# Hazards Resulting from the Burning Wood Impregnated with Selected Chemical Compounds
## 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). \begin{table}
\begin{tabular}{c c c c} \hline \hline  
The European wood preserving industry produces annually about 6.5 million m\({}^{3}\) 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 [PERSON] 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.