A motorcycle helmet doesn’t do you much good if it comes off in a crash or fails to absorb the impact and protect your brain. The United States Department of Transportation has created a set of standards designed to set the safe helmets apart from the unsafe. Among those standards, the DOT asks whether hte helmet absorbs a significant amoutn of the impact, whether its shell prevents penetration, and whether the fastening system that holds it on your head stays intact when struck with a significant amount of force.
One important feature of the DOT approved helmets is that they include an expanded polystyrene (EPS) foam liner. If you took this apart, it would look a lot like styrofoam, but it absorbs much more energy. However, it is a non-rseilient liner. This means that after one crash, the EPS liner becomes crushed and will not return to its original form. For this reason, if you have been involved in one motorcycle accident, you should throw away your current helmet and purchase a new one.
The DOT posts results of it’s pass/fail test on the National Highway Transportation and Safety Administration website, here. Only those helmets that meet DOT standards are eligible to place the “DOT Approved” sticker on the back.