I know down hill riders reach high speeds. How strong are down hill helmets? Any one ever seen one broken from a crash? I know it probably isnt advisable but i am considering wearing one of the full face down hill helemets for SLOW speed off road motorcycle single track. I would only be wearing it at slow speed stuff. My top speed would be around 20 mph on the moto. I have a herniated disk at C6 and would like to try riding again very slow low impact stuff. The MTB helmets are about 1.5 pounds lighter which is a lot of stress off of my neck. As my neck gets stronger I can transition back to my moto helmet when speeds pick up.

I know most of you guys are going to say moto helemt for moto, MTB helmet for mtb. But i will decide if i am willing to take the risk. Down hill MTB guys reach much higher speeds on the same terrain i will be on.

Having a helmet you will wear is the best kind of helmet. Go for it.

Get something Snell rated.
They do a tougher drop test than D.O.T.

Just get a bump cap, no OSHA regs and the those that really know will just shake their head at you.

I whacked a tree in a Bell Helmet during a stream crossing.  I was carrying my bike and running across the small stream and stood up under a low thick tree branch.  The helmet was fine, but it rung my bell and made my neck hurt.  

I would worry more about health insurance than your quality MTB helmet.  The helmet did nothing to protect me against broken ribs, a groin pull, and a broken foot.  They do nothing for cactus.

On a motorcycle, I never went cheap on a helmet, and mostly wore Arai. I have a Nolan right now.

Since no one has answered your question: They are super tough, but as opposed to moto helmets they are only for impacts, not compression/compacting. Meaning that they offer very little protection to getting run over by something heavy.

The cert you want is:  ASTM DH.

Personally if I was going to buy something, I'd go with Fox because they have by far the most crossover experience between moto and mtb, and their DH helmets are direct descendants of their moto helmets. Just IMO.

Helmets have come a long way with bike parks and mtb in general getting much, much bigger and faster in the past ten years. If you're not going over 20-30mph you'll be more than fine.

Thats a good point. A lot of moto rides ride or race in groups and there are times you could get hit by another moto. Fortunately (or unfortunately) for me i really only have one riding partner and we are pretty chill with speed. We give eachother lots of room. Ussually to avoid dust and enjoy the scenery.. only real risk of getting hit would be if our own bike fell on us or if we hit someone else head on in a two way trail. Which could happen. But would be bad in so many ways that i am not too sure how much the helmet would help.

Look up the Virginia Tech helmet ratings.  They are doing instrumented testing of helmets and the forces they allow to reach your brain.

Buy a Fox motocross helmet.  They're cheap, and they're ECE 22.06.

As to MTB helmets, I know a good number of dedicated MTBers - the kind that have thousands of hours of riding.  Every single one of them has knocked themselves out cold in a crash WEARING A HELMET.  MTB helmets are better than nothing, but I don't put a ton of stock in them.  Downhill would be better than enduro, which would be better than XC.

Once your neck is better, a Fox MX helmet is cheap, and is ECE 22.06.

I haven't watched it, but does it say that a half-helmet is useless?  Because they kind of are.  When people crash, they look up and forward to see where they're flying, and they land on their chin/lower face, taking the initial, hardest impact on that part of their skull, with no help from the half-helmet sitting up on top of their noggin.   If they landed face-down or chin-down, the helmet would help some, but most people don't land that way.

Multiple dudes I know IRL have been knocked clean out in half-helmets, without even taking much damage to the helmet.   One of them was riding alone, and has no memory other than he became aware that he was walking down the trail bleeding.  Another was in a group ride, but in back, people realized he wasn't in the group, went back, and there he was not even knowing where he was or who he was.  Both times, the initial impact was to their lower skull, the helmet was too high to do much (or anything).  I also know multiple dudes that weren't knocked out, but got some definite facial scarring from the initial impact being to their lower face, not the helmet.