One surprising answer is floating in the air.
Ozone exists in the atmosphere at extremely low concentrations, but it is remarkably aggressive toward many types of rubber. When ozone attacks the double bonds in stretched rubber, tiny cracks can form. Given enough time, those cracks grow—and a material designed to be flexible gradually begins to fail.
Rubber chemists developed an ingenious defense.
Instead of trying to make rubber completely unreactive, they add small molecules that are even more willing to react than the rubber itself.
Global Chemical Database