Snell Foundation has not published an article (or if it has, it is not easily accessible) comparing tests on a helmet that is 5 years or older and a new helmet. They seem just to repeat rumors of effects of UV light and sweat on helmets. Here is an excerpt from helmets.org of what seems to be a bicycle helmet safety organization. They say that EPS is resistant to even salt water and the outer helmet shells have UV resistant chemicals added and call statements about EPS degradation 'rumors':
"Is it newer? With what standards sticker inside?
Newer helmets from the late 1980's and the 90's may or may not need replacement. First look to see what standards sticker is inside. If it's ASTM or Snell, the helmet was designed to meet today's standards for impact protection, and you may even find that Consumer Reports tested it in one of their articles. 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. But if you ride thousands of miles every year, five years may be a realistic estimate of helmet life. And helmets have actually been improving enough over time to make it a reasonable bet that you can find a better one than you did five years ago. It may fit better, look better, and in some cases may even be more protective. For an alternate view that agrees with the manufacturers, check out the helmet FAQ of the Snell Foundation. Snell knows a lot about helmets and their views on this subject should not be dismissed lightly, even though we disagree with them.
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. Your helmet will get a terminal case of grunge before it dies of sweat. UV can affect the strength of the shell material, though. 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. Otherwise, try another brand next time and let us know what brand faded on you.
At least one shop told a customer that the EPS in his three year old helmet was now "dried out." That is highly unlikely, unless the EPS is placed in an oven for some period of time and baked. The interior of your car, for example, will not do that, based on helmets we have seen and at least one lab crash test of a helmet always kept in a car in Virginia over many summers. EPS is a long-lived material little affected by normal environmental factors. Unless you mistreat it we would not expect it to "dry out" enough to alter its performance for many years.
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."
On the other hand, there has been some failure analysis on helmets in which the foam is pre-compressed: "Influence of environmental factors on energy absorption degradation of polystyrene foam in protective helmets." De-Shin Liu , , a, Chia-Yuan Changa, Chin-Ming Fanb and Shu-Lin Hsub . Pre-compressing the foam does cause some deterioration of the EPS. I am not certain of that implication - perhaps the compression from wearing it over time may reduce protective capacity.
Ron