Moldy books (C) Daniel FriedmanMildew Contamination on Books
Are there mildew stains on books & papers?

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Mildew contamination on or in books, magazines, photographs or other paper products should be called mold.

This article explains the difference between mildew and mold and points out why that distinction matters to paper conservators and book restorers.

This article series explains how to deal with mold on books and papers, and what options we have for cleaning or storing moldy books. The moldy books in a college library (photo above) were in the opinion of some people "an old inactive mold problem" but see our warning below about "dormant mold". Sorry, it's not mildew.

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Distinction between "mold" and "mildew" Questioned & Answered

Moldy books (C) Daniel FriedmanAs a book collector and once-upon-a-time mycologist I found your web site of interest.

However, I do not believe there would be uniform acceptance among professional mycologists of your distinction between "mold" and "mildew". The organisms involved are all fungi in the classical sense. Those attacking dead organic matter like cellulose are not all that different biologically from their relatives that may have a preference for the living cells of plants and animals.

The fact that some parasitic species may be facultative heterotrophs (feeding on non-living materials) supports this view.

I believe your cause is best served by promoting the idea that the fungi, a diverse and highly successful breed, will exploit any environment where nutrients and moisture are available whether or not it is living or dead. Books and their bindings in a high humidity environment are sitting ducks.

One final point: since fungi and their spores and hyphae are ubiquitous in nature it should be recognized that there is little chance of getting rid of all of them by any practical means (HEPA filters included).

Since fungi (including species that attack damp books) play such an important part in our ecosystem we would be unwise to eliminate them entirely. We can, however, slow them down. - Chris 9/7/2012

Reply: Mildew doesn't grow on books, nor on leather shoes, nor on wallpaper, nor on other indoor building surfaces, though lots of other mold genera/species might

Mildew on a jasmine plant (C) Daniel FriedmanChris,

Thank you for the interesting comments about book mold, mildew, HEPA vacuuming, and the important role of the fungi in our environment.

I'm grateful to read your opinion and want to emphasize that we welcome polite, informed discussion or debate about this or any other topic found at

Indeed even among expert book restorers the term mildew is often used loosely and technically incorrectly to refer to the role of certain fungi or mold genera/species in the cause of foxing on books and papers.

A few points need clarification: by no means do I suggest that mildew is not a fungus, as mildew is indeed a proper subset of the huge kingdom of fungi. But the fungi appearing on books are different genera/species from the two fungi properly named mildew. There are some important distinctions to be made.

Mildew doesn't grow on shoes, nor on paper, though many other mold genera/species can grow on these materials

I learned about the distinctive properties of mildew as a living plant pathogen among other members of the Fifth Kingdom from Dr. John Haines, my friend and mentor, when John was still serving as the NY State mycologist.

Discussing some mold samples I'd brought along for us to examine, I mentioned that I'd just collected some white mildew from leather shoes found in a moldy home.

Like many people I just bandied the word mildew about willy-nilly. John asked why I thought it might be mildew, allowed me to embarrass myself, and then kindly explained that mildew grows on plants, not shoes.

Perhaps it's technical nitpicking, but mildew a subset of "mold" that only grows on living plants. Mildews are a small group of fungi found among the Basidiomycota, Ustilaginales if I recall correctly, and any "mildew" if properly identified, will be either Oidium-Erysiphe or Powdery Mildew or Peronosporaceae or Downy Mildew. Mildew, then, has nothing to do with and won't be found growing on books nor on other building surfaces unless the item in the building is a live or recently-live plant.

The fungi identified as growing on or in paper materials included the following

In research on foxing stains (see FOXING STAINS on BOOKS & PAPERS. ) experts have identified at least ten species of fungi and one yeast have been identified as growing on/in or "hosted by" paper, [16][17][17a][29]

In addition, when we add consideration of the different (from paper) materials used in book bindings, covers, etc., it is likely that there are additional mold genera/species that may be found growing on books. In our own field and lab experience, while there was some variation in mold genera/species present, the dominant fungus contamination found on the exterior of books in a moldy library basement was Aspergillus sp.

Indeed, a literature search confirms that among scholarly and research papers we researched for this article, not one authority detected the presence of either of the two types of mildew (Oidium-Erysiphe - powdery mildew, or Peronosporaceae - downy mildew) among the various fungi found growing on books, papers, photographs, stamps, or other paper based works. [1]REFERENCES op. seq.

Fungal species commonly identified as found growing on or in books, papers, photographs, stamps, etc. include:

The fungi identified as mildew include these two groups

Below I include photos of Oidium or powdery mildew that I collected from a jasmine plant (below left) that was growing indoors (we moved it outside before this photo was taken) along with a photo of the same mildew from that plant under my lab microscope (below right).

Typical library bookshelf dust (C) Daniel Friedman

Below: microscopic view of mildew spores of Oidium:

Typical library bookshelf dust (C) Daniel FriedmanWhat's wrong with misidentifying fungi types or genera/species

You make an important point, that fungi are very versatile and that many genera/species adapt well to growing on stuff found indoors, including books. On the other hand, if my "mold expert" insists on calling white or light colored molds on books or on other indoor building surfaces "mildew" it makes me nervous on several scores:

Why does the Mold / Mildew Distinction Matter? Implications for Building Testing

Very moldy home (C) Daniel FriedmanFrom one point of view, since the remedy for an indoor (or book) mold problem does not depend much on the mold genera or species, understanding mycology might not seem to make much difference in improving a moldy indoor building area.

But that's not quite so. Having the luxury of my own forensic lab along with performing field investigations for many years, I have been able to make a wide study of which mold genera/species seem to best like different indoor building surfaces and materials.

I've enjoyed collecting mold samples from every single type of surface and material in very moldy homes, having cataloged (MOLD GROWTH on SURFACES, TABLE OF)

and I've photographed quite a few, nearly 100 of which can be seen online at MOLD GROWTH ON SURFACES, PHOTOS.

This work has suggested that when building investigators or "mold experts" survey a building for mold contamination, a lot depends on where and how the sample is collected.

For example, in pulling a tape sample of mold growth on a hollow-core luan door in a very moldy home we find completely different genera/species preferring the edge of the door (probably southern yellow pine wood) from that growing on the door surfaces (luan mahogany).

If the investigator does not realize this, his/her report that claims to characterize what molds are present or potentially harmful in the building might be quite inaccurate.

Watch out: In a library of moldy books, if by careless language we informed building management that we thought the dominant mold present was just a bit of mildew (say Oidium) we would erroneously conclude that other than a possible allergic response or perhaps a problem for nearby houseplants, there was no health risk to building occupants.

But in point of fact what I find on moldy library books is typically a potpourri of fungi dominated on book jackets by several species of Aspergillus - far more likely to present an IAQ and health hazard to occupants.

HEPA Vacuuming is Not a Mold Cure: the two key steps are remove the mold and fix the cause of mold growth

You are of course also completely correct that HEPA filtering of air is never a cure for mold problems on books or other indoor surfaces. Such cleaning is, however, an important step in mold remediation in general and in the cleaning of visibly moldy books and papers - a topic which we discuss in greater detail at PREPARE TO CLEAN BOOKS / PAPERS.

The effective cure for indoor mold problems involves two basic efforts:

  1. Remove the problem mold - this means physically cleaning it off (some remaining stains may be harmless)
  2. Find and fix the cause of high indoor moisture or leaks that caused the mold growth and fix those conditions

The reference to HEPA filtration that you may come across in reading about mold remediation in buildings typically involves either a HEPA filter on a machine used to create negative air pressure in the infected area in order to protect other building areas from cross contamination during the period of mold cleanup and repair work, or a HEPA filter used during dust cleaning in other building areas as a means of reducing indoor dust levels without just stirring up worse dust than before.

Why We Need Mold in the Environment

I also agree with and appreciate your observation that the fungi are a crucial part of our ecosystem - without them and their ability to break down cellulose (not just books, but leaves and dead wood) I doubt that their partner in cellulose decay (some bacteria) would alone be up to the job. Imagine the earth being buried in all of the dead trees, leaves, grass and similar stuff that had ever died since the Jurassic period because we didn't have the fungi to break that matter down!

Or on a smaller scale, imagine if Dr. Florey hadn't found P. notatum on an orange in the marketplace and hadn't hidden some in his coat during the development of the antibiotic Penicillin? And on a still smaller scale, we enjoy both blue cheese and Ustilago maydis (Huitlacoche or "corn smut") in various food preparations here in Mexico.

Nevertheless, as a book collector you probably will agree that we don't particularly need nor want mold on books nor on other important papers or documents, a point of view that quite a few experts have made amply clear. [1][2][3][4][5][6][7][8]9][10][11][12][14] REFERENCES

Thank you again for the discussion, we'd be glad to hear further from you in this matter, particularly I'd like to know more about foxing on books and papers.


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