I try to stay away from Snell-certified helmets.
It's all about the deceleration rate of your head when you hit something, because brain damage results from your brain impacting the inside of your skull when your head suddenly stops, such as when you hit the ground.
The Snell standard includes the ability to protect against more than one impact. For example, if your head hits the ground, then runs into the curb. Sounds good, except that it means that for a specific thickness of styrofoam, all of the deceleration must occur within
half the styrofoam thickness so that half the thickness remains for the second impact. Thus more energy is transferred to your head than if you have the full thickness of the styrofoam absorbing the impact. As mentioned in the previous message, this is accomplished using harder styrofoam in Snell-certified helmets.
Studies have shown that most motorcycle accidents result in a single head impact. Actually, your head may hit a couple of times, but it's the first impact that typically involves severe energy. So, in playing the what-if odds, the probability is weighted towards protecting against a single hard impact, versus multiple hard impacts. In a single impact, the DOT non-Snell helmet will provide greater protection.
This is discussed in great detail at
http://www.motorcyclistonline.com/gearbox/motorcycle_helmet_review/, including comments from Dr. Harry Hurt, who has been the guru of gurus for motorcycle safety studies for decades.
So, everyone has an opinion, but for my money, I buy DOT non-Snell helmets. I've been down twice in the past 30 years, and fortunately was wearing a full-face helmet both times.
Rick Federmann
Cedar park, Texas