JAIC 1986, Volume 25, Number 1, Article 5 (pp. 49 to 55)
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Journal of the American Institute for Conservation
JAIC 1986, Volume 25, Number 1, Article 5 (pp. 49 to 55)


John H. Haines, & Stuart A. Kohler

ABSTRACT—O-phenyl phenol (OPP) was evaluated in tests designed to simulate its proposed use as a fumigant by archives and libraries to control fungus growth. It was compared with thymol (isopropyl-meta-cresol) for its ability to render the spores of seven species of common fungi non-viable, and it was found that although each fumigant had the ability to stop or retard fungus growth, neither was totally effective in preventing fungus spores from germinating.


THE PROBLEMS CAUSED BY FUNGUS growth on books and paper are a major concern of archivists. Veritable disasters can be brought about when floods, leaking roofs, seeping walls or condensation bring moisture into archival collections. Fungus spores are ubiquitous in our atmosphere, both indoors and outdoors (Burge 1980); and the materials found in archives, such as paper, starch paste, hide glue, sizing leather and cloth are all suitable substrates for fungus growth. Enzymatic breakdown and discoloration from fungal growth inevitably result when moisture reaches books or paper.

No satisfactory cure for this condition has been effected, although many treatments have been tried. There are conflicting reports as to the potential damage to cellulosic materials in using gamma radiation at the intensity needed to be fungicidal (McCall 1983, Calvini 1979, Flores 1976). Impregnation of paper with solutions of fungicidal compounds is an effective deterrent to fungi, but would also be damaging to many archival materials.

Fumigation would seem to be a solution to the problem of killing the fungi without damaging the paper. Various fumigants have been used (Byers 1983, Strassberg 1978). Some effective fumigants have been demonstrated to be significant health hazards and to require special handling or permits. For this reason neither ethylene dibromide nor ethylene dichloride (in the form of Dowfume 75) are now considered safe for use in public buildings except with very special precautions and mechanical installation (Strassburg 1983).

It was decided to test o-phenylphenol because it has been effectively incorporated as a fungitoxic agent in paper for storing and protecting fruit and was considered less toxic than thymol (isopropyl-meta-cresol). Several authorities have recommended the substitution of OPP for thymol in every application (McComb in Nagin 1982). Thymol is described as “moderately hazardous” while OPP is described as “a slightly toxic irritant” (Strassburg 1978). The Merck Index (10th ed., 1983) lists OPP as having an LD50 orally in rats at 2.48 gr./kg and Thymol as having an LD50 orally in rats at 980 mg/kg. Thymol also has a strong odor which some people find objectionable.

The fungi chosen as test organisms are commonly found in indoor and outdoor air and represent a range of taxonomic groups, spore morphology, spore production and growth rates. They all grow well on paper and have noticeable pigments capable of discoloring paper.

The term “mold”, or its alternative spelling favored in Great Britain “mould”, does not refer to a classification within the fungi but is a term of convenience for the downy or furry growth form many fungi take when growing on an exposed surface. The term “fungus” will be used here as it is a definable and inclusive term for the organisms that make paper “moldy”.


THE FUNGI used in these tests were prepared by simulating the conditions encountered with water-damaged paper. A heavy concentration of spores from mature agar-grown cultures was transferred to filter paper wedges. Some were allowed to dry to simulate airborne or direct contact contamination. Others were kept moist for 48 hours to encourage germination. Theoretically, the most resistant part of the fungus life cycle is the mature spore because its cell wall is the least permeable and its metabolism the lowest. The freshly germinated spore should be the most vulnerable as its metabolism and cell wall permeability are high. Thousands, or perhaps millions, of spores were used in each sample. This large concentration reflects that to be found on moldy paper.

The paper wedges were fumigated and then placed on a fresh nutrient agar plate. Some of the spores were transferred onto the agar, the rest left on the paper which was placed on the agar. In the absence of toxic substance, both sets of spores would germinate and grow well. If the toxin is fungistatic, that is if the fungus remains viable but is prevented from germinating only in the presence of the toxin, then the spores on the agar would germinate and the spores on the toxin-impregnated paper would not germinate until the fumigant disperses. If the fumigant is fungicidal, no germination would take place on either the agar or the paper no matter how long they are incubated. To be an effective fumigant for archival purposes, it must be fungicidal instead of fungistatic.

2.1 Materials and Methods

The fungus strains tested here were obtained from stock cultures maintained at the Mycology Laboratory of New York State Health Department in Albany. Their species and stock numbers are listed in Table 1. They were grown for 7 to 10 days on 2.5% Fisher's Malt extract in 1.5% agar. Whatman 110 mm, #1 filter paper discs were cut into four wedge-shaped segments and steam sterilized. The wedges were pressed lightly onto the mature fungus colony to pick up enough spores to color lightly the surface. The inoculated papers were placed on “V”-shaped glass tubing in sterile plastic petri plates. Sterile water was added to one set of samples while the other was kept dry. Control spores were treated the same way but placed outside the fumigation cabinets during the test. They all germinated and grew. All the paper wedges were transferred to 2.5% malt again after fumigation by pressing them upside down on the agar surface on one side of the petri plate to dislodge some spores and then placing the paper with the spore side up on the other side of the plate. The plates were examined for 15 days after inoculation.

Table 1. Fungus Strains Used for Tests

The chemicals tested were ortho-phenyl phenol flake crystals (as Dowicide 1) obtained from Dow Chemical Company, and thymol crystals purchased from Talas Company. Two identical fumigation chambers were constructed of uncoated poplar wood, 16⅜″ high, 13″ wide and 10⅜″ deep. At the bottom of the chambers was a 15 watt light bulb immediately above which was a watch glass to hold the fumigant. Two shelves constructed of plexiglass strips so as not to restrict air circulation were placed at 4″ and 8″ above the bulb. One of these chambers was used only for thymol and the other only for OPP.

The lights were operated by means of an automatic timer, set to go on four hours per cycle of twenty-four. Ten grams of Crystalline OPP or thymol was placed in a watch glass and weighed after each experiment (Table 2).

Table 2. Fumigant Use During Tests

To make sure that the lack of penetration of the fumigant was not a problem, the paper wedges were placed on a grate over the fumigant. This led to some cross contamination of the samples but contaminant colonies were not counted in the results.

Samples of the first series of experiments were fumigated in a single, large storage cabinet. When it was discovered that this was not lethal to all the fungi and since there was an uncertainty of how much residual fumigant may be built up in the cabinet, two separate, smaller cabinets were used for the tests reported in the tables. Tests were made in the small cabinets when they were new and again after 30 days of continuous fumigation. In this way the effects of the two fumigants were assessed without chance of cross contamination from residual fumigant. The efficacy of the chamber increased slightly after it became saturated with fumigant. The amount of fumigant used through sublimation from the heated crystals was measurably less in the cabinets after they had been in continuous use than that used when they were new (Table 1).

To make certain the chemicals retained their toxicity, cultures were tested by dipping them in 10% solutions of thymol and OPP in ethyl alcohol. These tests were completely effective for killing the fungi in all cases. Control tests were made in sterile water and ethyl alcohol alone. In fungi both control samples retained viability, although the alcohol visibly retarded the fungus development.

2.2 Results

The fungi were tested in germinated and ungerminated condition with different lengths of fumigation with either OPP or thymol, and the results were interpreted from growth vs. no growth on nutrient agar. In no case was growth of all the test species prevented by either fumigant under all conditions. This indicates that neither of the fumigants is totally fungicidal nor totally fungistatic. Table 3 shows the results of 3 days of fumigation of germinated (wet) and ungerminated (dry) spores with OPP and thymol in small chambers which had been in continuous use for 30 days prior to the test. The growth of fungus colonies on the agar surface and on the paper was recorded separately. Those colonies which were slow to develop are indicated. Table 4 shows the results of 10 days of fumigation.

Table 3 Viability of Fungi After 3 Day Fumigation with Ortho-Phenyl Phenol or Thymol

Table 4 Viability of Fungi After 10 Day Fumigation with Ortho-Phenyl Phenol or Thymol


TO BE EFFECTIVE in stopping fungus growth in moldy paper the fumigant would have to produce no growth (0) across all seven species under all conditions. The “dry” colonies on “paper” are analogous to the fungus colonies which would survive on moldy paper and the “dry” colonies on “agar” are analogous to those which could be spread from moldy paper.

The inability of OPP fumigation to render all of the fungus colonies inviable after three or even ten days of exposure indicates that it is not an effective method of treating moldy books and papers. Similar results were obtained for thymol in this study. The results obtained here are in contrast to those obtained by Florian (1975) in a different set of tests using thymol as a fumigant. The Florian study indicated that the fumigant was fungicidal for the fungi tested.

There are, however, significant differences between the two studies; the fungus species, the preparation of the fungi before fumigation and the amount of fungus spores in a sample. The present authors used very large amounts of dried spores, instead of smaller amounts of spores and hyphae for their tests. They believe that this accounts for the difference between the two studies.

The percentage of spores killed in the present study was high. Perhaps it was more than 99%, but that is an almost insignificant loss to a fungus which can produce hundreds of thousands of spores in a small colony started from a single spore.

To rid books and paper of mold problems by a non-destructive chemical application with a minimum of human contact would appear to be an attractive course of action. The problems with this are that most fungicides are either hazardous gasses which pose a health risk to the user or solutions which may damage cellulosic material. Even if the perfect, non-hazardous, non-destructive fungicide is found, there is a fallacy in the “one-shot cure” of fungus problems. Any “cure” which does not leave a residual toxin and which does not change the conditions for fungus growth is a temporary cure.

A second fallacy is that dead fungus spores are not a health hazard. If a spore is an allergen when it is viable it is still an allergen when it is nonviable, but if it was treated with a toxin it now has a coating of toxin in addition to its allergenicity.

It appears that there is no procedure in use at the present time which will allow paper to be stored in high moisture conditions and remain immune to fungus attack. Every effort to maintain lower relative humidity of collection materials and storage areas will be rewarded by diminished fungus growth.


MARY- LOU FLORIAN, for her helpful discussions of her studies and Ira F. Salkin, Infectious Disease Center, New York State Health Department, for the use of his cultures.


Burge, H. P., et al., “Fungi in Libraries: An Aerometric Survey,” Mycopathologia, Vol. 64, No. 2, pp. 67–72 (1980).

Byers, Barry, “A Simple and Practical Fumigation System,” Abbey Newsletter, Vol. 7, No. 4Supplement (September 1983).

Calvini, P.L.Santucci, “Alcuni dati sugli effetti dell' irradiazione gamma sulla carta.” Bollettino Dell'instituto Centrale per la Patologia del Libro, Vol. 35, pp. 55–62 (1978–1979).

Flores, Sar C.Pavon “Gamma Radiation as Fungicide and its Effect on Paper,” Bulletin of the AIC, Vol. 16, pp. 15–44 (Winter 1975–1976).

Florian, Mary-Lou, “The Thymol Cabinet-Fungicidal Efficiency,” (Research Report), Canadian Conservation Institute, 1975.

McCall, Nancy, “Gamma Radiation as Sterilant, “SAA Newsletter, p. 6 (November 1983).

Nagin, Deborah, and MichaelMcCann, “Thymol and O-Phenyl Phenol: Safe Work Practices,” NY: Center for Occupational Hazards, 1982. 4 pp.

Strassburg, Richard, “The Use of Fumigants in Archival Repositories,” American Archivist, Vol. 41, pp. 25–36 (January 1978).

Strassburg, Richard, “Further Information on the Use of Ethylene Oxide as a Library and Archival Fumigant,” SAA Leaflet (1983).

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Copyright 1986 American Institute for Conservation of Historic and Artistic Works