Final answer:
New or revised NFPA standards are implemented after a final rule is signed and a specified effective date is reached, but previous standards may still be active until certain criteria are met, such as time from designation or approval of an implementation plan.
Step-by-step explanation:
New or revised NFPA standards are not implemented until they go through a specific process. According to the information provided, once a final rule is signed, there is typically a period before it becomes effective. For example, a final rule signed on October 1, 2015, may not become effective until December 28, 2015. Even after the new standards are effective, previous standards may remain in effect in certain areas until specific conditions are met, such as the passage of a certain period since the designation under the current standards or the submission and approval of an implementation plan.
In the case of the previous sulfur dioxide (SO2) standards, these remained in effect in certain areas until it was not yet 1 year since the effective date of designation under the new standards, or an implementation plan for attaining the new standards had not been submitted and approved. This indicates that there can be an overlapping period where both old and new standards may remain active in certain areas, depending on regulatory and implementation requirements outlined by entities like the Environmental Protection Agency (EPA).