Sunscreen shields against the sun’s ultraviolet (UV) radiation, protecting skin from damage like premature aging and skin cancer. This protective function relies entirely on the stability of its chemical formula. While it is convenient to leave sunscreen in a car, the answer to whether this is safe is generally no. Extreme heat buildup inside a parked vehicle rapidly compromises the sunscreen’s integrity, reducing its effectiveness.
Why Extreme Temperatures Are Problematic
The temperature inside a parked car can rise dramatically, even on a moderately warm day, rapidly degrading the sunscreen formula. On an 80-degree Fahrenheit day, the interior temperature of a vehicle can exceed 109 degrees Fahrenheit in just 20 minutes, and may reach 123 degrees within an hour. Surfaces like the dashboard can become even hotter, sometimes reaching 160 to 170 degrees Fahrenheit, due to the greenhouse effect of the car’s windows.
Sunscreen is regulated by the Food and Drug Administration (FDA) as an Over-the-Counter (OTC) drug, requiring strict standards for safety and effectiveness. Manufacturers must ensure the product remains stable for at least three years, but this guarantee applies only when stored under normal conditions, typically standard room temperature. Exposing the product to prolonged high heat significantly accelerates chemical reactions within the formula. This heat stress quickly breaks down the active ingredients and compromises the emulsion that keeps the product mixed.
The Impact on Sun Protection Factor (SPF)
The core function of sunscreen is to provide the Sun Protection Factor (SPF) stated on its label, which relies on the stability of its UV filters. Heat exposure directly attacks these active ingredients, causing them to break down faster than they would under normal storage conditions. Chemical sunscreens, which absorb UV radiation and convert it to heat, are particularly vulnerable; ingredients like avobenzone and oxybenzone can lose their UV-filtering capabilities when exposed to high temperatures.
Physical sunscreens use mineral filters like zinc oxide and titanium dioxide to reflect and scatter UV rays and are generally more heat-stable. However, the extreme heat of a car can still compromise the non-active ingredients, such as the oils and emollients, that keep the mineral particles evenly suspended. When the suspension agents break down, the mineral particles clump together, leading to a non-uniform application on the skin. This heat-induced degradation reduces the stated SPF level, meaning the user is left with a false sense of protection.
Recognizing Degradation and Proper Storage Guidelines
A heat-compromised sunscreen will often show visible and tactile signs that its protective efficacy is gone. One of the clearest indicators of product degradation is the separation of its oil and water phases, which may make the product appear more watery, oily, or clumpy. A change in the consistency, such as a grainy or gritty texture, also suggests the formula’s emulsion has been compromised by heat exposure.
An unusual or rancid odor, or any discoloration like a yellowish or brownish tint, are other signs that the chemical ingredients have broken down and the product should be discarded. If a sunscreen looks or smells different from when it was purchased, it should not be used. The protective ingredients are likely no longer evenly distributed or fully effective.
To maintain the product’s integrity, sunscreen should be stored in a cool, dark, and dry place, such as a cabinet at home. When traveling or spending a day outdoors, keep the product in a cooler bag or insulated container. This shields it from intense heat and direct sunlight.