Preventing the growth of mildew and
other types of molds in book collections
Magnified mold spores, courtesy the EPA.
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.
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.
About molds and mildews:
The National Library of Australia describes mold and mildew and
"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."
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.
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."
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
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
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 40º and 100º F (equivalent to 4º and 38ºC
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
||22.7 ºC 29%
||19.0 ºC 34%
||22.9 ºC 28%
||18.8 ºC 34%
||21.4 ºC 33%
||18.9 ºC 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.
Various sources recommend:
|Books should never be shelved directly against an outside wall. Moisture
can develop on such walls and locally increase the RH.|
|Have few or no houseplants near the books.|
|For libraries below grade level, have the outside walls waterproofed.|
|Maintain adequate air circulation in the library.|
|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."
This report also suggests ways of drying and treating wet books and
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.
The following information sources were used to prepare and update the above
essay. The hyperlinks are not necessarily still active today.
- "PRESERconVersATION," The National Library of Australia, at:
- "Disaster Plan of the Thomas G. Carpenter Library University of North
Florida," (1991), at:
- Sandra Nyberg, "Invasion of the Giant Mold Spore," at:
- Virginia Pearl, "How to prevent and remove mildew," University of
- Beth Patkus, "Emergency salvage of moldy books and paper,"
Northeast Document Conservation Center, at:
- Radio Shack® Model 63-1032.
It runs on two AA batteries.
- "Caring for Important Papers - Steps To Take Before And After A Flood,"
National Ag Safety Database, at:
- Pam Atteberry, "Removing Cigarette Smell," at:
http://www.cgrove417.org/ 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
- "Appendix C: Moisture, Mold and Mildew," Environmental Protection
www.epa.gov/ This is a PDF file. A free reader
Copyright © 2004 to& 2007 by Ontario Consultants on Religious
Originally written: 2004-APR-8
Latest update: 2007-NOV-21
Author: B.A. Robinson