The hospital I volunteered at just required a drug test, background check and a couple of references. Given you aren't being paid I don't see why they would care. You're here legally and providing them a service free of charge. Now, how much they allow you to actually do as a volunteer varies greatly by state law or the facility's policy. Even if they won't allow you to do a lot of hands on work, its still a great experience just being there, seeing what happens, and hearing all the terminology used.