Vertalot,
Firstly I would recommend reading the regulations for they contain the answer to the question that you have posed. In particular Appendix 1 to JAR-OPS 3.005(d) paragraph (c)(2) and ACJ to Appendix 1 to JAR-OPS 3.005(d) “The HEMS philosophy”.
In practical terms 212man is correct in his statement that you have to meet the standard performance requirement of JAR-OPS 3 - but, as modified by the Appendix referenced above.
The following is therefore permitted:
If operating over a non-hostile environment operations can be conducted in PC3;
at a hospital in a congested hostile environment (city centre); operations conducted in PC1 except when an approval has been given under the Public Interest Site alleviation - which permits exposure on take-off and landing (a modified PC2 with exposure);
at the accident site; no performance requirement but pilots are expected to minimise exposure;
at the operating base; normal performance requirements apply - i.e. PC1, 2 or 3 as required by JAR-OPS 3.470.
As stated in the ACJ referenced above; the operating base is where the crew and helicopter are stationed and from where most sorties are started and ended. Complications occur when the operating base is in a congested hostile environment and specifically at a hospital; if this is the case then the local NAA will have to rule on what the requirement is. Basically, this situation should not occur as the base should have been risk assessed and located with safety in mind.
However, all are aware that as the HEMS helicopter becomes the trauma team mode of transport (see London HEMS), it becomes economical to base the helicopter at the hospital so that the trauma team can be gainfully employed when HEMS is not in action.
Jim