take longer to fade back to a clear state once indoors.
Protection Strategies for Eyewear in Vehicles
The most effective way to protect your glasses from temperature damage is to remove them from the vehicle entirely when parked. If temporary storage is unavoidable, choose a location that offers insulation from direct sunlight and thermal buildup. The dashboard is the worst possible place, as it absorbs the most heat and exposes the glasses to direct solar radiation.
A hard-shell protective case is an inexpensive yet effective defense against physical damage and temperature fluctuations. Storing the case in a center console or glove box provides a buffer from the sun and the most extreme thermal zones of the car interior. This practice helps moderate the temperature changes the glasses experience, whether from heat or cold. Treating your glasses like any sensitive device and removing them during prolonged periods of extreme weather will ensure their longevity.