After a significant impact
Even if no damage is visible to the naked eye, a helmet that has suffered a violent impact must be replaced immediately. The internal EPS foam absorbs the shock energy by deforming, which permanently compromises its ability to protect you during a new impact.
Safety rule: One helmet = one impact.
After any accident involving a head impact, systematically change your helmet, even if it appears intact. It is recommended to replace your ski helmet every 5 years even without apparent impact. External elements such as UV rays, humidity, and natural wear gradually reduce the protective effectiveness of the materials.