Mountain Bike Helmets
You are probably aware that most serious injuries from biking are due to a lack of adequate head protection. In fact, in one year the US Consumer Product Safety Commission reported that 150,000 head injuries were sustained while riding bikes. If you wear no other safety equipment, you should at least wear a helmet. After all, you can pick yourself up and continue riding after getting scraped up, but you may never get up again after fracturing your skull.
In the vast majority of head injury cases, a good helmet is the difference between a headache and a more serious injury. The modern mountain biking helmet can be very protective without being all that heavy. It is important for you to get one that fits you, as well as being compatible with the type of riding you do.
If you're a leisurely rider, you may be able to get away with a helmet that is merely good at absorbing impact. If you're a downhill racer or you like to do jumps and stunts, you need some extra protection. For the leisurely mountain bike enthusiast, a helmet can have an open face. But, for the aggressive mountain biker, a protective face shield in some form should be used.
Of even more importance, however, is how well the bike helmet fits. In many cases you will be able to get a good fit with an adjustable helmet. A helmet with an adjustable fit system may make use of a tightening dial and strap system, or the classic cinch it yourself strap and buckle system. The latter can typically be found on all non-fitted helmet systems and systems that don't have more advanced adjustable fit systems.
You should take consideration the strap buckle you use. You need one that will not break and will not pinch you during use. A surprise pinch of a helmet buckle can easily distract you and lead to an accident. If you have a beard, you'll need to make sure that the strap and buckle system will not catch your beard. There are good enough adjustable fit systems to avoid these types of problems altogether.
You should keep in mind that even adjustable fit system helmets are not one size fits all helmets. The adjustments are merely with the strap systems. There is a universal fitting system used now, but it typically leans toward those with smaller crown sizes. You need a helmet that is at least the right size for your crown. In particular, if you're an XXL, you should fit any helmet before you buy it.
A properly fitted helmet will sit low on the forehead without wobbling and without blocking forward vision at all. Air gaps around your head are dangerous during an accident, so make sure you have a head-conforming fit. The better helmets can still protect your head well, while not blocking too much of your peripheral vision. A good rule of thumb is to have at least 150 degrees of visibility.
As a good starting point for fit, you can use a cloth tape measure to go around your head right above your eyebrow line. Compare the circumference of your head to the brand sizing charge of the helmet you wish to purchase.