The U.S. National Archives has updated its 2002 guidance on acceptable relative humidity ranges, pollutant levels, security requirements, responsible program areas, and supplements to the standard.
In particular, this directive has been updated with information based on NARA Heritage Science research on collection materials that supports moving away from T/RH set points toward bands. It also reflects research showing that different materials have different requirements, and that the storage conditions a material was exposed to in the past can affect its storage requirements for the future. The guidelines for temperature and relative humidity conditions balance long-term preservation of the holdings and energy efficiency.
The directive and its attachments can be found here: https://www.archives.gov/foia/directives/nara1571