By Richard D. Smith, Ph.D

President, Wei T’O Associates, Inc.



Recent regulatory restrictions on the use of ethylene oxide have spurred the search for substitutes for this chemical and ways to use it safely.1  The single-minded passion with which library conservators and conservation scientists debate the hazards and future of chemical fumigation is amazing.2,3  The debate should not revolve around “Are conservators or materials endangered?”  but rather around “Is fumigation necessary?” and “What alternatives exist?”

These two questions cannot be discussed sensibly until the purpose of fumigation is decided.  Fumigation is a last resort measure which uses toxic gases to bring an insect or fungi infestation under control.  In practice, fumigation has become the standard treatment for books suspected to contain insects or fungi or books in which an insect or fungus has been observed.

It seems obvious, since fungi and insects are living creatures, that any method which could kill human beings might be useful for extermination.  Some possibilities are intolerable changes in relative humidity, temperature, or high pressure; use of a non-toxic fumigant; or combinations of these alternative methods.

Some additional background should be considered before discussing alternative treatments.  We need to:  (1) examine libraries as places to live, (2) review what librarians are trying to accomplish, and (3) recall society’s standards on extermination years ago when ethylene oxide was chosen as the library fumigant.

All libraries operate as highly efficient, automatic extermination machines.  Libraries are perpetually being invaded by insects, fungi, and rodents; and while they are continually killing off these invaders, pests are always in residence.  This situation, though undesirable, is preferable to the hazards that continual exposure to pesticides pose for librarians.

Libraries offer almost everything living creatures need for a good life.  Books contain plenty of carbohydrates as well as adequate quantities of protein, vitamins, and minerals.  Library buildings provide shelter and a healthful environment in terms of temperature and air quality.  The only thing libraries lack for a good life is water.  The overwhelming importance of water is obvious when one stops to think of the consequences from an accidental wetting:  insects, fungi and rodents flourish!

From the perspective, it is clear that librarians have not been trying to exterminate library pests; but rather, have endeavored to keep them under control.  There is an unspoken understanding that the number of insects, fungi, and rodents normally present in libraries is so small, and the damage they inflict is so insignificant, that it can be ignored . . . .if, and only if, good storage conditions are maintained.

This modern “common sense” view of libraries both as living institutions and as relentless extermination systems is directly comparable to the ways that people maintain good health.  We have countless numbers of bacteria, fungi, and insects living on and in our bodies.  Libraries differ only in having more forms of life present.

We have an operation when a body part breaks down; and libraries have a leaky pipe repaired or a roof or wall fixed.  We take medicine when we become sick to kill the “bug” which has infected us; libraries have infested books fumigated.  The first line of defense for our bodies, however, is a balanced life; friends and a job we like, adequate exercise, enough sleep, and a nourishing diet.  Libraries also defend their collections by establishing a proper environment, i.e., clean air and appropriate temperature and relative humidity.

Society’s approach to extermination was different during the 1920s and 1930s when ethylene oxide fumigation was introduced.  Public health required not only isolation of persons with infectious diseases, but also sterilization of books they had used.  Indeed, many public libraries actually maintained a special collection of books for use by children with infectious diseases like mumps, chicken pox, and scarlet fever which were duly sterilized after each use.  Libraries are no longer required to sterilize books, and indeed, never needed to do so to protect patrons against infectious diseases.  Though records are not available, my opinion is that (1) ethylene oxide was not selected as a fumigant but rather as a sterilant and (2) the convenience and effectiveness of ethylene oxide has kept it in use over fifty years.

After extensive investigation, the Occupational Safety and Health Administration (OSHA)1  published a new Standard on Occupational Exposure to Ethylene Oxide in the Federal Register on June 22, 1984.  It sets the Permissible Exposure Limit for ethylene oxide at one part per million in air as an 8-hour time-weighted average.  It also establishes an action level of one half part per million; if this level is exceeded, most of the standard’s provisions become effective.  If exposures are above one part per million, a schedule for reducing exposure and a written emergency plan must be developed.

The last figures suggest your institution should establish a maximum permissible working and monitoring level.  Though one-fourth part per million would be my suggestion, I would consult an ethylene oxide expert.

High concentrations of ethylene oxide can cause central nervous system damage, skin, eye and respiratory irritation, and nerve damage (neuropathy).  Lower concentrations can cause reproductive problems affecting both men and women, including chromosomal damage and spontaneous abortion, and leukemia and other cancers.

The level of employee exposure must be regularly monitored.  Monitoring records must be kept for at least thirty years, and medical surveillance for the duration of employment plus thirty years.

These are just summarial comments.  The full text of the Standard should be consulted.


Before discussing alternative treatments, I want to make it clear, I think:  (1) Libraries are no longer required to sterilize books, and indeed, never needed to do so to protect patrons against infectious diseases.  (2) Library collections are always infested with small numbers of fungi, insects, and rodents.  (3) The damage from their attack is insignificant when the collections is properly stored.


The standard recommendation of 50 percent relative humidity is selected by libraries because paper has its optimal physical properties at 50 percent R.H.4 The variation ranges plus or minus 5 percent in a well-run library.

Severe fungus and insect infestations do not occur normally in libraries until the relative humidity reaches 65 percent P.H.  The lower relative humidity’s are too low for these destroyers to multiply and cause trouble under ordinary storage conditions.  Therefore, moving infested portions of any collection to locations with lower relative humidity (when immediate extermination is not necessary) will bring the infestation under control.  In other words, air at 50 percent R.H. and 72°F (22°C), aided perhaps by a fan to improve circulation, will reduce the numbers of fungi, insects, and rodents to an ordinary (i.e., virtually non-existent) level in a short time.


As carbohydrates, books and documents can be considered food.  Pasteur showed that a few minutes at 135°F (57.2°C) is adequate to kill many of the fungi, bacteria, and viruses found in food.  Pasteurization will kill insects too.  Unfortunately, many deteriorated materials, e.g., leather book bindings, the use of lower temperatures and longer treatment times might be effective and deserves investigation.

A better and proven way to kill insects is to expose infested books to deep-freeze temperatures, a technique pioneered at the Beinecke Rare Book and Manuscript Library of Yale University.5,6  The effective temperature range is -20°F to -40°F (-29°C to -40°C).  The question under discussion is not whether the treatment will work, but rather; what are the optimum temperature and time for deep-freezing.  Yale University recommends three days at -20°F (-29°C).  Wei T’o Associates suggests one day at -40°F (-40°C) for the Book Dryer-Insect Exterminator.  The advantages of deep-freezing are that it is an effective treatment, chemicals are not needed, and staff members can operate the freezers safely with minimal training.


Many common gases like nitrogen are lethal as asphyxiates because they dilute the air and reduce the oxygen present to a level where normal life is not possible.  Unfortunately, this approach is not a practical extermination method because, given warning, fungi and insects avoid asphyxiation by going into a state of suspended animation called stasis where very little oxygen is required to sustain life.

Fortunately, carbon dioxide at a 60 percent concentration does not induce stasis, is effective, and has been recommended by the U.S. Dept. of Agriculture as a non-toxic fumigant for foods (such as cereals in grain elevators).  My literature review is not complete, but what I have learned convinces me that carbon dioxide will meet the fumigation needs of libraries as set out in this article.  Moreover, research is underway to develop a low cost unit to convert existing ethylene oxide fumigators into carbon dioxide fumigators.

Carbon dioxide is not perfect; it has advantages and disadvantages.  On the one hand, carbon dioxide is safe to eat (for example, it is a main component in carbonated beverages), non-flammable, non-explosive, inexpensive, and readily available.  The use of carbon dioxide does not require a clean-up procedure after treatment, does not deposit a hazardous residue, nor require periodic health examinations or medical follow-up for personnel who have been exposed.  On the other hand, carbon dioxide is a high pressure gas which must be handled carefully.  It is a slower acting fumigant, and the literature suggests one to three days may be required for a fumigation cycle compared to only several hours for an ethylene oxide fumigation cycle.


The increase and the decrease of pressure can be lethal.. Through research, we know that cell structures rupture (literally explode) when pressure drops to vacuum levels; and life forms whether mammal, insect, or microscopic fungi, ooze to death.  Deep space is not the only place that this can occur.  An ordinary vacuum drying oven capable of reaching 100 milletorr (one lb./sq. inch equals 51,700 milletorr) probably would be effective.  An advantage of this method would be used to dry damp books and return them to their proper moisture content in a short time.  The moldy odor of damp, decaying books would probably also be removed.

Unfortunately, the “water-ring” type of vacuum pumps used in ethylene oxide fumigators must be replaced because they only reach about 88,700 milletorr (sea level air pressure is 760,0009 milletorr).  The vacuum pump and most, if not all, of the piping and controls would probably have to be replaced to make existing ethylene oxide fumigators into efficient vacuum fumigators.

Swimmers utilizing aqua-lungs either dive for short periods, or decompress before returning to the surface.  If they do not, the nitrogen gas dissolved in their blood can form gas bubbles that induce a heart stoppage.  I am pessimistic, but I think a high pressure treatment of 100 to 300 pounds per square inch (equivalent to diving 100 to 300 feet) for several hours, followed by a fast pressure drop, deserves investigation as a possible method of making existing ethylene oxide fumigators useable.


Libraries are both (1) continually being invaded by fungi, insects and rodents, and (2) extremely effective at exterminating these life forms.  The goal of librarians for fumigation is not exterminating all life in libraries but rather keeping the numbers of fungi, insects, and rodents so low that no perceptible damage occurs.  A variety of alternative extermination methods may replace ethylene oxide.  Theses alternatives include (1) substituting carbon dioxide, (2) reducing relative humidity with fresh air, (3) deep-freezing at -20°F to -40°F, (4) vacuum drying, and (5) high pressure treatment.


1 Occupational Safety and Health Administration (OSHA).  “Occupational Exposure to Ethylene Oxide,” Federal Register (June 22, 1984.  pp. 25, 734-25, 809.  A summary prepared by the Center for Occupational Hazards, 5 Beekman St., N.Y., N.Y.  10038 is reprinted as “OSHA Issues Ethylene Oxide Standard,” SAA Newsletter (November 1984) p.4.

2 How to comply with the OSHA Ethylene Oxide Standard, A one day conference sponsored by the Center for Occupational Hazards and held at the Metropolitan Museum of New York City on November 20, 1984.

3 Ethylene Oxide Fumigation Meeting, Sponsored by the New England Archive, Library & Museum ETO Study Group and held at the New Bedford Whaling Museum, 18 Johnny Cake Hill, New Bedford, MA  on October 17, 1984.

4 The ability of paper to resist aging is greatly improved by lowering the relative humidity where the books are stored.

5 Nesheim, K. “Yale University’s Method of Exterminating Insects by Deep-Freezing,” Restaurator, International Journal for the Preservation of Library and Archival Material.  In Press.

6 Smith, R.D. “Use of Redesigned and Mechanically Modified Commercial Freezers to Dry Water-Wetted Books and Exterminate Insects,”  Restaurator, International Journal for the Preservation of Library and Archival Material.  In Press.



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