Is this the thousandth time this topic has come up??
from
http://www.helmets.org/replace.htm
Most manufacturers now recommend that helmets be replaced after five years, but some of that may be just marketing. (Bell now recommends every three years, which seems to us too short. They base it partially on updating your helmet technology, but they have not been improving their helmets that much over three year periods, and we consider some of their helmets since the late 1990's to be a step backwards, so we would take that with a grain of salt.) Deterioration depends on usage, care, and abuse.
Occasionally somebody spreads rumors that sweat and ultraviolet (UV) exposure will cause your helmet to degrade. Sweat will not do that. The standards do not permit manufacturers to make a helmet that degrades from sweat, and the EPS, EPP or EPU foam is remarkably unaffected by salt water. Since helmets spend a lot of time in the sun, manufacturers usually put UV inhibitors in the plastic for their shells that control UV degradation. If your helmet is fading, maybe the UV inhibitors are failing, so you probably should replace it. Chances are it has seen an awful lot of sun to have that happen. EPS is a long-lived material little affected by normal environmental factors.
In sum, we don't find the case for replacing a helmet that meets the ASTM or Snell standards that compelling if the helmet is still in good shape and fits you well.
I should add that Randy Swart (the individual most responsible for the content on the Bicycle Helmet Safety Institues' website) uses a helmet that is 13 years old. He emailed me in March and said,
"The helmet I wore today was made in 1992. I believe its smooth round profile, impact performance and coverage are better than anything on the market at the moment. I am not concerned about the EPS deteriorating."
I can't help but think helmet manafacturers are just trying to move more stock by recomending frequent repacement.
About the recomendation of manafactures for frequent replacement Randy said,
"CPSC are probably being told by their lawyers not to recommend longer use for fear sombody with a beat-up helmet may get injured and sue them. I don't know anything more about their position than that bit of speculation. There are millions of riders out there using older helmets and we have not seen any evidence of a problem with the older ones. And I don't think anything on the market outperforms the older helmets in impact protection or coverage, since the designers are just designing to the standard. We don't care if others want to recommend replacement more often--most riders won't do it anyway."