Preventing Mold in Archives:
Humidity, Temperature & Air Quality Solutions

1. Why Mold Is a Collections Emergency

Mold doesn't damage collections — it destroys them. Fungal hyphae penetrate paper fibers, digest cellulose, and leave behind irreversible staining. Even after the mold is dead, the stains remain permanently. And mold spores — microscopic, airborne, and extraordinarily resilient — can remain dormant for decades, waiting for the right humidity conditions to reactivate.

A mold outbreak is not just a preservation problem. It's a health hazard for staff (respiratory issues, allergies, asthma), a public relations risk (news of a moldy archive travels fast), and an operational crisis (affected areas must be sealed off, sometimes for weeks).

2. The RH Threshold: When Does Mold Start Growing?

Mold growth depends on three factors: available moisture (RH), temperature, and a food source (paper, leather, adhesives — all abundant in archives).

Temperature accelerates the process. At 25°C and 75% RH, common archive molds like Aspergillus and Penicillium can complete a full growth cycle in under a week. At 15°C and the same RH, growth slows to weeks — which buys you time to respond, but does not eliminate the threat.

3. Early Detection: What to Look For

Mold doesn't announce itself with a sign. It starts quietly, in the darkest, most stagnant corners of storage rooms. Train staff to recognize these early indicators:

Visual Signs

• Fuzzy or powdery deposits on book spines, box exteriors, or document surfaces — white, grey, green, black, or orange
• Foxing: Small rust-colored spots on paper, often the residual metabolic byproducts of past mold activity
• Water stains or tide marks — evidence of past moisture events that create high-risk zones

Olfactory Signs

• Musty, earthy smell: Microbial volatile organic compounds (MVOCs) produced by active mold metabolism are detectable by the human nose well before visible growth appears
• If a storage room "smells old" or "smells like a basement," you likely have mold activity somewhere — even if you can't see it

Environmental Indicators

• RH consistently above 60% for more than 72 hours
• Condensation on windows, pipes, or cold walls
• Temperature differentials creating microclimates (cold corners, areas near exterior walls)

4. Immediate Response Protocol When You Find Mold

Based on protocols developed by the UK National Archives and field-tested at archives including Somerset Archives and Local Studies:

Step 1: Isolate (First 2 Hours)

• Seal off the affected area — close doors, tape gaps, minimize air movement
• Do NOT move affected items through unaffected areas — you'll spread spores
• Wear PPE: N95 mask minimum, nitrile gloves, protective clothing

Step 2: Stabilize the Environment (First 24 Hours)

• Deploy a HEPA-filtered dehumidifier in the affected area immediately
• Target RH below 50% to stop active growth
• Increase air circulation (but contain spores with HEPA filtration)
• Lower temperature if possible — every 5°C drop roughly halves metabolic rate

Step 3: Assess and Triage (24–72 Hours)

• Document affected items — photograph, inventory, note severity
• Separate into three categories: dry/salvageable, wet/active mold, destroyed
• For dry mold: vacuum with HEPA-filtered vacuum, clean surfaces with 70% ethanol
• For wet/active items: freeze (-20°C) to halt growth, then handle when resources allow

Step 4: Treat and Return

• Dry cleaned items thoroughly before returning to storage
• Replace any shelving or boxes that show mold penetration (not just surface cleaning)
• Validate that environmental conditions are stable at <55% RH for minimum 7 days before restocking

5. Designing a Mold-Resistant Storage Environment

Prevention is dramatically cheaper than remediation. A properly designed storage environment makes mold outbreaks virtually impossible:

1. Maintain RH ≤55% year-round: This is the single most important number. Size your dehumidifier for worst-case seasonal conditions — not average conditions.
2. Eliminate cold surfaces: Condensation forms where warm moist air meets cold walls, pipes, or windows. Insulate exterior walls, insulate cold water pipes, and use double-glazed windows in collection areas.
3. Avoid dead air zones: Stagnant air in corners, behind shelving, and inside closed cabinets creates microclimates where RH can be 10–15% higher than the room average. Use low-velocity circulation fans and maintain spacing between shelving and walls.
4. Monitor at collection level: A wall-mounted sensor at head height tells you nothing about conditions inside a box on a bottom shelf. Place sensors where collections actually sit. Monitor at 15-minute intervals with automated alerts.
5. Use mold-resistant materials: Steel shelving instead of wood. Alkaline-buffered archival boxes. Avoid adhesives and coatings known to support fungal growth.
6. Plan for failure: What happens when the power goes out during a summer storm? Have a contingency plan: backup generator, portable dehumidifier, clear protocol for who does what.

6. Case Study: Somerset Archives Mold Recovery

The Somerset Archives and Local Studies service in the UK provides a well-documented mold recovery case study, shared through the UK National Archives' sector guidance program. When a significant mold outbreak was discovered affecting historical parish records, the team's systematic approach offers transferable lessons:

• Containment first: Affected materials were bagged and moved to a separate cleaning area — never processed in the storage room itself
• HEPA at every stage: Dehumidifiers with HEPA filtration ran continuously during the drying phase; vacuuming used HEPA-filtered units to avoid redistributing spores
• Staged return: Items were only returned to storage after environmental monitoring confirmed stable sub-55% RH conditions for over a week
• Root cause: The outbreak was traced to a combination of inadequate dehumidifier capacity during an unusually wet summer and poor air circulation in a corner of the storage vault

The key takeaway: mold is almost always an environmental control failure, not a collection hygiene failure. Fix the environment, and the mold doesn't come back.

Further reading: Complete Humidity Control Guide · ISO 11799 Standards Explained