A Wake-Up Call for Preservation Environments
When we think of mould in museums, most of us imagine damp basements or water leaks. But this new fungal threat is different. Aspergillus section Restricti is part of a group of xerophilic moulds — species that don’t need moisture in the way we usually think of it. These moulds can survive and grow at relative humidity levels as low as 60%, and in some cases, even lower.
That’s well below the traditional 70–75% RH threshold used by many museums, archives, and libraries as a warning level for mould risk. The implications are clear: what was once considered “safe” may now be vulnerable.
The mould thrives in areas where objects have been in contact with human hands — even if they haven’t been handled for many years. (Photo: © Camilla Jul Bastholm)
Why Traditional Limits No Longer Apply
The problem is rooted in how we understand relative humidity (RH) and a concept called water activity (aw). Water activity is essentially how much “available” moisture there is in a material or environment — and xerophilic moulds are unusually adept at drawing out the smallest amounts. In real terms, this means even a cool, clean, and seemingly dry archive can harbor the ideal conditions for mould growth if the RH hovers in the mid-60s for long periods.
This is exactly what Danish conservation experts have recently confirmed. Despite RH levels below 60% in some museum storage areas, colonies of A. restricti have still taken hold, as reported in the DR article. This doesn’t just challenge our assumptions — it demands a shift in strategy.
An attack by these extreme moulds is taken seriously — especially when it comes to the conservators' own safety here at Anchers Hus in Skagen. (Photo: © Jacob Jul Nørup)
The Cotes Perspective: Why Precise Humidity Control Matters
At Cotes, we’ve long advocated for proactive humidity management — not just to meet standards, but to stay ahead of risks. The lesson from Aspergillus restricti is clear: margin matters. Keeping RH at or below 60% is no longer a “best practice” — it’s essential.
Our dry-air solutions are already being used in sensitive storage environments, helping to:
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Keep RH consistently in the 40–50% range, even in fluctuating seasonal conditions
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Prevent microclimates caused by cold surfaces or poor air circulation
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Protect irreplaceable organic materials like wood, paper, textiles, and artwork from mould colonisation
Best Practices Going Forward
Based on international guidelines (ISO 11799, ICOM, and the National Park Service) and the latest Danish findings, we recommend the following for museums, archives, and other vulnerable environments:
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Target RH: 45–55% year-round, with a hard upper limit of 60%
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Temperature: 18–20°C is ideal; cooler temperatures help, but only if RH is also controlled
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Avoid RH peaks: Even short periods above 65% RH can trigger mould spore germination
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Monitor locally: Use dataloggers and sensors not just for the room, but near objects and walls
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Respond quickly: Temporary dehumidification or HVAC adjustments can prevent outbreaks after leaks or equipment failures but the response should be swift: www.cotes.com/WaterDamage
The Stakes Are Higher Than Ever
Our cultural institutions are the custodians of history, identity, and science. The objects they protect cannot be replicated. As this new mould threat spreads — invisibly, silently, and under the radar — our response must be equally precise.
Humidity may seem like just a number. But when it creeps too high, it opens the door to irreversible damage. The difference between 63% and 58% RH could mean the survival of a 400-year-old manuscript or the loss of a centuries-old textile.
At Cotes, we help you close that gap.
