Hazards Resulting from the Burning Wood Impregnated with Selected Chemical Compounds

Abstract

In the construction industry, a variety wooden products have been used for thousands of years, according to demand, accessibility/availability, and customers’ requirements. Wood is a preferred material due to its large range of properties, depending on the type of wood. It is an easily available and economically competitive material, and it is also extremely strong in relation to its weight. Therefore, it is used in the production of construction materials, building parts, and finishing components, as well as for furniture and decorative elements. Each of these products is commonly additionally chemically treated in order to improve its performance parameters. However, impregnated wooden products such as furniture and fence boards are often misused, including for house heating, waste incineration, bonfires, etc. For this reason, among the products of combustion, there is a whole range of different chemical compounds, frequently carcinogenic, and dangerous for health and the environment, for example, heavy metals. Knowledge in this field is important for professions, such as: firefighter, lifeguard, people dealing with environmental management, and units responsible for waste landfills. On the other hand, important recipients of this information are ordinary residents who, due to a lack of knowledge, use such materials as e.g., heating material.

1. Introduction Wooden products are used in every aspect of life. They are used as building and decorative materials, an energy source, or a starting material for the production of other elements. Depending on demand, various types of wood are used, which can additionally be chemically treated.

Wood is an organic material, and it is exposed to many harmful biotic and abiotic factors, such as fungi, insects, termites, and external conditions, including damage by water, UV radiation, and fire. Some applications require additional wood protection in order to protect wooden material from these harmful effects, and to extend its service life [1,2]. In addition, the increasing demands placed on products in their field of use, including, e.g., durability, colours, and the possibility of using them for various purposes, mean that products made of wood and appropriately modified, including impregnated goods, are becoming increasingly important on the market. Industrial treatment with protective chemicals compounds is the most-common method of protecting wood from damage. The chemicals used penetrate the wood, which extends the life of the wood and wooden products [1,3,4,5]. However, it should be noted that the compounds that are used for the impregnation and protection of wood and wood-type products are subject to the legal regulations in force in any given area. In the case of European Union countries, the legal basis in this regard is:

• Regulation (EC) No 1907/2006 of the European Parliament and of the Council of 18 December 2006 on the registration, evaluation, authorisation and restriction of chemicals (REACH) covered by the European Chemicals Agency, the changing requirements of 1999/45/EC and repealing Council (EEC) No 793/93 and Commission (EC) No 1488/94, as well as Council Directives 76/769/EEC and Commission Directives 91/155/EEC, 93/67/EEC, 93/105/EC and 2000/21/EC,
• Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on the classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC and amending Regulation (EC)) No. 1907/2006, and
• Regulation (EU) No 528/2012 of the European Parliament and of the Council of 22 May 2012 concerning the making available on the market and use of biocidal products.

The requirements in this regard are important due to the possibility of contact during machine impregnation or independently, by a person. Wood preservatives belong to the group of 23 different types of biocides specified in The Biocides Directive [6]. In the group of active substances are admitted, among others 4,5-dichloro-2-octyl-2H-isothiazol-3-one (DCOIT), alkyl (C12-16) dimethylbenzyl ammonium chloride—C12-16 ADBAC, basic copper carbonate, boric acid, boric oxide, DDAcarbonate, hydrogen cyanide, disodium tetraborate, copper(II) oxide, copper hydroxide, and creosote. However, biocidal products can affect not only harmful organisms, but also humans, the environment, and endangered species. The active substances can be carcinogenic, toxic to reproduction, or disruptive of the endocrine system. Children and pregnant women are particularly vulnerable [6]. In addition, each country has its own internal rules and regulations, including requirements for approvals, certification, and products’ technical approvals to which manufacturers have to comply. The requirements relate to issues of the safe use of agents, their stability, and the reactivity, quantitative, and qualitative characteristics, and toxicity.

2. Chemism of Wooden Materials

Wood consists, by total weight, of over 99% organic substances, including cellulose, lignin, and hemicellulose. Absolutely dry wood, on average, contains 49% carbon, 44% oxygen, 6% hydrogen, and 0.1–0.3% nitrogen [7]. The rest comprise inorganic compounds, consisting of calcium, potassium, sodium, magnesium, and other elements. Polysaccharides, such as cellulose and hemicellulose, and lignin, belong to biopolymers, with varying degrees of polymerisation. Thus, they are characterised by different properties, both chemical and physical. Cellulose creates microfibres, among which are lignin, hemicellulose, and water.

In addition to basic organic substances, natural wood, depending on the species, also contains a relatively small amount of extractive substances, such as tannins, resins, pectins, fats that are soluble in water, alcohol, and ether. The pine and spruce walls of wooden houses contain active substances, such as phytocides, which can protect humans against fungi, bacteria, and viruses, and, thus, against infectious diseases [8].

Diverse requirements in the scope of the quality and durability of wood, as well as the diversity of wood species which require various modes of handling, mean that a whole range of different substances are available on the market. Of additional important significance is the possibility of using wooden products inside or outside, in industrial plants during technological processes in the production of a given product, or individually, during usage. For example, salt impregnations are used for protection against moisture, UV rays, and pests to protect the structural wood against insects, fungi, and fire; solvent preparations and colouring impregnating agents are fungicides and insecticides; while, water-borne impregnations allow for protection against moisture, insects, and microorganisms. In order to improve properties in the field of the reaction to fire there are used, e.g., boron compounds, while, to protect wood against fungi, insects, and termites, active substances, such as copper and chromium, are employed (Table 1).

The European wood preserving industry produces annually about 6.5 million m³ of pressure-treated wood, of which almost 1.5% is garden wood, 21% construction wood, 15% small round wood, and 6% mudsills, according to the report that was presented by Salminen et al. [23]. When considering the different types of impregnation, it was found that 71%, at most, of water-soluble products are involved, and definitely less, because only 18% are solvent-based products. As many as 11% of these products are impregnated with creosote [23]. It should be noted that these substances must be used in accordance with the applicable EU standards, which specify five classes of the use of impregnation agents to guarantee the durability of products [23]. According to these standards, impregnation agents in, for example, Class 1, may be used in situations in which wood or a wood-based product is covered, and not exposed to weather conditions and soaking. In the case of impregnations of Class 2, they may be used for products under cover and not exposed to weather conditions, but where high humidity in the environment can lead to sporadic, but not permanent, wetting. Class 3 impregnation should be used when the product is not covered, and it is not in contact with the ground and, thus, is exposed to weather conditions, or is protected from weather conditions, but can get wet. As for Class 4 impregnation agents, they are used when the product is in contact with soil or freshwater, and therefore is permanently exposed to wetting. Class 5, however, should be used when the wood or a wood-based product is permanently exposed to salt water [23,24,25]. Flame retardants should improve their fire properties without decreasing the performance of the material. An effective flame retardant must have significant ignition resistance, contribute to reducing the intensity of combustion, and reduce the rate of smoke generation; and, combustion products should have as low a toxicity as possible. The features and appearance have to be suitable for the area of application and should not significantly affect the cost of the product [1,25,26].

Substances, such as ammonia salts, phosphorus, and boron compounds, are added to reduce the flammability of wood. Additives are implemented to change the mechanism of the pyrolysis process. Cellulose, under ideal conditions, decomposes into coal and water, and the addition of the appropriate agents lowers flammability by reducing the amount of burning pyrolysis products, thereby reducing the amount of heat released by the product. The additives react with the C6 cellulose hydroxyl group, which leads to the formation of a C5 = C6 double bond. Reactions occur through dehydration or esterification processes. Flame retardants can also slow down pyrolysis reactions and stabilise the chemical structures of wood against decomposition, such as aluminium sulphate, which, when added to wood, forms bonds between the cellulose molecules at elevated temperatures, thus preventing thermal decomposition [1,27].

Esmailpour et al. [28] tested, for fire properties, i.e., the time to the onset of ignition, the time to the onset of glowing, the back-darkening time, the back-holing time, the burnt area, and the weight loss, involving samples of beech wood impregnated with graphene or nano-wollastonite (NW), using water-based paint. The research was conducted for improving the effects of graphene on the times to the onset of ignition and glowing. Graphene is characterised by a very low propensity to react with oxygen, and high and low thermal conductivity in the plane and cross-section. Thus, graphene has great potential for use as a flame retardant in solid wood species [28].

It should be noted that, depending on the chemical impregnation, we produce a different result in relation to the individual properties of wood and wood-like materials, including flammability, and even the combustion process itself. Paraffin, styrene, methyl methacrylate, and isocyanate, all materials that increase dimensional stability and improve hydrophobic efficiency, affect the flammability of wooden products, resulting in an enhancement to this parameter [1,29,30]. Impregnates, such as TiO₂, WO₃, or CaSiO₃, penetrate the structure of the wood and fill the pores and areolate pits, which affects both the amount of water absorbed in the equilibrium state and the kinetics of water sorption [31]. However, it should be added that fire-resistant chemicals have some negative impact on the physical and mechanical properties of wood materials [1,29]. One of the substances used to impregnate wood is creosote, a mixture made of coal tar, consisting of inter alia, compounds from the group of phenols, cresols, and xylenols, in various ratios, depending on the production process used [32]. Creosote is commonly used in railway factories and utility poles. Copper particles that are found in impregnating agents, such as Micronised Copper Azole (MCA) and Micronised Copper Quaternary (MCQ), are so small that they fill small holes in the wood structure, and accumulate in the wood, and do not chemically bind [33]. Research conducted by Platten et al. [33] showed that MCA-treated wood contained copper, primarily in the form of copper carbonate. However, it might also be present in other forms, including organocopper complexes, or in the form of particles of varying sizes [33], which affects their chemical and biological activity.

Alternatives to chemicals that are used to reduce the flammability of wood include natural and environmentally friendly materials, industrial by-products, and agricultural and food waste. Intumescent coatings containing bio-fillers, bio-based substances, such as ginger and coffee husks, egg shells, molluscs, tea saponin, and organically modified montmorillonite (MMT), are being developed [34].

3. Flaming and Smouldering Combustion Processes

Wooden and wood-like, wood-based, products emit various compounds into the environment, the composition of which depends on the type and chemical composition of the material, as well as external factors, including temperature, oxygen access, and the presence of other substances, such as radicals and catalysts. All of these elements determine the type of combustion process, which can include processes, such as smouldering (flameless combustion), or combustion with the production of flame (flame combustion). Flameless combustion, e.g., smouldering, is one of the slow processes occurring under relatively low temperature conditions, and it is the most persistent type of combustion phenomenon, characterised by the absence of a flame, and is therefore a threat to safety and the environment. Smouldering is one of the main causes of death in apartment fires, and it is a source of safety concerns in workplaces and other situations in which biomass and peat are burned, which causes environmental degradation [35,36,37,38,39]. As smouldering is a slow and sustained process, smouldering fires can lead to increased heat transfer and pollutants’ entering the soil over a much longer period of time [37,38,39]. In the case of a smouldering front’s moving in the direction of the oxidant flow, the fresh oxidant flows through the charred layer and reacts in the ignition zone, which causes oxidation reactions to occur at the rear of the ignition zone, and pyrolysis at the front. In the reverse situation, the oxidant travels through the primary fuel and reacts in the smouldering zone. As a result, both oxidation and pyrolysis reactions occur approximately in the same place [40]. Both smouldering and flame-burning combustion have a genesis from the same process as pyrolysis.

However, flameless combustion is the heterogeneous reaction of a combustible material with an oxidant, while flame combustion is a homogeneous reaction of a gaseous fuel with an oxidant that releases more heat. It should be remembered that, for each solid material, both smouldering and flame combustion can occur, and one process can also lead to another [35,36,37]. Under specific conditions, rapid oxidation can develop, and in a very short time, i.e., an explosion. Flameless combustion refers to a combustible material in a solid state, e.g., wood, and generally proceeds at lower temperatures and at a lower speed. Among such substances, the products of partial carbon oxidation predominate in comparison to the composition of the products of flame combustion. Flame combustion, on the other hand, is associated with the combustion process of the flammable volatile phase, and takes place during the combustion of substances that turn volatile during heating. This phenomenon is mainly characteristic for organic materials that decompose due to increases in temperature and produce flammable vapours and gases. Burning gases and vapours above the surface of combustible material create a flame. The combination of combustible material with oxygen is preceded by the thermal decomposition of molecules into atoms that are easier to react. The materials that contain organic carbon are burnt, but, depending on the conditions, it can be initiated by the appropriate external sources of ignition, e.g., an open flame, a spark, a hot surface, or the spontaneous ignition of the material. Inflammation refers to the even heating of the combustible material to a temperature at which it will spontaneously ignite in the whole mass, without the participation of a so-called point energy stimulus.

In the case of ignition, there is reference to igniting the combustible mixture with a point energy stimulus [41]. This process takes place in a limited space, with the flame front moving automatically on to the rest of the material, and this also applies to flammable liquids.

The last type of smoking conditioning factor is self-ignition, which is an exothermic process occurring as a result of biological, physical, or chemical changes. The heat thus created causes the material to ignite. Among the most frequently analysed substances in the context of fires are wooden materials, which, under the influence of temperature increases (pyrolysis), undergo thermal decomposition emitting large amounts of volatile substances. A delicate carbon coating forms on the surface of the wood, which is characterised by incandescence. The combustion process for wood strictly depends on its composition, construction, and fragmentation. What is important, dusts can burn with flames, flameless combustion and, in the event of a detonation, also explosively [42].

Subject to the composition of the material, different amounts of heat are emitted, which affects the stage of the combustion process. Various products can also be formed that determine the subsequent combustion process. Contingent on the presence of compounds, melting, evaporation, decomposition, oxidation, inflammation, or smoking might occur [42]. Compounds are emitted that have a different chemical nature and biological activity and, thus, various kinds of impact on humans and the environment, depending on the stage. Scattered small gaseous and solid particles result from the combustion of organic materials, giving their characteristic colour, smell, taste, density, and toxicity, and their ability to penetrate and move in the environment and create smoke. In the case of the same wooden products, but impregnated with other chemicals, other substances, more or less toxic, will be discharged into the environment. Therefore, smoke in blue, white, or yellow colours, with a bitter or sweet taste, indicates the presence of poisonous substances. Combustion products include volatile combustion substances, such as carbon oxides, methane, hydrogen, hydrogen sulphide, and sulphur dioxide, and solid combustion products, such as soot, ash, and slag, which differ in composition and properties.

The emission of wood combustion products and chemicals used to impregnate wood and wooden products negatively affect the air quality, causing environmental degradation and a threat to the health of humans and other organisms. According to the European Environment Agency, outdoor air pollution is the greatest threat to the populace, contributing to around 400,000 premature deaths in Europe each year [43]. Energy poverty is often the main factor in the combustion of wood and wooden products impregnated with various chemicals in low-efficiency furnaces for heating homes. This type of situation leads to high exposure of the low-income population to particulate matter (PM) and polycyclic aromatic hydrocarbons (PAH) [43], as well as other compounds that result from the combustion of impregnating substances at low temperatures, such as heavy metals. Therefore, impregnated wood and derived products should not be incinerated in uncontrolled conditions, but they must undergo the appropriate processes, including segregation and recycling/disposal.

Impregnated wooden products, due to additives, such as heavy metals, like As and Cu, or carcinogenic compounds, such as creosote oil, and some polycyclic aromatic hydrocarbons, constitute hazardous waste, and they must be covered by measures that are addressed to hazardous waste. One example is the waste that is generated during the modernisation of railway lines, such as impregnated railway sleepers. Wood obtained from such waste can be managed through storage in places suitable for hazardous waste, through incineration or other chemical or biological treatment. However, it cannot be used indoors, it must not come into contact with the skin [44].

It should be noted that used and waste wood impregnated by various chemical compounds is classified as hazardous waste and it requires appropriate handling. Combustion is only possible in properly prepared installations due to the emission of hazardous substances. In every country as well as special protection areas, such as recreational zones, special protection areas, and cross-border regions, are covered by guidelines for dealing with hazardous waste. For the European Union, the classification of waste is based on the European List of Waste (Commission Decision 2000/532/EC—consolidated version) and Annex III to Directive 2008/98/EC (consolidated version). The properties that make waste hazardous are laid down in Annex III of Directive 2008/98/EC, and they are further specified by Decision 2000/532/EC, establishing a List of Waste, as last amended by Commission Decision 2014/955/EU [45]. On the other hand, the Environment Agency published Guidance: Classifying waste wood from mixed waste wood sources: RPS 207 in May 2020, which states that treated waste wood is any waste wood, processed wood, or wood fuel that contains, in any quantity, wood that has been preserved, varnished, coated, painted, or exposed to chemicals [46]. However, the emission of combustion products of wood impregnated with various chemical compounds is related not only to the inappropriate handling of wood as waste. The risk is also associated with situations of uncontrolled combustion, such as fires. Additionally, the next chapter in this study shows why it is so important to properly handle this type of material.

4. Emission of Pollutants and Methods of Measurement

The characteristics of emitted pollutants generated during the combustion of impregnated wood depend on the type of impregnate and the burning conditions. As is known, much larger amounts of toxic gases, including CO, are emitted during the smouldering combustion process of non-treated wood when compared to the flaming combustion of such wood [47]. Karpovic et al. [48] performed detailed toxicity tests based on measurements of CO release during smouldering and flaming pine timber, testing both impregnated fire retardant and non-impregnated samples. It was found that, during the smouldering, combustion of treated pine timber CO emission was higher in the initial seconds of the tests as compared to the emission of CO from the non-treated smouldering pine timber. During the tests, the amount of CO that evolved from impregnated samples varied slightly, while the CO emissions from non-impregnated samples increased markedly. In addition, when comparing the results of measurements for pine timber treated and non-treated, it can be stated that the total CO emission from treated pine timber was more than four times higher when compared to the results that were obtained for non-impregnated samples. During incomplete wood burning, in addition to CO, other combustion products—methanol, formaldehyde, and acetic acid—are released, as well as more complex products derived from the depolymerisation of the lignocellulosic structures of wood [49]. Depending on the type of wood, polycyclic aromatic compounds (PAH) [50,51], polychlorinated biphenyls (PCB) [52], polychlorinated dibenzo-p-dioxins (PCDD), and polychlorinateddibenzofurans (PCDF) [51,52,53,54] can also be released.

According to legal requirements, impregnates used to protect wood, especially those exposed to weather conditions, should not emit toxic products during thermal decomposition at high temperatures [55]. However, the composition of pollutants released during the impregnation of wood is usually different from those of non-impregnated wood. Depending on the type of impregnate, various reactions can take place during combustion processes, including those catalysed by metal ions and atoms contained in the impregnate, especially those that are intended to protect against microbial and fungal attack. The effect of the various combustion conditions during flaming and smouldering combustion processes of impregnated wood on the composition of combustion products is evident.

Until the end of the 20th century, wood impregnates based on chromium and arsenic were used. The situation changed with the advent of regulations prohibiting the use of arsenic compounds for wood impregnation [4,56,57]. However, the problem of utilising wood impregnated with such impregnates and, above all, chromated copper arsenate (CCA), still remains.

The chemicals used for preservation are relatively simple; however, inorganic reactions occurring during the wood preservation process contribute to the formation of complex inorganic compounds and complexes [58,59]. Helsen et al. [60] found that pure As₂O₅ aq does not decompose or volatilise at temperatures below 500 °C. However, arsenic is already released at 320 °C due to the pyrolysis of CCA-treated wood. It was also found that, although arsenic is present in wood in a five-valued state, As(III) is present in the pyrolysis residue. The presence of wood, charring, and pyrolysis vapours thus influences the thermal behaviour of nitrogen oxides [60].

Burning wood impregnated with arsenic compounds causes the release of arsenic in quantities, depending on the conditions of this process. McMahon et al. [61] report that 13–27%, 22–44%, and 70–77% of arsenic is released at temperatures of 400, 800, and 1000 °C, respectively [61]. Similar values were confirmed by other authors [62,63]. Kakitani et al. [64], based on the detailed pyrolysis of wood impregnated with CCA, found that, depending on the seasoning of such wood waste, there are two modes of arsenic release. CCA impregnated samples, dried for 21 days at room temperature, and ground to a particle size below 20 mesh, were pyrolysed under an N₂ atmosphere in the temperature range 135 to 500 °C, and in time from 0 to 60 min. In addition, part of the ground wood was annealed at 60 °C in order to achieve the complete conversion of arsenic compounds, leading to arsenic immobilisation in the wood [64].

Before the pyrolysis, it was confirmed that both of the wood samples contained the same amount of arsenic. The incorporation of CCA into the wood structure was accompanied by a reduction of Cr⁺⁶ to Cr⁺³, followed by a further reaction of reduced chromium with As₂O₅. As a result of this reaction, the poorly soluble CrAsO₄ salt was formed [65,66]. The described process was not gone through completely, and in seasoned wood some unreacted As₂O₅ might have remained, which would have been converted to As₂O₃ in the initial stage of pyrolysis [58]. In annealed wood, all of the arsenic is present in the form of CrAsO₄, decomposing into As₂O_5, and reduced during pyrolysis to As₂O_3, with its subsequent separation as As₄O₆, at a temperature of about 400–500 °C. In order to reduce the release of arsenic during pyrolysis, CCA-impregnated wood should be carefully pyrolysed at temperatures around 300–350 °C only if the content of the unreacted arsenic compound is low. In the following years, CCA-impregnated wood utilisation methods were developed to reduce arsenic release through low-temperature pyrolysis [67,68,69], and by the incorporation of a sorbent compound [70].

Keskin et al. [71] found that the type of impregnation determines the method of combustion, including the burning time and the presence or absence of a flame or glow; thus, also the products of combustion. The wood test samples, which were prepared from rowan wood materials, were impregnated with Tanalith-E, Vacsol-Azure, Imersol-Aqua, and Boron compounds (Borax and Boric acid). The burning time ranged from 4.112 to 6.888 min. for samples impregnated in the sequence Vacsol Azure, Tanalith-E, Boricacid, Imersol Aqua, and Borax, and at a 3.110 min. burning time for the control samples. The highest combustion temperature was obtained for materials that were impregnated with Imersol Aqua (458.686 °C), and the lowest with Borax (439.023 °C). It was also found that boric acid reduced material losses during combustion, which indicates that boron impregnation increases the combustion temperature and provides additional fire resistance and safety [71].

The most effective preparations of refractory wood-based materials contain halogens. Thus, combustion can produce toxic and irritating gaseous products [34]. Aqlibous et al. [34] conducted research on the flammability and combustion of softwood treated with intumescent coatings that contained different ratios of industrial fillers, TiO₂ and Al(OH) ₃, and/or bio-fillers, eggshells, and rice husk ash. The flame retardant effect of the samples is the result of the decomposition of the additives used, as in the case of Al(OH)₃, from which water vapour and Al₂O₃ are released. The resulting aluminum trioxide contributes to the formation of a protective layer, promotes the oxidation of carbonising carbon, and increases the production of water vapour, carbon dioxide, and carbon monoxide. The amount of emissions varied, depending on the ingredients used in the coatings and the heat flux to which they were exposed [34].

Inorganic salts, such as (NH₄)₂HPO₄ and K₂HPO₄, when wood is doped with these, reduce the intermolecular interactions and the interactions between the chains, and change their crystallinity. However, all of the ammonium salts are possible sources of ammonia [72].

A number of alternative impregnates have been developed for wood treatment: copper boron azole (CBA), alkaline copper quaternary salts (ACQS) [73], and chlorinated pesticides, which are analogues of the naturally occurring compound pyrethrum in certain plants of the aster family, especially in Chrysanthemum cineraria folium. Protective impregnates containing these substances are approved for use. However, studies on the effects of these preparations on the emissions of combustion products have shown that they can contribute to emissions dioxins and furans—see Figure 1.

Fires in wood impregnated with copper-based preservatives can increase the amount of PCDD/F. The formation of PCDD and PCDF during fires is favoured by low-temperature combustion with limited oxygen supply. PCDD and PCDF can be formed by different reactions depending on the degree of copper oxidation and the combustion conditions, as illustrated in detail in Table 2.

The catalytic effect of the Cu(II) ion in the form of CuO and CuCl₂ was the most thoroughly tested. The experimental results indicate that Cu(II) participates in various stages of PCDD/F formation. The effectiveness of CuCl₂ is attributed to the propensity of copper to oxygen coupling, which reduces the temperature of exothermic oxidation by the chlorination of carbon. The catalytic efficiency of a number of CuO, CuSO₄, Al₂O₃, AlCl₃, Fe₂O₃, NaCl, and KCl salts in the formation of C-Cl bonds, and the promotion of carbon degradation, were also tested [79]. It has been found that copper is constantly the most effective metal for catalysing the formation of PCDD and PCDF. The mechanism can be described as the formation of carbon-chlorine bonds, followed by the oxidation of the carbon matrix and volatilisation of chlorinated aromatics [80]. The effect of the deposition of CuO on silica to increase the contact surface, which can have a positive effect on the efficiency of chlorophenol pyrolysis [75], was tested in order to increase the catalytic activity.

Can et al. [1] examined the effect of a substance, called Firetex, on improving the fire resistance of copper-impregnated materials, i.e., ACQ and CuA. Samples of fir (Abiesnordmanniana subsp. Bornmulleriana) were treated with copper azole (Tanalith E-3492) and copper-ammonium acid (ACQ) at a concentration of 2.4% and Firetex (FT) at a concentration of 100%, in five different samples, which were characterised by different ratios of individual impregnation agents. The obtained test results showed that the highest mass reduction—of up to 100%—was observed for the unimpregnated control sample, and the sample impregnated with ACQ and CuA compounds, whereas the lowest was for the sample only impregnated with Firetex (17.15%). The highest temperature (479.63 °C) was observed for samples that were impregnated with CuA. In addition, impregnation with Firetex, by using the full-cell method, facilitated a reduction in the temperature by an average of 80% [1].

It was also found that the critical factor in the formation of PCDD/F in the combustion of wood was the temperature at which combustion took place—see Figure 2.

The formation of PCDD and PCDF is favoured at low temperatures conducive to smouldering, especially in the case of reduced oxygen access from the air.

In conclusion, it can be stated that the composition of wood-burning products strongly depends on the temperature. In various publications, the impact of wood-impregnation agents was not taken into account, because, when fully burnt at high temperatures, the impregnates decompose, and copper and other metals remain in the ashes that result from the combustion [81]. On investigating the impact of combustion conditions on the release of toxic products, it was found that, when burning at lower temperatures, the volatile products which could be formed were: substituted benzenes and phenols, and lignans leading to the formation of phenol and dibenzofuran. PCDD/F can also be released, but in significantly smaller amounts. However, in the case of wood containing pesticides, chlorinated aromatics, and copper-based impregnates, the formation of PCDD/F under fire conditions could be much more intense.

In addition, the composition of products determined as a result of wood burning also depends on the methods of measurement [82]. For this purpose, a number of measuring methods and techniques are used, including thermogravimetric analysis, cone calorimetry, and the single burning item test. The results of such tests are often highly dependent on various parameters, including changes to the gas composition, temperature, heating rate, and sample shape size. Thermogravimetric analysis, differential thermal analysis, cone calorimetry, lateral ignition, and the flame spread test (LIFT) are the most commonly used methods. For the tests necessary to classify the resulting smoke and its toxicity by large-scale calorimetry, as well as a steady-state tube furnace [83,84] and an NBS smoke density chamber [85], are used.

5. Conclusions Impregnated wood is commonly used in building interiors, both as a building material and as finishing, decorative, and utility elements. Impregnation agents contain in their composition organic compounds, carboxylic acids, esters, and inorganic compounds, including mainly heavy metals, such as Cu, Zn, and Cd. The variety of impregnating compounds means wooden and wood-based products may be used in various conditions, outside and inside buildings. They reduce the risk of fire hazards in standard conditions by changing the pyrolysis process and reduce material degradation under the influence of water, sunlight, microorganisms, or other factors. However, this diversity determines the potential risk in the event of using impregnated wood as an energy material, or if there is a fire. Compounds that are added to wood as a result of high temperatures undergo thermal changes, releasing toxic carcinogenic compounds. It should be noted that there are not enough research results in the literature that would allow the conclusion that the issue of the effect of burning wood material, depending on the substances used for impregnation, has been thoroughly understood. However, it is necessary to gather knowledge of the mechanisms of thermal degradation, the combustion efficiency of impregnated-wood material, and the volume of emissions of combustion products due to the potential threat to humans and the environment in the event of fire from impregnated-wood materials. Knowledge in this field will facilitate the development of the necessary tools to increase security and take the appropriate precautions. The knowledge about chemical compounds, the combustion conditions, and amounts of emission, as well the effect of these compounds on humans and the environment, is indispensable. It allows for proper preparation of a rescue operation, securing, and developing protective measures that minimize the risk.

Figure 1. The effect of impregnates chemistry on dioxins and furans emissions.

Figure 2. The effect of combustion temperature on PCDD/F formation.

Name of the Chemical	Chemical Composition	Application	Ref.
Altax impregnation for structural wood	(2-methoxymethylethoxy) propanol,(RS)-1-[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-ylmethyl]-1H-1,2,4-triazole,3-Iodo-2-propynyl butyl carbamate, zirconium 2-ethylhexanoate, permethrin (PN)	Protects wood against biocorrosion: mold, house fungi, blue stain, and insects feeding on wood; outside and inside	[9]
Altax wood oil	Naphtha (petroleum), hydrotreated heavy; low-boiling oil fraction treated with hydrogen; 2-butanone oxime; zirconium 2-ethylhexanoate	Protects wooden furniture, floors, terraces, platforms, bridges against weather conditions	[10]
IMPRAPOL PQ40	Cu(OH)2:CuCO3 (1:1), ethanolamine, alkyl dimethyl benzylammonium chloride, boric acid	For industrial protection of wood against mold, house fungi, technical insects, wood pests, atmospheric factors, hazard classes I, II, III and IV (ground contact); the preparation is solidified in the wood	[11]
PENETRIN	Hydrocarbons, C9-C11, n-alkanes, cyclic isoalkanes, <2% aromatic hydrocarbons, hydrocarbons, C15-C20, n-alkanes, isoalkanes, cycloalkanes, <0.03% aromatics, (1RS)-cis, trans-3—3-phenoxybenzyl((2,2-dichlorovinyl)-2,2-dimethylcyclopropanecar-boxylate,(RS)-1-[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-ylmethyl]—1H-1,2,4-triazole,3-iodo-2-propynylbutyl carbamate,2-butanone oxime,1-methoxypropan-2-ol	Solvent wood impregnation, protection against fungi and insect larvae; for terraces, facades, log houses, gazebos, roof, and floor constructions	[12]
Cabinet & Wood Cleaner -3063	Petroleum distillates, hydrotreated light, butane, propane	Liquid for removing dust, grease, dirt from wardrobes, furniture, doors. and other wooden surfaces	[13]
AUSPLAST	Sodium fluoride, paraffin, inert filler—calcium sulphate, thixatropes	Wood preservative to protect timber structures from wood decay.	[14]
CREOSOTE	Creosote	Biocide, wood impregnation, wood preservation (for outdoor use)	[15]
ACQ Pressure-Treated Lumber	Monoethanolamine, copper complex expressed as copper oxides, didecyl dimethyl ammonium carbonate/bicarbonate	Compound to protect wood from decay	[16]
MCQ Treated Wood—Other	Copper carbonate, expressed as copper oxide, didecyl dimethyl ammonium carbonate, and didecyl dimethyl ammonium bicarbonate	Preservative-Treated Wood for various exterior applications, including above ground, ground contact, and fresh water exposure	[17]
Topcoat	Nitrocellulose	Topcoat can be used for many interior wood applications as a high-quality nitrocellulose lacquer. This product may be used on all types of wood	[18]
Vidaron	Naphtha treated with hydrogen (petroleum), xylene, tebuconazole, tolilofluanid, permethrin	Protect them from the harmful effects of the agent in biological and atmospheric conditions, and the damaging effects of moisture. Element used for painting external and internal carpentry, rafters, battens, roof trusses, wooden claddings of buildings, arbors, fences, etc.	[19]
ROXIL—10-YEAR WOOD PROTECTOR	A mixture of 5-chloro-2-methyl-2h-isothiazol-3-one [EC No. 247-500-7] and 2-methyl-2h-isothiazol-3-o	Waterproofing liquid for wood;reduced organic growth; reduction of water uptake; improved dimensional stability	[20]
Zeroflame Fire-Retardant Treatment	Ferric(III) phosphate; citric acid; polyoxyethylene (21) stearyl alcohol; water	Achieves Euroclass B (BS Class 0) fire propagation and the spread of flame-fire protection on solid timbers; for internal and external use	[21]
MERIT ZIRCON EXTRA 25	Butyl acetate, ethanol, nitrocellulose, urea formaldehyde resin, melamine formaldehyde resin, isobutanol, isopropanol	Lacquer for both sealing and top lacquering; the lacquer may be used for furniture, doors, and other wooden surfaces	[22]

Reaction Type	Catalyst	Reference
Heterogeneous condensation of 3-chlorophenol, 3,4-dichlorophenol, and 2,4,6-trichlorophenol	CuCl2	[74]
PCDD formation from 2-chlorophenol according to the Eley-Rideal mechanism, and PCDD/F formation according to the Langmuir-Hinshelwood mechanism	Copper oxide	[75]
Pyrolysis chlorophenols with PCDD and PCDF formation	CuO dispersed on silica surfaces	[75]
Reaction of carbon, oxygen, and chlorine sources	Copper or iron ions	[76]
Chlorination of dibenzofuran and dibenzo-p-dioxin vapour at temperatures between 200 and 400 °C	CuCl2	[77]
Dechlorination of higher homologues on heating under oxygen-deficient conditions	CuO	[78]

Author Contributions

A.R.: My individual contribution is the concept development and the development of a large part of the material for the chapter: "Chemism of Wooden Materials" as well as such parts as: Introduction, Conclusions. I have read and agreed to the published version of the manuscript. M.Z.: My individual contribution is the development of a large part of the material for the chapter: "Emission of pollutants and methods of measurement". I have read and agreed to the published version of the manuscript. D.M.: My individual contribution is the development of a large part of the material for the chapter: "Flaming and Smouldering Combustion Processes". I have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education as part of a granted subsidy for maintaining research potential in CNBOP-PIB-research work No. 025/BW/CNBOP-PIB/MNiSW "Reaction to fire tests for construction products, interior finishing materials and cables".

Conflicts of Interest

The authors declare no conflict of interest.

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