Did you drop it hard enough to crack the foam? Replace.
Is it from the 1970's? Replace.
Is the outside just foam or cloth instead of plastic? Replace.
Does it lack a CPSC, ASTM or Snell sticker inside? Replace.
Can you not adjust it to fit correctly? Replace!!
Did you crash in it?
For starters, most people are aware that you must replace a helmet after any crash where your head hit. The foam part of a helmet is made for one-time use, and after crushing once it is no longer as protective as it was, even if it still looks intact. Bear in mind that if the helmet did its job most people would tell you that they did not even hit their head, or did not hit their head that hard. And the thin shells on most helmets now tend to hide any dents in the foam. But if you can see marks on the shell or measure any foam crush at all, replace the helmet. (Helmets made of EPP foam do recover, but there are few EPP helmets on the market. Yours is EPS or EPU unless otherwise labeled.) You can also crack the helmet foam or damage it by dropping the helmet on a hard surface. The cracks may be small and hard to see, so you need to look carefully. Cracks in the foam always require replacement of the helmet. You may be reluctant to replace a helmet that looks almost as good as new, but if you did hit, you don't want to take chances on where you will hit next time. If the foam is cracked under the thin shell, it will be more likely to fly apart in your next crash. Many manufacturers will replace crashed helmets for a nominal fee, and most will also inspect crashed helmets to see if they need replacement. Call them if you are in doubt. For contact info check our list of manufacturers. (You can also ask them if they think the advice on this page is valid!}
Is it from the 70's?
If you still have a helmet from the 70's without a styrofoam liner, replace it immediately. That would include the Skidlid (with spongy foam), 1970's Pro-tec (spongy foam), Brancale (no foam) and all leather "hairnets." They just did not have the protection of helmets made after 1984 when the ANSI standard swept the junk off the market. The better 1970's helmets were reasonably good ones, but were not quite up to current standards. It is probably time to replace that old Bell Biker, Bailen, MSR, Supergo or similar model from the 70's or early 80's. (We have a page up on replacing the Bell Biker.) The hard shells were great, but the foam liners were not thick enough to meet today's ASTM or Snell standard. The Bell V-1 Pro was designed to today's standards, but the foam is very stiff, and if you are over 65 you probably should replace that too. If you have one of the 1980's all-foam helmets with perhaps a cloth cover, we would recommend replacing that one. Lab tests showed some years ago that bare foam doesn't skid well on pavement, and could jerk your neck in a crash. The cloth doesn't help much. In addition, some of them had no internal reinforcing, and they tend to break up in a crash. That's not serious if you just fall, but if you are hit by a car the helmet can fly apart in the initial contact and leave you bare-headed for the crack on the pavement.
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. Sunlight 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 or showing small cracks around the vents, the UV inhibitors may be 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." Other sales people refer to "outgassing" and say that the foam loses gas and impact performance is affected. Still others claim that helmets lose a percentage of their effectiveness each year, with the percentage growing with age. All of that is nothing but marketing hype to sell a replacement helmet before you need it. There is some loss of aromatics in the first hours and days after molding, and helmet designers take account of that for standards testing. But after that the foam stabilizes and does not change for many years, 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. Helmet shells can be affected by car heat, but not the foam. The Snell Memorial Foundation has tested motorcycle helmets held in storage for more than 20 years and found that they still meet the original standard. 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.
An honest manufacturer: MET
The Italian company MET says in their 2010 catalog:"We are often asked 'For how long is a helmet safe?', or 'how often should I replace my helmet?”' Until now it has been difficult to find any reliable figures to help answer these queries. MET have now developed a series of tests which are conducted on aged helmets to determine a 'best before' date (unless the helmet is involved in an accident. In that case it should be replaced immediately.). The results indicate that, if used properly accordingly to our owner manual, our helmets will still do their job up to eight years after they have been made. Not only is that good news for the customer, it’s great news for the environment!" We applaud MET for undertaking an actual testing program on helmet life and for making that statement. We regard it as a triumph of integrity over marketing. MET's helmets are made with industry standard shells and liners, so there is no reason we can see that their recommendation should not be good for many other helmet brands as well. If another manufacturer comes up with a testing program that shows earlier deterioration in the protection from their products we will review this page. 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.
Are you using it for non-bicycle activities?
Since 2003 helmets have been available that are actually certified to skateboard or ski standards as well as the CPSC bicycle helmet standard. If you are using a bicycle helmet for skateboarding or any other sport where you crash regularly, see our writeup on helmets for the current season for more info on that. Otherwise, we would recommend buying another helmet designed for the activity you are pursuing, whether or not you replace your bike helmet. We have more on that subject on our page on other helmets. Note that most "skate-style" helmets currently on the market are actually bicycle helmets certified only to the CPSC bicycle helmet standard. They have CPSC stickers inside, but no ASTM Skateboard standard sticker.
Do you still like wearing it?
Your helmet is of course a piece of wearing apparel as well as a safety appliance. If you consider yourself a stylish rider and your helmet is not as spiffy as the new ones, go for it. There is nothing wrong with wanting to look good, and if you do, fashion is a valid reason to replace a helmet.
Is it a better helmet than the ones available today?
As new styles have become more "squared-off" and designers have begun adding unnecessary ridges and projections that may increase the sliding resistance of a helmet shell, there is good reason to stay with one of the more rounded designs of the early to mid 90's. Those round, smooth shells like the original Bell Image that Consumer Reports rated highly in 1993 are more optimal for crashing than some of the newer designs. So think twice about "moving up," and look for a rounded, smooth-shelled design when you do. We have a lot of info on the new ones up on our page on helmets for the current season.