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Preventing the growth of mildew and
other types of molds in book collections

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Sponsored link.

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Magnified mold spores, courtesy the EPA.

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The author of this essay has no expertise on book preservation. The essay is simply reporting information gathered from the Internet related to the preservation of books. If you have a significant investment in your library, we suggest that you hire a qualified expert to advise you on preservation matters. We cannot be responsible for any financial or other loss as a result of anyone acting on the data in this essay.

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Our problem:

The owner of this web site, the Ontario Consultants on Religious Tolerance, currently has a collection of almost 3,800 books which deal, directly or indirectly, with religious topics. Together, these books represent an investment of tens of thousands of dollars -- a large amount for an organization like ours that operates on a very small budget. We were concerned about the preservation of our books -- particularly the prevention of the growth of mildew or other molds. Mildew often forms in house basements, such as the one where our office and library are located.

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About molds and mildews:

The National Library of Australia describes mold and mildew and their differences:

"Mould is a type of fungus. It grows on surfaces in masses of branching threads which resemble dense cobwebs. The fertile threads, those which produce spores (minute reproductive cells), often stand up from the surface into the air to release their spores. Spores are carried by air currents or by adhering to insects or animals. Active mould can be any colour, depending on the species and what it is growing on."

"Mildew is another type of fungus, similar in structure to mould, but distinct as one species of fungus is distinct from another, such as yeasts, rusts, and mushrooms. The terms "mildew" and 'mould' are most often used in the common names of various fungi, but they are not interchangeable."

"Unlike plants, which produce their own food, fungi absorb nutrients from dead or living organic matter. There are over 100,000 known living species of fungus and mycologists estimate that there may be as many as 200,000 more unidentified species. The spores of fungi that become mould or mildew are always present in the air and on objects.
" 1

The solution is apparently not to prevent mold and mildew from landing on our books. They are all pervasive in our environment. The solution is to make certain that they will not grow into colonies. "...mould and mildew will begin to grow in 48 hours" if the temperature and RH conditions permit. 2

Moud and mildew are not merely a nuisance. They eat books and paper. They are also harmful to humans. "Those with allergies, asthma or other respiratory problems should stay away from infested areas, as many fungi will seriously irritate and inflame lungs. Some fungi can cause skin and eye irritation and infections. Prolonged exposure to germinating molds in closed areas (which exist in many library collections) can damage the lungs, mucous membrane, cornea, respiratory tract, stomach, intestines, and skin. Some varieties of mold are highly toxic." 3

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Recommended ranges of temperature and RH:

We searched the Internet for web sites that recommend optimum or limits on room temperature and relative humidity (RH) to assure that mildew will not grow. We found the following data:


The National library of Australia recommends a temperature range of 19 to 23 degrees Celsius (66 to 73 degrees Fahrenheit), and a relative humidity 50% with a maximum change of plus or minus 5% per month. 1


The University of Florida notes that a relative humidity less than 62% stops all chances of mold growth. A RH below 70% inhibits most moud growth. The optimum growth temperature range for molds is 20 to 30 degrees Celsius (77 to 88 degrees Fahrenheit). 4


The Southeastern Library Network (Solinet) states that mold grows in environments which are have a relative humidity higher than 70%. However, some books will become brittle at RH values under 40%. They recommend that RH be kept in the range of 45% and 65%. 3


The Northeast Document Conversion Center recommends a relative humidity of under 56% and a temperature of less than 21 degrees Celsius (70 degrees Fahrenheit). 5


The U.S. Environmental Protection Agency states that mold growth can occur on surfaces whose temperature ranges between 40 and 100  F (equivalent to 4 and 38C

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Our plan:

Actually, we don't have one yet.  We purchased two battery-powered instruments that measure indoor/outdoor temperature and relative humidity. 6 We locate the outdoor sensor directly behind the device, so that we obtain two measurements of room temperature. We then keep a simple log:

Date in 2004 Office Temperature and RH Library Temperature and RH
MAR-14 22.7 C  29% 19.0 C  34%
MAR-26 22.9 C  28% 18.8 C  34%
APR-08 21.4 C  33% 18.9 C  36%

We plan to initially monitor all four data points a few times a month. If the RH values get too high, we will investigate what corrective actions we can take. We will regularly inspect our books for mold or mildew.

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Various sources recommend:

bullet Books should never be shelved directly against an outside wall. Moisture can develop on such walls and locally increase the RH.
bullet Have few or no houseplants near the books.
bullet For libraries below grade level, have the outside walls waterproofed.
bullet Maintain adequate air circulation in the library.
bullet In case of a flood in which books become soaked, "If you don't have the time to clean and dry them immediately, consider putting them in the freezer to prevent mildewing. Place wax paper between layers of paper bundles or books so they can be separated easily when removed." 7 This report also suggests ways of drying and treating wet books and papers.

We found by accident that a magazine -- and presumably a book -- should not be placed on a concrete floor in a basement. We had a stack of magazines sitting on the floor and the bottom ones became mildewed. We suspect it was caused by underground moisture escaping into the basement.

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References used:

The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.

  1. "PRESERconVersATION," The National Library of Australia, at:
  2. "Disaster Plan of the Thomas G. Carpenter Library University of North Florida," (1991), at:
  3. Sandra Nyberg, "Invasion of the Giant Mold Spore," at:
  4. Virginia Pearl, "How to prevent and remove mildew," University of Florida, at:
  5. Beth Patkus, "Emergency salvage of moldy books and paper," Northeast Document Conservation Center, at:
  6. Radio Shack Model 63-1032. It runs on two AA batteries.
  7. "Caring for Important Papers - Steps To Take Before And After A Flood," National Ag Safety Database, at:
  8. Pam Atteberry, "Removing Cigarette Smell," at: This is a collection of home remedies by a librarian, most of which refer to  removing mildew smell from books. This is a Word document. A free reader is available.
  9. "Appendix C: Moisture, Mold and Mildew," Environmental Protection Agency, at: This is a PDF file. A free reader is available.

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Copyright 2004 to& 2007 by Ontario Consultants on Religious Tolerance
Originally written: 2004-APR-8
Latest update: 2007-NOV-21
Author: B.A. Robinson

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