By Karen E. Burke, MD
Dr. Burke is a dermatologist in private practice in New York City.
Q: Does a sunscreen become ineffective with age? How long can I keep a sunscreen before I have to throw it out?
A: Expiration dates are often stamped on sunscreen containers to specify the time limit for a product’s stability and efficacy. For optimal sun protection as well as texture, stability, and sterility, use the sunscreen prior to the date listed. If you can’t find a date on a new tube or bottle, write the month and year you purchased it in permanent marker on the tube.
To test shelf life, manufacturers store a product at 40°C with 75 percent humidity; then at 40°C with 25 percent humidity; and then test it at 0, 1, 2, and 3 months. Stability for three months in these laboratory conditions is comparable to three years in normal ambient environments. So your sunscreen should be good for up to three years after purchase.
Of the 17 sunscreen ingredients approved in the U.S., 15 are organic, or chemical, sunscreens: They work by absorbing damaging ultraviolet radiation (UVR). These chemical sunscreens consist of innately unstable molecules, but in the past few years manufacturers have started adding stabilizers like octocrylene.
Zinc oxide and titanium dioxide are physical, inorganic sunscreens; they reflect and diffuse UVR. Both are photostable and do not change molecular structure when exposed to UVR. Physical sunscreens once had an opaque, paste-like consistency, but in the past two decades manufacturers have developed cosmetically elegant formulations using micronized (tiny) particles. Since micronized titanium dioxide and zinc oxide can clump together over time, often the particles are coated with dimethicone or silica to keep the ingredients stable and smooth. And in addition to active compounds, sunscreens usually contain preservatives, emulsifiers, fragrances, and other additives.
Sunscreens are tested in their actual containers, since plastics in the container may leach into the sunscreen and cause a chemical interaction. UVR changes the molecular structure of chemical sunscreens, so sunscreen containers should be opaque.
For best results, use your sunscreen before the stated expiration date, and store it in a cool place.