Your helmet is made to absorb some of the potential energy of a blow. Even though damage may not be visible or apparent, it is infinitely important that any helmet having suffered from a major impact in an accident, or has received a similarly severe blow, should be replaced. An inspection of the helmet might not reveal any apparent damage, but it might be there.
Sweet Protection also advises that any helmet should be replaced after approximately three years of normal use, or even sooner if exposed to long periods of sun, hard use, or any contact with chemicals. There are many variables to how often you should replace your helmet depending on how the helmet is treated (use, carefulness, travel, vibrations etc) what kind of conditions they are used (sun, heat, cold etc) as well as storing the helmet (temperature, humidity). For these reasons, we recommend all of our customers to change out their helmets for safety measures.
The helmet must also be replaced if it does not fit anymore.