D-31 June 2002
7
Humidity Control
Control of relative humidity is required for
preservation of permanent records.
Air Conditioning
Properly controlled air conditioning of the storage
area is definitely recommended for the optimum
long-term preservation of permanent records.
The air should be filtered to remove dust,
cleansed of gaseous contaminants, if present,
and controlled to the desired relative humidity and
temperature. Slightly positive air pressure should
be maintained within the storage room or vault.
Film should be conditioned to this environment
and then placed in containers.
Where air conditioning is not available and high
humidities are likely to be present, the humidity
of the storage area can be lowered by electrical
refrigeration-type dehumidifiers. These are readily
available and inexpensive. The storage space
should first be vapor-sealed by covering the walls
with asphalt or aluminum paint or, better yet,
paper-laminated aluminum foil or other water-
vapor barriers. Painting the storage area is
suggested, but follow the painting precautions.
A humidistat set at the desired level of humidity
should be used to control the dehumidifier. The
humidity level should be checked frequently with a
reliable hygrometer, such as a sling psychrometer.
For added protection against short-term moisture
effects, such as dehumidifier failure, tape the
containers as described in the following section,
“Moisture-Tight Containers.”
Dehumidifiers using desiccants should be used
with caution. They may create a danger of fine
dust particles getting on the film and causing
abrasion when the film is used. Also, when some
chemical-desiccant particles are trapped in rolls
of microfilm, they may form bleached spots.
Therefore, inert desiccates should be used, and
it is essential that the system be designed so
that particulate material cannot enter the storage
area. The use of molecular sieve packets is
recommended, as this is an efficient desiccant.
Chemically pure silica gel may also be used as
a desiccant, provided that filtration is used to
remove particle layers larger than 0.3 micron.
See ANSI/PIMA IT9.11-1998 and ISO 18911-
1998
5
or their latest revisions.
Moisture-Tight Containers
If humidity control of the storage area is not
possible, the storage of film in moisture-tight
containers is required to maintain the film’s
humidity limits. The film must be conditioned to
the required humidity before being sealed in the
container. This requires the temporary use of an
air-conditioned cabinet or room, or an electrical
dehumidifier. Smaller quantities of film can be
conditioned for two or three weeks in a desiccator
containing activated silica gel, and then removed
and quickly sealed.
Only properly sealed metal or glass containers
are considered moisture- and gas-proof. They
are preferred when long-term air conditioning is
not practical, when gaseous impurities may
regularly be present, or when low-temperature
storage is used.
Plastic boxes, such as Kodak Microfilm Plastic
Storage Cartons (CAT No. 108-6867—16 mm;
CAT No. 841-8741—35 mm), can be used as
protective containers in controlled areas. They
provide satisfactory protection against short-term
problems of moisture or gaseous impurities,
such as might occur with floods, dehumidifier
failures, smoke from a fire, or moisture released
by insulation (upon exposure to heat) in certain
fire-resistant safes and cabinets.
With both metal and plastic containers, all
materials (including any gaskets and protective
paints or lacquers) must be non-corroding and
free of peroxides, reactive fumes, and exudations
during storage. Closed containers with friction-
type or threaded twist-on lids may require no other
seal, but should be tested for imperviousness.
If the container is in an area having the prescribed
humidity and known to be free of harmful gases,
taping is not required. If these conditions are not
met, taping will provide satisfactory protection.
Plastic boxes or cans are preferable, as they
provide more insulation in case of fire.
