Biocides

Contributors

Magdalena Grenda-Kurmanow

Introduction

This website deals with biocides, which are present in biocultural collections, with particular emphasis on herbaria. Although the information is based on research related to botanical material,[1] most of these substances may be present in a number of other types of objects, belonging to zoological, ethnographic and other collections. The following list, therefore, because it was created for herbaria, does not constitute a complete list of toxic substances that can be identified in other biocultural collections. Moreover, for the herbaria themselves, it may not be an exhaustive source, either, due to the increasingly new discoveries resulting from research carried out with modern sensitive analytical techniques.

Herbaria with desiccated plants, composed of organic material combined with other organics such as glues, pastes and other cellulose-based and proteinaceous materials are particularly vulnerable to pest infestations. Several chemical compounds have been used either to prevent infestation or to fight an existing pest attack. Most of them are now phased out or restricted in use and production. Biocides pose numerous threats both to artefacts and people working in the heritage sector, such as conservators.

The use of biocides in herbarium collections

Popular and easily accessible biocidal substances are widely used for pest control. Unfortunately, several compounds proved to have negative impacts not only on insects and microorganisms, but also on other forms of life, including humans, and on the artefacts themselves. As the history of the use of biocides in institutions and conservation studios is poorly documented, this article focuses on the issues related to their use. Herbaria with desiccated plant material can be regarded as paper-based artefacts, but they also contain biological (plant) material, often mounted with natural adhesives. This combination of components makes herbaria particularly vulnerable to damage caused by insects and rodents. Unstable storage conditions, climatic fluctuations or random events involving higher moisture levels (such as dampness, flooding) also create conditions for mould growth. Herbaria in which the plants are kept loose between pages are also not free from potential damage by insects and microorganisms. Stored with other collections containing materials “attractive” to pests (i.e. natural adhesives), herbaria also suffered damage by pest attacks. Some plant species are more susceptible to insect attacks than others. The most endangered groups of specimens are petaloids, belonging to the monocotyledons, many dicotyledon species, particularly Asteraceae, and fungi. On the contrary, bryophytes and lichens are more resistant. Problems concerning insect damage were well known already at the beginning of the creation of herbaria. The first publication on herbarium preparation, Isagoge in rem herbariam libri duo by Adriaan van den Spieghel (1606), already mentions biocidal and repellent substances. Spieghel describes cloves powder and aloe as additions to the glue he used to mount specimens. In another archival source by Joseph Pitton de Tournefort, there aloe extract (aloe hepatica) is mentioned, as well as a decoction of common wormwood (Artemisia absinthium L., “absinth”) and santonica (Artemisia cina O. Berg.), which, however, altered the colour of the botanical specimens.

Natural biocidal products

Other plants and plant compounds that were generally used as natural pesticides and repellents were camphor, extracted from camphor tree oil (Cinnamomum camphora (L.) J. Presl.), cassia oil from Chinese cinnamon (Cinnamomum cassia (L.) J. Presl.), thymol and carvacrol – present in thyme oil (Thymus vulgaris), wild thyme (Thymus serpyllum L.) and carom (ajwain, Trachyspermum ammi L. or Carum copticum L.), neem (Azadirachta indica A. Juss.), pyrethrin, extracted from Dalmatian pellitory (Tanacetum cinerariifolium (Trevir.) Sch. Bip., previously called Chrysanthemum cinerariifolium (Trevir.) Sch. Bip.), linalool, from lavender oil (Lavandula angustifolia Mill., previously called Lavandula officinalis Chaix), turpentine, distilled from pine resin (Pinus sylvestris L.), leafy sprouts of wild rosemary (Rhododendron tormentosum Harmaja, syn. Ledum palustre L.), sweet flag (Acorus calamus L.), cassumunar ginger (Zingiber cassumunar Roxb.), cultivated tobacco (Nicotiana tabacum L.), strychnine from the strychnine tree (Strychnos nux-vomica L.), Javanese long pepper (Piper retrofractum Vahl), maidenhair tree seeds (Ginkgo biloba L.), citronella oil (Cymbopogon nardus (L.) Rendle or Cymbopogon winterianus Jowitt), rotenone, present in the roots of many plants of the Fabaceae family, menthol, cedar oil and bitter almond oil. Several of these compounds were at some point synthesised to expedite their production and to standardise their composition and effects. Several are still used as biocides, such as pyrethrins, both in natural and synthetic form.

Miraculous chemicals and health risks

Over time, users and custodians began to notice that the use of biocides is detrimental not only to insects and microorganisms, but also to human health. Our civilization is still struggling with the use of “miraculous” biocidal chemicals that facilitate agricultural production in many areas, reducing its costs, but also cause irreversible changes in natural environments. The most frequently cited example is DDT, which is now conditionally phased out. The production of DDT provided its developer, Paul Hermann Müller, with the 1948 Nobel Prize in Physiology or Medicine because of its insecticidal properties and effectiveness in fighting malaria and yellow fever. Today, DDT is still used to treat malaria. At the time of its intensive use in agriculture, this chemical compound almost led to the extermination of entire bird populations in the USA; being one of the most persistent polluting pesticides, it is still detected in populations of animals inhabiting the Arctic. Such examples remind us, as conservators, that an awareness of the threats connected with pesticides is crucial to responsibly and effectively care not only for the safety of historical items, but also to protect both the natural environment and human health.

Classification of biocides

Biocides can be grouped according to their mode of action: stomach poisons that must be consumed by pests, contact pesticides that must come into direct contact with an insect, residual products, desiccant insecticides causing dehydration and, eventually, death, insect growth regulators and fumigants. They are available in different formulations: in mixtures with solvents or inert substances, as oil concentrates, as emulsifiable concentrates, wettable powders, dusts, baited insecticides, microcapsules diffusing poison and plastic strips impregnated with insecticide. The best known and most commonly used pesticide in herbaria is mercury chloride. The first reference to its use is the description of Tournefort (1694), who used mercury compounds in both monovalent (mercure doux) and divalent form (sublimate, French sublimé corrosif) as additives to the glue used to mount the specimens. However, the list of toxic substances used as preventive measures is considerably longer. Synthesised biocides, also synthetic versions of natural compounds, are listed in Table 1. This table contains references to publications that provide information on individual substances used in herbarium collections. The literature on biocides themselves is far more extensive, in addition to general studies, in which the use of pesticides is discussed in relation to all museum objects, also covering ethnographic, taxidermy, fluid entomological and economic botany collections. These publications mention all biocides listed in the table below, as well as other substances such as borax, methoxychlor or glyphosate, which were, however, most likely not used in herbaria collections. It cannot be excluded that in the past, these biocides might have been used for the preservation of herbarium specimens, and the development and availability of sensitive analytical techniques, mainly chromatographic, still provides new discoveries. Pesticide residues and degradation products are less evident in herbaria than in zoological items. The most extensive literature concerns the use of the above-mentioned mercury chloride, which is one of the earliest known biocidal substances and one of those pesticides that permanently contaminate and damage specimens and are dangerous to human health, even many years after the original application. In addition, the CAS (Chemical Abstract Service registry) identification number is listed under the name of each pesticide. When an entire group of compounds (e.g. pyrethrins or carbamates) is listed, the CAS number for the representative substances is given. However, this does not mean that there are no other substances with different registration numbers in the same group. The third column of the table indicates whether the compound has a negative effect on the genetic material of objects and artefacts. Due to the limited number of studies in this area, information on the impact on genetic material has been generalised and concerns a wider group of natural history collections, not only herbaria. In some cases, the results of research in this area are not clear or consistent. In such situations, the table provides a less “optimistic” result, assuming that if in one study, a given compound showed a destructive effect, it is a potential threat to other historic objects. The last column contains information on the genotoxicity (mutagenicity or carcinogenicity) of the listed materials, based on the EPA (United States Environmental Protection Agency) and IARC (International Agency for Research on Cancer) databases. All listed pesticides are toxins with a variety of risks to human health. Some of them have a confirmed muta- and carcinogenic status, and for others, genotoxicity is defined as “possible” or “probable”, according to the classification implemented in the above-mentioned sources. In some cases, although a compound is known to be a neurotoxin and interferes with identification of genetic material in museum objects (as in the case of mercury chloride), there are insufficient data to classify it as a carcinogen or mutagen. There is also often a lack of research on chemicals currently phased out (at least in some regions of the world and most previous uses), such as DDT. Descriptions of individual pesticides indicate whether they are listed in the Stockholm Convention, an agreement governing the use of persistent organic pollutants (POPs). The Stockholm Convention divides toxic substances into three groups: group A – intentionally produced toxic agents that should be eliminated, with specific exemption for certain uses; group B – toxic agents, of which international production should be restricted to manufacture and use to fulfil specific purposes (e.g. DDT for the treatment of malaria); group C – unintentional toxic agents that are products of the decomposition of other substances (e.g. production process side effects) and which should be kept to a minimum.

List of biocides used in herbaria.

Characteristics of selected biocides used in herbaria

The following review is based on a number of cross-sectional studies on pesticides21 and selected other publications, which describe from one to several selected biocides. The basic names of biocidal products are accompanied by summary formulae as well as variations of chemical names and trade names to facilitate the recognition of compounds by users of this summary. The toxicity of the biocides is signalled in general terms.

Inorganic compounds

Mercury chloride, (I) Hg₂Cl₂ and (II) HgCl₂ (sublimate)

An inorganic chemical compound, occurring in the form of calomel, where mercury is at degree I oxidation (Hg₂Cl₂), and the more common so-called sublimate, where mercury is at degree II oxidation (HgCl₂). The first evidence of the use of both forms of mercury chloride can be found as early as 1794. Calomel under the influence of UV radiation may decompose into pure mercury and mercury (II) chloride, i.e. the sublimate. The sublimate is volatile at room temperature and has been used in solutions in spirit, in various proportions, sometimes with the addition of phenol, or mixed with arsenic or lauryl pentachlorophenate (LPCP). Whole botanical specimens were immersed in the solution or it was applied on them with a brush. After removal from the solution, the specimens were sometimes fragile. Mercury chloride may leave a white layer in the form of hairy crystals, which may have been confused with the taxonomic characteristics of the specimen. To avoid recrystallisation, kerosene was added to the alcoholic solution of the sublimate. Mercury chloride at second degree oxidation slowly sublimates from the area where it is applied, and therefore, treatment was generally repeated several times. This was also because it was long claimed that after some time, mercury chloride lost its biocidal properties. However, the opposite is true, because mercury chloride settles permanently in the structure of objects. The mercury present in mercury chloride reacts with sulphur, present both as an air pollutant and a component of paper substrates and animal adhesives. It forms black mercury sulphide (metacinnabar, HgS) and leaves a trace on the paper in the form of concentrated or merged black-edged spots. Over the years, it is reduced to pure mercury, showing a tendency to disproportionation, i.e. self-oxidation and self-reduction. The reduction of mercury accelerates the acid hydrolysis of cellulose. Mercury chloride therefore has a destructive effect on both plant tissues and paper substrate, causing brittleness and structural changes. In specimens, it can cause the breakdown of the cuticle (layer saturated with lipid substances, protecting the plant from water loss). Since it was most often used in alcoholic solutions, repeated application of the solution to specimens and sheets additionally caused rinsing of chlorophyll. Mercury chloride also has a corrosive effect on aluminium. In institutions with a long tradition of using mercury chloride, mercury vapour in the air can be expected, especially in storage cabinets and their immediate surroundings, because pure mercury is highly volatile. Back in the 1980s, mercury chloride was used in the Netherlands and at the Royal Botanic Gardens in Kew (according to a recipe called the Kew mixture). In 1988, mercuric chloride continued to be used in African herbarium collections; until the end of the 20th century, it was also used in France. It is a strong neurotoxin.

Arsenic (diarsenic trioxide), As₂O₃, As₄O₆; lead arsenate, PbHAsO₄; other arsenic compounds

Arsenic has been known since ancient times and has been used in museum collections since the beginning of the 18th century. In the past, arsenic-based substances were often used as insecticides, herbicides and rodenticides. In natural history collections, they were mainly used on taxidermy objects in the form of arsenic and sodium arsenate. Herbarium collections were dusted with arsenic, lead arsenate (PbHAsO₄) or potassium arsenate (sel de Macquer, KH₂AsO₄). Arsenic was also used in solution with mercury chloride and phenols. Arsenic settles permanently in the structure of objects and is sometimes visible as a white deposit. It is highly toxic not only as a substance consumed, but also inhaled or absorbed through the skin. It is also a carcinogen. Withdrawn from the 1960s onward, it was used in some institutions until the 1990s.

Hydrogen cyanide, HCN

Other names: Prussian acid, Zyklon B.

Hydrogen cyanide was has been used as a biocide since the 19th century. It was included in Zyklon B according to a patent in force since 1922. It was later used by the Nazis in extermination camps during World War II. In the post-war period, it was used in disinfection chambers and to fumigate entire rooms. Hydrogen cyanide is well absorbed by porous structures such as paper and is desorbed over a long period, posing a constant threat to human health. In reaction with water molecules, it forms formic acid and ammonia and is therefore particularly harmful under conditions of increased humidity, causing damage to certain pigments (containing copper) and plant dyes, yellowing of paper, brittleness of animal adhesives and corrosion of base metals.

Phosphine, PH₃

Other names: Phostoxin; Celphos, DeliciaGastoxin. Phosphine is a pesticide intended for use in agriculture, but has also been used in herbarium collections. Only a few years ago, it was still being used in Poland as a disinfectant in the collection of herbaria. It is effective in combating insect eggs and occurs in the form of powder compounds of aluminium phosphide and magnesium phosphide. With moisture from the air, these substances release hydrogen phosphate gas, a strong reducing agent. As a product, it also occurs in combination with ammonium carbamate, which decomposes to ammonia and carbon dioxide. It is extremely flammable and highly toxic. Hydrogen phosphate itself oxidises in the presence of water, forming phosphoric acids. Under conditions of elevated temperature and humidity, it can cause corrosion of copper and its alloys, as well as of silver, gold, aluminium, nickel and sulphur compounds. Therefore, it has a destructive effect on pigments such as ultramarine or copper greens, and gilding fades in reaction with phosphoric acid. Exposure to hydrogen phosphide causes many health effects, but no carcinogenic effect has been found.

Barium fluorosilicate BaSiF₆, sodium fluorosilicate, Na₂SiF₆

Barium fluorosilicate was patented as a pesticide in 1827. Barium and sodium fluorosilicates were used interchangeably, mainly as an agent against silverfish. Barium fluorosilicate was more effective, but both agents were used as bait in a paste consisting of sugar, flour and Arabic gum in water. The paste was applied on wooden collection storage furniture. Sodium salt can damage the seeds of herbarium specimens.

Sulphuryl fluoride, SO₂F₂, sulphuryl chloride, SO₂Cl₂

Other names: Vikane; sulphuricoxyfluoride Used as an alternative to fumigation with methyl bromide and phosphine. Fumigation with sulphuryl fluoride causes yellowing and acidification of papers, especially those containing lignin. According to Whitten, sulphuryl fluoride does not destroy the genetic material in herbarium specimens, but Kigawa identified changes in the protein structure by examining specifically proteinaceous samples (from chicken muscles). Regarding the destructive effect on other museum objects, destruction of wallpapers and bronze handles was observed during fumigation because of gas condensation. In paintings, non-varnished paint layers containing the pigments azurite, malachite and cobalt and Prussian blue in linseed oil medium and protein binders were also destroyed. Changes in textile dyes, layers of polyvinyl acetate and epoxy resins have also been observed. Sulphuryl chloride shows similar effects on sulphuryl fluoride and was used in a similar way.

Silica gel, SiO₂

Silica gel is the most popular desiccant and commonly used in the museum environment. It occurs in the form of a fine powder and is often mixed with pyrethrins and pyrethroids, acting as repellents. In this mixture, it is also available as an aerosol. The use of an aerosol spray provides protection in areas that are more difficult to access (shelf joints, rear walls of shelves) and reduces silica gel deposits. Contact with finely ground silica gel can lead to dryness and irritation of the skin and irritation of the respiratory system. Although spraying is not carried out directly on specimens, excess silica powder residue can enter the object and cause micro-damage, acting as an abrasive layer. The agent is used against many species of insects, including the varied carpet beetle (Anthrenus verbasci L.), the silverfish (Lepisma saccharina L.) and psocids (book lice, belonging to the order Psocoptera). The effect of silica powder is to destroy the outer layer of the insects’ cuticle, leading to fatal dehydration.

Organic compounds

Naphthalene, C₁₀H₈

Naphthalene was first synthesised in 1821 and has been used in collections since 1887. It is a polycyclic aromatic hydrocarbon and occurs in the form of white powder or balls, used in storage cabinets and inserted into packaging as a passive fumigant. It can be confused with paradichlorobenzene, which occurs in a similar form and is used in a similar manner. Under storage conditions, an insect repellent smell is slowly released from the naphthalene balls. It can recrystallise on the surface of specimens and can be confused with taxonomic features. Naphthalene deeply penetrates the structure of paper and is therefore difficult to remove from objects. Contact with naphthalene can cause many adverse effects on human health. Despite suspicions, neither carcinogenic nor mutagenic properties have been confirmed. In 2008, the use of naphthalene was banned in the European Union.

Dichlorvos, dichlorovinyl-dimethylphosphate,Cl₂C=CH(CH₃)₂PO₄

Other names: DDVP, Dichlorfos, Vapona. Dichlorvos occurs in the form of emulsions, aerosols, pesticide impregnated strips and oil concentrate. It is soluble in many organic solvents, also chlorinated, and in alcohols and was placed on the market as an insecticide in the 1960s. Dichlorvos is used for dusting, spraying or as a passive fumigant. As a product used in museum collections, it can leave a film on object surfaces and can cause colour changes in the dyes, especially red, and in the paper itself. Under conditions of increased humidity, it has an acidic reaction and causes corrosion of silver, tin, lead and soft steel. It is a highly toxic substance, and working in the vicinity of a dichlorvos source (e.g. a toxin-releasing dispenser) causes numerous adverse health effects.

Malathion, C₉H₁₉O₆PS₂

Other names: Carbofos, Maldison, Mercaptothion. Pesticide from the group of organophosphates, used in agriculture, horticulture and healthcare, are mainly applied to control mosquitoes and lice. Malathion occurs in the form of a liquid, dust, emulsion and powder to form a paste and has been introduced in the 1950s. It is a compound from the group of phosphoric acid esters and succinic acid derivatives.

Chlorpyrifos, C₉H₁₁Cl₃NO₃PS

Other names: Dursban; Lorsban; Dowco, Chlorpyriphos-ethyl; Brodan, Bolton, Detmol, Eradex, Empire, Hatchet, Nufos, Piridane, Scout, Stipend, Tricel, Warhawk. Chlorpyrifos was introduced as a pesticide in 1965 and occurs as a liquid for spraying. It has a corrosive effect on copper and bronze and causes colour changes in red dyes. It is strongly toxic, and exposure to this agent causes a number of adverse health effects, especially in children. Withdrawn from 2014, it is still detected in fruit plants.

Organochlorides

Organochlorine compounds were introduced to the market in the mid-20th century.

DDT, dichlorodiphenyltrichloroethane (C₆H₄Cl)₂CHCCl₃

Other names: Chlorophenothane, Dicophane. Its use in the treatment of malaria and typhus during the Second World War and its popularity in agriculture after the war made DDT generally available and widely used. It was used in collections from the 1940s, at least until the 1970s, when it was banned in the USA due to significant environmental contamination. It was used for direct dusting of objects as well as in solution, for spraying, sometimes mixed with Lindane or mineral oils. It can have a destructive effect on botanical specimens. The DDT decomposition products, mainly DDE, dichloro-diphenyl-dichloroethylene, are also harmful to human health. Until 2008, it was produced in China and is currently still produced in India. Almost phased out because of the Stockholm Convention, it is classified as a group B toxic compound. In certain regions of the world, it is approved for the treatment of malaria.

Lindane, gamma-hexachlorocyclohexsane, C₆H₆Cl₆

Other names: Gamma-BHC, Gamma-HCH, Gammexane, Forlin, Gamophex. Lindane was first synthesised in 1825, and its insecticidal properties were discovered in 1942. It is soluble in many organic solvents. During fumigation, the gas decomposes to phosgene (COCl₂) and hydrochloric acid (HCl), which is a powerful catalyst for acid hydrolysis of cellulose. It is therefore particularly unsuitable for use in cellulose-based artefacts, such as paper substrates or herbarium specimens. Lindane is a neurotoxin with an ambiguous effect on genetic material. It is classified in Group A of the Stockholm Convention.

LPCP, lauryl pentachlorophenate C₁₈H₂₃Cl₅O₂

Other name: Mystox. It was first used in 1962 in the Natural History Museum in London; used for fumigation as well as for individual use, preventive soaking, lubrication and spraying incoming specimens. It was used at a concentration of 5%, in solution with paraffin or mineral turpentine. When using the solution, it was recommended to carry out resistance tests of the handwritten elements beforehand. Coated papers as well as herbarium grass and fern specimens had to be removed from the collection. Application of LPCP onto the specimens was usually performed more than once. The LPCP solution left stains on the paper, which were supposed to fade after several years. A substance used in herbarium collections in both temperate and tropical zones. Withdrawn in 1984.

Pentachlorophenol (PCP), C₆Cl₅OH, sodium pentachlorophenate, C₆Cl₆ONa

Other names: Santophen, Pentachlorol, Chlorophen, Chlon, Dowicide 7, Pentacon, Penwar, Sinituho. First synthesised in the 1930s, it was used mainly as liquid for soaking and spraying specimens. Pentachlorophenol, its salts and esters were placed on List A of the Stockholm Convention in 2015.

Carbon tetrachloride, CCl₄

Other names: Tetrachlorometan, Benziform, Benzinoform, Carbon chloride, Carbon tet, Freon-10, Refrigerant-10, Halon-104, Methane tetrachloride, Methyl tetrachloride, Perchloromethane, Tetraform, Tetrasol. Colourless liquid with a sweet smell and formerly used as a solvent, extinguishing and cooling agent. First synthesised in 1839, it was used on a mass scale between 1950 and 1980. In the 1980s, it was withdrawn from general use due to its toxicity to humans and its depleting effect on the ozone layer.

Paradichlorobenzene, C₆H₄Cl₂

Other names: P-dichlorobenzene; 1,4-dichlorobenzene, p-DCB; 1 ,4-DCB; PDB; PDCB. Paradichlorobenzene has been used in museum collections since 1913 for passive fumigation and as repellent, fungicide and rodenticide. Generally, it is used in a similar way to naphthalene, inside cupboards, but also as a solution. It is more volatile than naphthalene and five times heavier than air and should therefore be placed on top shelves, never in direct contact with specimens. Paradichlorobenzene is a toxic substance that causes numerous adverse health effects upon exposure. It reacts with some pigments and dyes by changing their colour: zinc white, lithopone, scarlet pigments, ultramarine, acetyl cellulose-based dyes. It also causes yellowing of paper, fading of inks and increased shrinkage of some resins and plastics. By dissociating, it can have a whitening effect on specimens and archival documents, even when they are tightly packed. It is highly volatile, making it difficult to maintain an even concentration in storage cabinets. When paradichlorobenzene vapour leaks from the storage area, clouds of vapour in the air form next to the sources of the leak, posing a health risk. In some institutions, it is still being used.

Para-chloro-meta-cresol, C₇H₇ClO

Other names: 4-chloro-3-methylphenol, p-chlorocresol, PCMC, rachit, Preventol CMK. Para-chloro-meta-cresol is a chlorinated phenol used for passive fumigation and by tamponing with 0.3% solution, either directly or by soaking the interleaves with 10% PCMC solution in 70% ethanol.

Ethylene dichloride, C₂H₄Cl

Other names: 1,2 dichloroethane, EDC. Ethylene dichloride is a chlorinated hydrocarbon. It was used in a 7:3 ratio in mixture with carbon tetrachloride for fumigation and was deregistered from use in 1984.

Aldrin, Dieldrin, Endrin

Aldrin, C₁₂H₈Cl₆, other names: HHDN, Octalene. Dieldrin, C₁₂H₈Cl₆O, other names: HEOD, Octalox. Endrin, C₁₂H₈Cl₆O, other names: Mendrin, Endrex. Aldrin, dieldrin and endrin are compounds invented in the mid-20th century. They were used more or less until the 1990s, being gradually banned since the 1970s in various countries. They are among the most persistent pesticides, are highly toxic and are potential carcinogens. Dieldrin is an effective pesticide, while aldrin is oxidised inside the insects to dieldrin. Endrin is a dieldrin stereoisomer. They are all classified as Group A in the Stockholm Convention.

Chlordane, C₁₀H₆Cl₈

Other names: Heptachlor, Octachlor, Chlorindan, Kypchlor, Toxichlor, Belt, Ortho-Klor, ChlorKil, Chlorotox, Dowchlor, Oktaterr, Starchlor, Topiclor, Sydane, Termex, Intox. Chlordane is a compound from the group of dienes, such as aldrin, endrin and dieldrin. It is a mixture of isomers of chlorinated hydrocarbons and decomposes to phosgene, chlorides, hydrochloric acid and carbon monoxide. It is a highly persistent compound, classified under Group A of the Stockholm Convention.

Phenol, C₆H₅OH, thymol, C₁₀H₁₃OH, creosote C₇H₈O₂

The antiseptic properties of phenol have been known since the 19th century. Phenol and creosote were added to a solution of mercury chloride in alcohol, sometimes in combination with arsenic. They were also placed in liquid form in cupboards as passive fumigants. Thymol, previously obtained from natural resources, was first synthesised in the 19th century and used as a fungicide for application as a solution or passive fumigation. A solution of 1% thymol in ethanol was sprayed or applied on specimens affected by microbiological infection. Fumigation takes place at elevated temperatures by heating thymol in the solid form. Thymol dissolves some inks and adhesives and may leave stains on paper. As a toxic substance, it can cause adverse health effects upon direct contact and ingestion.

Pyrethrins and pyrethroids

Pyrethrins are substances of natural origin and are extracted from the Dalmatian pellitory (Tanacetum cinerariifolium (Trevir.) Sch. Bip., once called Chrysanthemum cinerariifolium (Trevir.) Sch. Bip.). The group of pyrethrins includes six natural compounds, which are components of the raw extract of the Dalmatian pellitory flower. Their insecticidal properties have been known since the 19th century. Pyrethrins are prone to biodegradation under the influence of oxygen and ultraviolet radiation. Since the 1970s, pesticides based on pyrethroids have been available – a synthetic version of pyrethrins, including permethrin (C₂₁H₂₀Cl₂O₃), cypermethrin (C₂₂H₁₉Cl₂NO₃) and deltamethrin (C₂₂H₁₉Br₂NO₃). Pyrethroids are chemically more stable than pyrethrins but are also easily biodegradable. Pyrethrins and pyrethroids are contact insecticides and used in the form of microcapsules, powders, oil solutions, in liquid form, sprayed and, in the case of herbaria, mixed with silica gel. They are also combined in piperonyl butoxide to increase their durability and effectiveness. They are widely used to this day, both for prevention and intervention.

Carbon disulphide, CS₂

Carbon disulphide has been used in collections since 1887, mainly in disinfection chambers, but also as a fumigant for specific rooms; it is suspected of destructive effect against varnishes. It is highly toxic and was deregistered in some countries in 1984. According to Pfister, some institutions still use it.

Ethylene oxide, C₂H₄O

Other names: 1, 2-epoxyethane, oxirane, Rotanox, Carboxide, ETO. Used for fumigation in specific fumigation chambers, usually in libraries, against microorganisms and insects. However, it was also used as a fumigant for storage rooms. Ethylene oxide was used in a mixture with carbon dioxide (CO₂), methyl bromide (CH₃Br), freons and phosphine (PH₃). If a vacuum can be maintained in the chamber, ethylene oxide also kills insect eggs. Ethylene oxide is accumulated by objects made of leather, rubber, vinyl and fats and is gradually released. It cross-links free hydroxyl groups in materials such as wood and paper, causing a reduction in hygroscopicity. It may have a negative effect on some pigments (lead-tin yellow) and plant dyes, as well as on chicken egg casein. It oxidises copper and its alloys (for example bronze). It also reacts with proteins present in leather and parchment, destroying their structure – they become stiff, brittle and less resistant to biological damage. Reaction with textiles that were previously disinfected with sodium pentachlorophenate has been observed. Polyvinyl chlorides strongly absorb ethylene oxide while losing their tensile strength. Ethylene oxide is a mutagen and a carcinogen.

Propylene oxide, C₃H₆O

Other names: 1,2 epoxypropane, 2-methyloxirane, PPO. Propylene oxide is weaker than ethylene oxide and not as effective. It has been mixed with methyl bromide (CH3Br) and used as a fumigant.

Methyl bromide, CH₃Br

Other names: bromomethane, organobromine, Meth-O-Gas, Maltox. Its insecticidal properties were discovered in 1932. In its pure state, it is odourless; it disperses quickly and penetrates walls and floors. It is more effective under conditions of increased humidity and temperature. In reaction with water, it forms aggressive bromine water, which acts corrosively. It is highly reactive and reacts via methylation and bromination of various substances. It should not be used for sulphur-containing materials, including herbarium specimens, leather, coated paper, photographic and parchment. As a result of the reaction with methyl bromide, the colour of these substrates will change, and they will emit a strong odour of thiols (thialcohols), which are acidic substances. It has a corrosive effect on zinc, tin, iron and lead and is therefore destructive to pigments such as lead white, Neapolitan yellow, lead yellow and lead red, which blacken in reaction with methyl bromide. It is a strong solvent for rubbers and other organic materials and may cause brittleness of animal glues and softening of natural resins and varnishes. It is often mixed with chloropicrin (CCl₂NO₂), which is phytotoxic. It is highly toxic and has carcinogenic effects. It was widely used for fumigation of herbarium collections, also for passive fumigation. It was phased out in 2004 as an ozone-depleting agent.

Methyl iodide, CH₃I

Used as a substitute for methyl bromide, for room fumigation and passive fumigation and inside storage cabinets.

Carbamates and thiocarbamates

These substances are derivatives of carbamic acid, invented in the 1950s and used in institutions at least until the 1970s. Poorly soluble in water, more soluble in organic solvents, they are also used as fungicides. Examples of products in this group are Bendiocarb, Carbaryl and Propoxur, which are contact or stomach insecticides. They are available in the form of solutions, dusts, powders for making paste, granules and emulsions. They are highly toxic, and the presence of a pesticide from this group was detected in one of the herbaria from the first half of the 18th century.

Formaldehyde (methanal), CH₂O; formalin, aqueous solution of formaldehyde

Formaldehyde and formalin were used for fumigation. Paraformaldehyde, a formaldehyde derivative, was used to perform fumigation in institutions, leaving a white deposit on objects, including those that were not directly exposed, but were in the vicinity of fumigated objects. It reacts with free amine groups present in protein-binding media, causing hardening. It also hardens parchment and leather and accelerates the degradation of wool, fur and hair. Formaldehyde was also used as a passive fumigant. It is a strong reducing agent.

Alcohol, mainly ethanol (C₂H₅OH)

Alcohol was often used as a solvent in solutions of mercury chloride, arsenic, thymol and PCMC, but also alone. Usually, the solution was applied to the specimens or they were soaked in alcohol. Alcohol can cause discolouration of the plants (especially chlorophyll) and make them brittle.

Petrol

Petrol was used to soak the specimens. It has a destructive effect on xanthophylls (dyes from the carotenoid group).

Pyridine, C₅H₅N

First synthesised in the second half of the 19th century, it was used for fumigation of collections, also passively. It is a starting product for various pesticides, including chlorpyrifos.

Quaternary ammonium cations

An example of a product belonging to this group is Sterinol (or Atoxin) (10% aqueous solution of dimethylolaurylbenzylammonium bromide). The group consists of ionic agents working as surfactants, which were synthesised for the first time in 1890. They increase the permeability of the cell membrane of botanical specimens, destroying their walls. In contact with the cell membrane, ammonium cations are adsorbed by a negatively charged cell wall, which allows the compound to penetrate the cytoplasm. They can irritate the skin and cause contact allergy.

Thiosemicarbazide (rodenticide); Caratane/Dinocap (fungicide) – dinitrophenol derivatives

the presence of these pesticides was detected in a herbarium from the first half of the 18th century.

Summary

Herbaria are a narrow group of historic objects on a paper substrate, but the substances used to protect the botanical collections are also present in many other museum and archival collections. As shown in the characterisation of these substances, biocides have a harmful effect on the preservation of individual elements and layers occurring in historic artefacts. They also pose a health risk to users. An obstacle to a realistic assessment of the risks associated with the protection of collections and occupational safety is certainly scarcity or lack of documentation, which is a common problem in many institutions around the world. This makes it even more important to know the potential risks that conservators encounter in their daily work, as well as the processes that affect the state of preservation of the collection items. In addition, many of the biocides mentioned above are encountered in everyday life. Toxic substances are unfortunately commonly used in agriculture, horticulture and various industrial sectors. Many of them have been phased out, but have not disappeared from the environment, often contributing to irreversible damage to the ecosystem and human health.

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