Factories often invest heavily in dehumidifiers, believing that maintaining indoor relative humidity (RH) below 60% will guarantee mold prevention. On paper, the numbers look good. In practice, however, mold still appears. Why? Because air humidity does not equal surface moisture.
Spores Need Surfaces, Not Numbers
Mold does not grow in the abstract concept of “air”—it germinates on surfaces. Even if average RH looks safe, condensation or absorption can create micro-wet zones invisible to the eye. Studies confirm that mold can germinate when surface moisture persists despite moderate RH. For fungi, these wet spots are far more decisive than overall air readings. A single layer of condensation on a wall or pallet can turn a “dry” warehouse into a biological hotspot.
Condensation and Porous Materials
The most common source of surface wetness is condensation. When humid air meets a cooler surface below the dew point, droplets form. These droplets may linger long enough to trigger germination. Materials like cardboard, wood, and textiles amplify the problem by absorbing water vapor. Even at seemingly safe RH levels, their internal moisture remains high, making them hidden reservoirs for mold growth (material susceptibility research). In food factories, stacked cartons or wooden pallets are frequent starting points for contamination, precisely because their surfaces never truly dry.
Airflow and Dead Zones
Air circulation is another overlooked factor. Corners, stacked goods, and poorly ventilated shelves often trap humid air, creating “dead zones” beyond the reach of dehumidifiers. Field surveys show that localized conditions in storage areas can support mold even when central monitoring systems indicate compliance. In other words, sensors tell only part of the story. Without airflow management, drying systems treat the average but fail to address the extremes.
Misleading Numbers and Costly Failures
The obsession with air humidity leads to blind spots in audits and risk assessments. Factories often rely on RH data alone to demonstrate compliance, while ignoring whether surfaces are actually dry. The result: humidity reports look excellent, yet pallets, cartons, or product surfaces still develop mold. In one leather warehouse, RH was consistently below 60%, yet Aspergillus flavus was isolated from products after condensation formed on untreated walls. The financial loss exceeded millions in spoiled goods and damaged brand reputation.
Global Standards and Local Reality
International bodies like ASHRAE and WHO recommend controlling both RH and condensation to minimize microbial risks. Critical moisture levels for building materials have been established in laboratory settings, showing that mold can develop on gypsum, wood, or paper even at 75% RH if surface conditions allow. Yet many factories implement only half of the solution, focusing on RH numbers without evaluating surface wetness.
Toward Surface-Oriented Prevention
True prevention requires shifting focus. First, surface moisture detection tools—such as infrared thermography or moisture meters—should complement RH monitoring. Second, airflow design must minimize stagnant zones, ensuring dehumidification reaches every corner. Third, packaging choices matter: switching from untreated cardboard to coated or synthetic alternatives reduces risk. Finally, staff training is essential: workers must understand that “dry readings” do not equal “dry surfaces.”
Rethinking Mold Control
Mold is a local problem, not an average one. Overreliance on air humidity data is like reading the weather forecast without noticing puddles on the ground. To prevent costly failures, factories must integrate both air and surface perspectives. Only then can drying systems truly deliver, and only then can mold be kept at bay.
References
- Gorny, R. L. (2004). Fungal fragments as indoor air biocontaminants. Applied and Environmental Microbiology. NCBI
