No one has mentioned it, but ,collar bone fractures - they are common in both street and dirt riding - and I can tell you ,they suck ! I wasnt wearing my chest protector and crashed, thus , breaking my left wrist, left elbow, rt. wrist, rt. clooat bone ( need a rod to put it back together ) , 3 ribs and a slightly punctured lung - lesson learned ! I now have a Alpinestar bionic siuth that I wear in the cooler months and an Alpinestars chest protector that I wear during the warmer months -
As far the the street , Id venture to say the vast majority of injuries happen while sliding and sudden stop /cou-counter-cou ( I know my spelling is off here, butm basically its like if yo ushake a water bottle then hold it still - the bottle stops , but, the water keeps moving - this can and does happen with your brian and organs, causing serious and majoy trauma or death in extreme cases ) , alot less falling/height impact related when compared to dirt, but, it does happen.
Whole point, wear your gear, regardless of what disipline your riding - If your in the dirt, wear dirt gear ( you will die from heat exhaustion if you ride in a full face helemet at a MX track ! ), if your doing a trackday , wear that gear !
And if you don't you will have no teeth left.
It all depends on what type of dirt riding you will be doing, and at the level you are doing it at. I have raced motocross for 15 years, and what I wear are a good pair of knee braces, as one wrong landing, foot stab, can destroy an ACL. Boots, of course are necessary. I used to wear a chest protector when I raced, but switched out to a soft protector under my jersey, if its a rockier track then I might use the chest protector. There have been a lot of spinal injuries lately, or brought into light more recently. The Leatt neck brace is a fairly new device that many riders are now wearing to help with spinal injury. In the case of going over the bars and spearing into the ground like a dart, this limits the head/spine from compressing. http://www.leatt-brace.com/index.php?page_id=14&id=3
If you are doing more woods riding, dealing with trees, elbow pads, chest protector would be a good idea.
Wear a dirt bike helmet. Besides looking like a gonn, you will have a ton of dirt in your eyes. Not to mention you will destroy a visor pretty quickly. A motocross helmet is a lot less bulky also, considering you are moving around a ton more on a dirt bike.