It’s common knowledge that a helmet should be replaced after a significant impact. Exposure to extreme heat or chemicals, like gasoline, ammonia and solvents, can also degrade a helmet. But aside from impact, heat or chemical damage, does a helmet need to be replaced just because it is old? Do the impact attenuating properties of the helmet diminish with time? Put more simply, do helmets have an expiry date?
Owners’ manuals for motorcycle, bicycle and snow-sport helmets often suggest that helmets should be replaced every 2 to 10 years. The scientific basis for these recommendations is unclear, and a cynical consumer might view the short end of this range as self-serving for helmet manufacturers.
Few published studies have evaluated the effect of age on helmet impact performance. Previous tests we did showed that unused 10-year-old motorcycle helmets attenuated impacts as well as new helmets (see this link). But showing that one old helmet model performs like an array of newer helmets does not answer the more general question of whether helmet impact performance degrades with age.
To address this bigger question, we acquired and tested hundreds of undamaged used bicycle helmets of various makes, models and ages. Our preliminary findings, presented at the World Congress of Biomechanics in July 2014, showed that helmet age does not have a significant effect on the impact performance of bicycle helmets. Based on these data, age-related degradation in helmet impact performance is not a compelling reason to replace your old helmet.
As with all scientific studies, our study has limitations. Helmet performance relies on more than just the energy absorbing properties of the helmet’s foam. The straps, buckles, and other helmet features also affect helmet performance, and we did not study how age affects these components. We also tested only helmets without visible damage or significant wear and tear, and therefore our results do not mean that any old helmet will provide the same protection as a new helmet. Nevertheless, our results show that the impact properties of bicycle helmet foam are not significantly affected by age.
Figure 1 – Exterior and interior of a damaged helmet
Figure 2 – Impact performance (head acceleration) does not change significantly with the year of helmet manufacture.