Mold thrives on paper. Cellulose, the main structural fiber in every sheet of paper, is an organic polymer that many common fungi can break down and feed on. A review of 54 published studies on mold in libraries found that fungi consistently colonize books and archival holdings wherever climate control is inadequate, with Aspergillus species turning up as the single most common genus in over half of those studies.1PubMed Central. Mold in Paradise: A Review of Fungi Found in Libraries The question is less “can it happen?” and more “how fast, how bad, and what can you do about it?”
Why Paper Is Such an Easy Target
Paper is made of plant fibers, and fungi have evolved over hundreds of millions of years to decompose plant material. Cellulose gives mold both a surface to anchor to and a food source to digest. But cellulose alone does not fully explain why some papers mold faster than others. The sizing compounds, coatings, and additives mixed into paper during manufacturing play a significant role. Starch-based sizing, gelatin, rosin, and even trace heavy-metal contaminants all influence how aggressively fungi colonize a sheet.2PubMed. Microfungal biodeterioration of historic paper: Preliminary FTIR and microbiological analyses Starch, for example, is an easily digestible carbohydrate that many mold species consume even more readily than cellulose itself.
Paper composition has also changed historically in ways that matter. Before about 1850, paper was made almost entirely from rags, using hemp, flax, and cotton fibers. After that, wood pulp became the standard raw material, and wood-pulp paper degrades more quickly on its own, even before mold gets involved.2PubMed. Microfungal biodeterioration of historic paper: Preliminary FTIR and microbiological analyses That built-in fragility makes wood-pulp paper doubly vulnerable: it is already weakening with age and offers fungi a substrate that is easier to penetrate.
Laboratory work has shown that even subtle differences in paper composition change what mold does once it arrives. When researchers inoculated paper of the same fiber origin but with different sizing treatments, the fungi produced markedly different stain patterns. In some cases the mold sporulated heavily; in others it spread without forming visible spore structures. The paper’s chemistry pushed the fungus into different physiological states.3Macromolecular Symposia. Biodeterioration of Paper: A SEM Study of Fungal Spoilage Reproduced Under Controlled Conditions
The Usual Suspects
A handful of mold genera show up again and again in contaminated paper collections. Aspergillus, Penicillium, Cladosporium, and Alternaria are the most frequently reported across libraries, archives, and museums worldwide.4PubMed. Fungi in archives, libraries, and museums: a review on paper conservation and human health These are not exotic organisms. They are the same genera responsible for bread mold, bathroom mold, and the fuzzy patches on forgotten leftovers. Their spores are essentially everywhere in indoor and outdoor air, waiting for the right moisture level to germinate.
Which species dominates depends on local conditions. A microbiological survey of old books in a fifteenth-century Romanian monastery found Penicillium species on about 41% of isolates, Cladosporium on about 33%, and Fusarium on roughly 30%.5PubMed. Assessing the microbiological contamination along with environmental factors of old books in the 1490-founded Bistrița Monastery, Romania Meanwhile, the broader library literature points to Aspergillus as the single most commonly isolated genus across dozens of studies.1PubMed Central. Mold in Paradise: A Review of Fungi Found in Libraries The takeaway is that no single species “owns” paper. Multiple mold species compete and co-colonize, and the mix depends on geography, humidity, temperature, and what the paper is made of.
Conservation scientists sometimes refer to these filamentous fungi as hyphomycetous molds, and they are considered the most important agents of biodeterioration in museums, storage facilities, libraries, and restoration studios.6PubMed. The revenge of time: fungal deterioration of cultural heritage with particular reference to books, paper and parchment They are so common in those settings that the question for archivists is never whether mold will arrive, but whether conditions will allow it to wake up and grow.
Humidity Is the Switch
Mold spores can sit dormant on paper for years without causing visible damage. What triggers active growth is moisture. Laboratory testing that exposed traditional and recycled book paper to controlled humidity levels showed just how steep the threshold is. At 100% relative humidity, mold colonized the vast majority of samples: Cladosporium grew on 74%, Aspergillus niger on 75%, Trichoderma harzianum on 72%, and Chaetomium globosum on 60%. Drop the humidity just five percentage points to 95%, and those numbers collapsed. Cladosporium appeared on only 9% of samples, A. niger on 1%, T. harzianum on 3%, and C. globosum did not grow on any sample at all.7International Biodeterioration & Biodegradation. Accelerated testing of mold growth on traditional and recycled book paper
Those numbers illustrate a point that catches many people off guard: the difference between “slightly damp” and “mold explosion” can be remarkably narrow. Most preservation guidelines recommend keeping storage areas below 60% relative humidity, with temperatures in the low 60s to low 70s Fahrenheit. Once relative humidity consistently stays above about 65–70%, the risk of mold activation rises sharply, and above 80% you are almost guaranteed problems on organic materials like paper. Temperature matters too, since warmth speeds fungal metabolism, but humidity is the dominant driver.
This is why water events are so catastrophic for paper collections. A roof leak, a burst pipe, or even prolonged condensation from poor ventilation can push local humidity to near-saturation levels. At that point, spores that have been sitting harmlessly on shelf surfaces or between pages for decades germinate within hours to days.
What Mold Does to Paper
Active mold does not just sit on the surface. It digests the paper from the inside out. Fungi that colonize paper produce cellulase enzymes that break down cellulose fibers, weakening the sheet’s structural integrity. They also generate organic acids as metabolic byproducts. In lab experiments with archaeological Egyptian papyrus, Aspergillus niger infections dropped the paper’s pH to 5.2, and all tested fungal species caused measurable darkening, with color-change values ranging from about 3.8 to 7.3 on a standard scale.8Archaeometry. Mold‐deteriorated archaeological Egyptian papyri: Biodeteriogens, monitoring the deterioration, and treatment approach In practical terms, that means acid damage, discoloration, and physical weakening happening simultaneously.
One of the most visible signs of fungal damage on older paper is foxing, those scattered reddish-brown spots that appear on the pages of old books and prints. Foxing has been studied for over a century, and researchers generally divide it into two types: biotic foxing caused by fungal activity, and abiotic foxing caused by metallic contaminants in the paper that catalyze chemical reactions.9PubMed. Foxing of Watercolor Paper and Environmental Control as Preventive Actions In many cases, both processes overlap: metal particles in the paper promote fungal growth, and the fungi in turn accelerate discoloration. The brown spots are permanent stains and cannot simply be brushed off.
Microscopic analysis of foxing spots in a nineteenth-century book revealed that fungi sometimes colonize paper at multiple stages of its life. Researchers found one fungal species growing on top of printed ink, while a second species was nestled beneath the ink, between the paper fibers, suggesting contamination both before and after printing.10PubMed. SEM analysis of irregular fungal fox spots in an 1854 book: population dynamics and species identification That finding is a reminder that mold is not always a late arrival. Spores can be embedded in paper from the moment it is manufactured, lying dormant until conditions shift.
Health Risks from Handling Moldy Paper
Moldy paper is not just a conservation problem. The same fungi that eat cellulose can affect your health. Aspergillus, Penicillium, and Alternaria species are all known to cause allergic reactions, respiratory irritation, and in some cases more serious infections in people with weakened immune systems.4PubMed. Fungi in archives, libraries, and museums: a review on paper conservation and human health Archivists, librarians, and restoration workers who regularly handle contaminated materials face chronic, low-level exposure that can add up over time.
Some molds produce mycotoxins, toxic secondary metabolites that pose more serious risks than ordinary allergens. A well-documented case involved office employees exposed to Stachybotrys chartarum growing on water-damaged, primarily cellulose-based materials. The fungus was producing satratoxin H, a potent macrocyclic trichothecene. Compared to unexposed controls, the exposed workers showed significantly higher rates of lower respiratory symptoms, skin problems, eye irritation, chronic fatigue, and abnormalities in white blood cell function.11PubMed. Health and immunology study following exposure to toxigenic fungi (Stachybotrys chartarum) in a water-damaged office environment While Stachybotrys tends to prefer materials with higher cellulose content like drywall and ceiling tiles, it can colonize paper products too, and the health consequences are not trivial.
If you are cleaning up a small amount of moldy paper at home, wearing an N95 mask and nitrile gloves is a reasonable precaution. Disturbing mold releases spores into the air, and breathing in a concentrated cloud of spores from a moldy box of documents is quite different from the trace amounts floating around in normal indoor air. For large-scale contamination, professional remediation is worth the expense, especially in poorly ventilated spaces.
Preventing Mold on Paper
Climate control is the single most effective defense. Keeping relative humidity below 60% and temperatures moderate will stop nearly all common paper molds from germinating, even when spores are already present. For home storage, that means avoiding attics, basements, and garages, the three spots where humidity swings are wildest. A climate-controlled interior closet or a room with consistent air conditioning is a far safer bet for anything you care about preserving.
Air circulation matters almost as much as absolute humidity. Stagnant air pockets, like those between tightly packed books on a shelf or inside a sealed cardboard box in a humid room, create micro-environments where moisture builds up against paper surfaces. Leaving a little space between items, using open shelving rather than sealed containers in humid climates, and running a fan or dehumidifier in storage rooms all help keep local humidity from spiking.
For valuable documents, archival-quality storage materials make a difference. Acid-free boxes and folders slow the chemical degradation that makes paper more hospitable to fungi. Silica gel packets or other desiccants inside sealed containers can buffer short-term humidity fluctuations. Interleaving important documents with acid-free tissue prevents moisture from transferring between stacked pages. None of these replace climate control, but they add layers of protection.
One emerging area of research involves essential oils as antifungal treatments for paper. Plant-derived volatile compounds from oils like thyme, oregano, and tea tree have shown antifungal activity in laboratory settings, and some conservators are exploring them as gentler alternatives to chemical fumigants.12PubMed Central. Essential Oils for the Conservation of Paper Items The research is still in early stages and the practical limitations are real: essential oils can stain paper, their effects are temporary, and dose control is tricky. But the concept is promising for low-intervention conservation contexts.
Removing Mold That Has Already Taken Hold
Once mold is actively growing on paper, the options range from simple to highly specialized, depending on the severity and the value of what you are trying to save.
For lightly contaminated everyday paper, the first step is to get it dry. Mold stops growing when moisture drops, so moving items to a dry environment with good airflow and, if possible, running a dehumidifier will halt active growth. Once the paper is dry, surface mold on hardcover book covers or coated paper can sometimes be wiped away with a soft cloth. Loose spores on uncoated paper can be brushed off gently outdoors, using a soft brush, to avoid releasing spores indoors. This does not undo existing staining, but it prevents further spread.
Washing contaminated materials is more aggressive but can be effective. A study that inoculated paper with high concentrations of several mold species, including Stachybotrys chartarum, Aspergillus niger, and Penicillium chrysogenum, found that washing completely inactivated or removed spores from paper for most species. The one exception was Chaetomium globosum, which resisted washing on paper even when it was removed from other materials. The same washing technique also inactivated all tested mycotoxins on paper.13Taylor & Francis Online (J Occup Environ Hyg). An investigation into techniques for cleaning mold-contaminated home contents That last point matters: even after spores are dead, residual mycotoxins can linger on surfaces, and washing can address both problems at once on paper substrates.
For irreplaceable or historically valuable documents, conservators sometimes turn to gamma irradiation as a sterilization method. Ionizing radiation kills fungi effectively, but it comes with a trade-off: radiation can break down cellulose, reducing its degree of polymerization and making the paper more brittle and discolored.14Journal of Cultural Heritage. The recovery of biodeteriorated books and archive documents through gamma radiation: some considerations on the results achieved That said, more recent research using carefully controlled dose rates on model paper samples detected no oxidation or change in cellulose crystallinity, suggesting that with the right parameters, the damage can be minimized.15Radiation Physics and Chemistry. Combined effects of gamma-irradiation, dose rate and mycobiota activity on cultural heritage – Study on model paper The debate among conservators about whether irradiation does more harm than good is ongoing, and the answer probably depends on the specific paper, the dose, and how much fungal damage has already occurred.
Everyday Paper vs. Archival Collections
Most of the published research on mold and paper focuses on libraries, museums, and historical archives, because that is where funding for conservation science tends to go. But the same biology applies to the shoebox of old photographs in your closet, the stack of tax returns in your basement, or the kids’ artwork pinned to a bulletin board in a humid room.
The practical difference is mainly about what you are willing to invest. For a box of old magazines, drying them out and tossing anything that is heavily stained is perfectly reasonable. For a one-of-a-kind family document, a handwritten letter, or an original print, the calculus changes. At minimum, move irreplaceable items to a dry, temperature-stable environment. If you already see mold growth, isolate the affected items in a sealed bag to prevent spore spread to clean materials, then assess whether the damage is cosmetic or structural. If pages are soft, crumbly, or stuck together, professional conservation help is worth exploring before you accidentally tear something apart.
Digitization is also worth mentioning as a parallel strategy. Scanning important documents does not stop mold on the physical originals, but it ensures the information survives even if the paper does not. For family photos and personal records, a high-resolution scan stored in multiple locations may be the most realistic form of long-term preservation, especially if you do not have access to ideal storage conditions.
Foxing Spots and the “Old Book Smell”
The characteristic musty smell of old books is partly a product of fungal metabolism. Molds growing on paper release volatile organic compounds as they digest cellulose and other substrates, and those compounds contribute to the smell people associate with used bookstores and aging libraries. Not all of the smell comes from fungi: chemical degradation of lignin, acids, and sizing materials also produces volatile compounds. But in humid or poorly ventilated collections, mold is a major contributor.
Foxing spots, as described earlier, are one of the most common reasons people notice mold-related paper damage. They tend to appear on paper that was stored in fluctuating humidity over long periods, giving fungi repeated windows of growth activity followed by dormancy. The spots often cluster near the edges of pages, where moisture exposure is highest, or in areas where metallic impurities in the paper create favorable microenvironments. Once formed, foxing stains are stable and permanent under normal conditions. Chemical bleaching can lighten them on some paper types, but it introduces its own risks of fiber damage and is generally only attempted by trained conservators on items of significant value.
If you are buying secondhand books and notice scattered brown spots on the pages, that is almost certainly foxing. The books are safe to handle and read. The fungi that produced the spots are typically long dormant, and the spore load on a dry, foxed book is not meaningfully different from the background level of spores in ordinary indoor air. The stains are a cosmetic issue, not a health hazard, unless the book has been stored in actively wet conditions and you can see fuzzy, three-dimensional mold growth on the surface.