Service animals should be permitted in the waiting areas of emergency rooms.
However, it is at the discretion of treating physicians whether a service animal should be permitted in a treatment area. This determination must be made on a case-by-case basis based solely on whether the presence of that particular animal, in that particular place, at that particular time poses a direct threat to a person. It cannot be a perceived threat ("sometimes dogs bite") or based on past experience ("another service dog acted up") but must be a cognizable real threat ("I have a patient with serious open wounds and the presence of any loose dog hair or dander may cause dangerous infection of the wounds").
A service animal may also be excluded if its presence fundamentally alters the ability of the hospital to provide emergency services, for example if there isn't room to safely work around the dog without tripping or slowing treatment.