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Lately I am wearing my helmet also on my daily commute to work.
Of course I don't wear it while sitting at my desk, and it sits on a drawer next to the window, where in the afternoon it is exposed to sunlight with little or no air flow.
Can this lead to long term damage of the helmet? The helmet shell is white.
I was able to find one study that comes close to addressing this: Age Does Not Affect the Material Properties of Expanded Polystyrene Liners in Field-Used Bicycle Helmets. You'd need to pay to read the whole thing, but the abstract and title are clear enough. It's not clear from the abstract whether the helmets tested had been exposed to UV or not.
According to the Bicycle Helmet Safety Institute, UV exposure is not a big concern because helmets are treated with UV inhibitors, and unless there's visible signs of cracking, talk of replacing helmets on a regular schedule is marketing hype.
The EPS foam makes up the bulk of the helmet, of course, but it's not the only part, and if the shell, pads, or straps are damaged or degraded, that might be a reason to replace it.
Helmets are intended to be worn while cycling in the sun, so they have to be ok. Now that in many places it's normal to leave helmets on bikes stored outside all day.
Plausible damage mechanisms are UV and heat.
Through glass there will be less UV than outside, so we can dismiss that indoors.
Heat is harder to dismiss but if you're in a temperate place it's not likely to get as hot as outside in full sun. Anyway your helmet is white so won't absorb much heat from the sun. The lack of airflow isn't a big deal. There's not much airflow riding slowly especially as your head blocks it.
Yes, sunlight may affect helmet performance. It catalyzes the material-aging process. Sunlight will not break your helmet after several rides, but years of exposure may substantially affect the performance of the helmet during an accident.
According to the Snell Foundation, a motorcycle helmet should be replaced every five years. Looks like this recommendation should also apply to bicycle helmets.
