Using the GetAssociateType() function your code can be made so that only types of creatures you want affected are affected. Using the GetMaster() function you can run a check to make sure the associate is aa associate of a PC. These are examples of what you can do to ensure only those you want affected by the damage are affected…
Get Master will return OBJECT_INVALID for the the master of others, since they have no master, run a check to sii if they are a PC and if so, damage them too. This way you could add the check for “winter clothing” in a freezing area or check for a type of equipped item for a hot area, such as metal armor in a HOT desert, etc…and be sure that only those creatures you want affected are the ony ones affected.