Accidental freezing of contact lenses, whether in their blister packs or storage case, is a common concern during cold weather or improper storage. Freezing poses a significant safety risk because the lens material and its surrounding solution are highly sensitive to temperature extremes. The primary concern is that the freezing process introduces microscopic damage invisible to the naked eye, which can cause serious injury or infection when the lens is worn.
Structural Integrity of the Lens
The physical structure of a soft contact lens relies heavily on its water content for flexibility and oxygen permeability. When the solution surrounding the lens freezes, water molecules expand into ice crystals. This ice formation causes mechanical stress on the delicate polymer matrix of the lens material.
The sharp edges of these expanding ice crystals can physically abrade the lens surface, creating microscopic fissures or scratches. Even minor surface alterations affect the lens’s ability to distribute oxygen and can lead to discomfort or corneal abrasion. The freezing process also draws water out of the hydrogel, known as cryo-concentration, which dehydrates the lens.
This loss of hydration changes the lens’s physical parameters, potentially leading to a change in its curvature or an overall stiffening. A dehydrated lens loses its intended shape, impacting the fit and comfort on the eye. If the lens is worn in this compromised state, the microscopic tears and shape changes can irritate the cornea, increasing the risk of inflammation or infection.
Compromise of the Contact Solution
The liquid environment in which the contact lens is stored is a carefully balanced chemical mixture designed for disinfection and hydration. Freezing this solution disrupts the chemical equilibrium by causing the components to separate or concentrate unevenly. As water freezes first, the remaining liquid becomes a highly concentrated mixture of salts, buffers, and preservatives.
This high concentration can damage the lens material and render the solution useless. The preservatives, designed to inhibit microbial growth, may be rendered ineffective after thawing because their chemical structure has been altered. Using a lens stored in a thawed, compromised solution increases the risk of introducing bacteria or other pathogens directly onto the eye.
The solution’s tonicity, or salt balance, is designed to match the natural salinity of the eye to ensure comfort and prevent the lens from shrinking or swelling. Freezing and thawing can alter this balance, making the solution hypertonic or hypotonic. Inserting a lens soaked in an improperly balanced solution can cause stinging and irritation to the sensitive corneal tissue.
What to Do After Thawing
If your contact lenses or their storage case have frozen, the first step is to allow them to thaw gradually at room temperature. Never attempt to accelerate the process by applying external heat, such as placing the case near a radiator or in hot water. Rapid temperature changes can warp the lens material and further compromise its physical structure.
During the thawing process, avoid shaking or disturbing the case, as any remaining ice crystals can act like sandpaper against the delicate lens surface. Once the solution has completely thawed, you must immediately discard all of the old solution. Even if the lens appears visually intact, the disinfecting and buffering properties of the solution are likely compromised and pose a risk of eye infection.
The thawed lens should be rinsed and stored in a fresh batch of new multi-purpose solution. However, because the damage is often microscopic and cannot be reliably detected, the safest recommendation is to discard the lens entirely. The potential for corneal damage or a serious eye infection always outweighs the cost of replacing a single pair of contact lenses.