Difference between revisions of "Biocides"

Revision as of 22:25, 2 December 2020

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.^[1] Problems concerning insect damage were well known already at the beginning of the creation of herbaria.^[2] The first publication on herbarium preparation, Isagoge in rem herbariam libri duo by Adriaan van den Spieghel (1606), already mentions biocidal and repellent substances.^[3] 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;^[4] being one of the most persistent polluting pesticides, it is still detected in populations of animals inhabiting the Arctic.^[5] 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.^[6] 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.^[7] 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.^[8] These publications mention all biocides listed in the table below, as well as other substances such as borax, methoxychlor or glyphosate,^[9] 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.^[10] 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.^[11] 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.^[12] 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).^[13] 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.

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.^[14] 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.^[15] The toxicity of the biocides is signalled in general terms.^[16]

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.^[17] 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).^[18] 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.^[19] Mercury chloride therefore has a destructive effect on both plant tissues and paper substrate, causing brittleness and structural changes.^[20] In specimens, it can cause the breakdown of the cuticle (layer saturated with lipid substances, protecting the plant from water loss).^[21] 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)^[22] or potassium arsenate (sel de Macquer, KH2AsO4).^[23] 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 invented in 1922 in Germany and was widely used as a rodenticide. 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.^[24]

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.^[25] It is effective in combating insect eggs^[26] 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.^[27]

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.^[28] According to Whitten, sulphuryl fluoride does not destroy the genetic material in herbarium specimens,^[29] but Kigawa identified changes in the protein structure by examining specifically proteinaceous samples (from chicken muscles).^[30] 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.^[31] 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.^[32] 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

=== Aromatic hydrocarbons

1. ↑ Hall 1988
2. ↑ The years 1530–1540 are considered to be the most probable beginning of the history of making herbaria with dried plants (Bridson, Forman 1999, p. 4)
3. ↑ Spieghel 1606, p. 81
4. ↑ Jagannath, Shore, Walker, Ferns, Gosler 2008
5. ↑ Doyle 2008
6. ↑ Dawson, Strang 1992
7. ↑ Tournefort 1694, p. 547–548
8. ↑ Examples of studies: Dawson, Strang 1992; Hawks 2001; Sirois 2001; Odegaard, Sadongei 2005; Odegaard, Zimmt 2008; Pfister 2008
9. ↑ Glastrup 1987; Hawks 2001; Odegaard, Zimmt 2008
10. ↑ https://www.cas.org/about/cas-content
11. ↑ The information in this column was obtained from publications on the effects of pesticides and preservatives on the genetic material of various biological objects in museum collections. Studies and compilations in this area were carried out, among others, by Brown (Brown 1999), Whitten (Whitten, Williams, Glover 1999), Kigawai Stranga (Kigawa, Nochide, Kimura, Miura 2003; Kigawa, Strang, Hayakawa, Yoshida, Kimura, Young 2011; Kigawa, Strang 2011; Strang 1999), Cartera (Carter 2003), Eklund (Eklund 2006), Espeland (Espeland, Irestedt, Johanson, Akerlund, Bergh, llersj 2010) and Purewal (Purewal 2012)
12. ↑ EPA: www.epa.gov; IARC: www.iarc.fr
13. ↑ http://www.pops.int/
14. ↑ Purewal 2012; Dawson, Strang 1992; Hall 1988; Drobnik 2009; Sadongei, Odegaard 2005; and Pfister 2008 – these publications will not be cited in each case when describing the individual substances. Information on the harmfulness of the compounds, as in the table, was obtained from the EPA and IARC databases (see footnote 19), which are also not always quoted.
15. ↑ These publications are given in footnotes.
16. ↑ The formulas were abandoned when describing whole groups of compounds (e.g. pyrethrins and pyrethroids) to maintain legibility.
17. ↑ Tournefort 1694, p. 547–548
18. ↑ Briggs, Sell, Block, I’ons 1983. Purewal quotes older, historical recipes in which the sublimate is also mixed with alcohol and phenol or its derivative – cresol (Purewal 2012, p. 20)
19. ↑ Collins 2014. The authors state that mercury reduction can be catalysed by the presence of naphthalene vapour, once a popular pesticide and repellent
20. ↑ Clarck 1986
21. ↑ Collins 2014
22. ↑ According to Purewal, the lead present in herbaria may also come from another pesticide, lead acetate [Pb(CH3COO)2], suggesting that lead arsenate was commonly used in herbaria (Purewal 2012, p. 20)
23. ↑ Péquignot 2006
24. ↑ Unger, Schniewind, Unger 2001, p. 279–280, 315; Hahn 1999
25. ↑ Information from an unpublished nationwide survey of the methods of protecting herbarium collections, conducted by the author in 2013
26. ↑ Unger, Schniewind, Unger 2001, p. 280
27. ↑ Hall 1988
28. ↑ Burgess, Binnie 1990a and 1990b
29. ↑ Whitten, Williams, Glover 1999
30. ↑ Kigawa, Strang 2011
31. ↑ Unger, Schniewind, Unger 2001, p. 316
32. ↑ The so-called aerogel with pyrethrins and pyrethroids in a concentration of 1%, silica gel occurs in a concentration of about 40%; trade names: Drione, Driaone, Dri-die and Silikil (Dawson, Strang 1992; Hall 1988 Bridson, Forman 1999, p. 21; Hall 1988; Schofield, Crisafulli 1980), currently also Evergreen Pyretrum Dust, CimeXa, Tri-Die Silica & Pyrethrum Dust.