It’s not the be-all and end-all but be aware there is a new standard out that is specific to DH helmets – ASTM 1952: My new Kali Avatar doesn’t conform to it, but only because it is an older design, and the helmet conforms to all the other standard, plus some other ones. The Kali Durgana conforms to it, as does the Bell Drop, plus a few others.
As I said, it is not a deal breaker, but is something you may want to look out for as I guess eventually all DH helmets will comply with it.
The Kali helmets are also ridiculously light; a factor which is worth bearing in mind.
Check out this thread: Helmets and goggles