In the US the Emergency Aid Doctrine is an exception to the Fourth Amendment, allowing warrantless entry to premises if "exigent circumstances" make it necessary.
Contents
Enforcement
A number of exceptions are classified under the general heading of criminal enforcement:
Community response
In deciding whether such entry was legal, courts will consider whether a reasonable and prudent person would have considered there was need to make an immediate entry.
References
Emergency Aid Doctrine Wikipedia(Text) CC BY-SA