I spoke with an Archival agency, and they also 
suggested that confining the cels and genga into 
an airtight environment would be a bad choice.

They had a great idea though.  To place pieces 
of special archival MicroChamber interleaving 
paper on the non-painted side of each cel.  This 
microchamber paper will capture some of the gas 
released from the cel and trap it so it cannot 
cause deterioration.

Regards,
Jason