## 2 Wood-Based Panels
### Emission Issues and Legislation  
Panels produced with formaldehyde-based adhesives are prone to release formaldehyde, which is a serious concern as the main applications of WBP are the construction and furniture sectors. In an effort to regulate the impact of these emissions, several countries impose emission limits on different types of panels. According to the European Standard EN 13986:2004+A1:2015 [15], panels with formaldehyde-containing materials as part of their production process must be tested and classified into the E1 or E2 categories, according to their formaldehyde content or emission. The formaldehyde content for panel classification is determined by the perforator method (ISO 12460-5) [33], while formaldehyde emission is determined by the chamber method (EN 717-1) [34]. To be classified as an E1 product, the formaldehyde content in the panel must be less than or equal to 8 mg formaldehyde/100 g of dry board, and formaldehyde emission must not exceed 0.124 mg\(\cdot\)m\({}^{-3}\) air. A panel with a formaldehyde content higher than 8 mg but not greater than 30 mg per 100 g of dry board or, formaldehyde emissions higher than 0.124 mg\(\cdot\)m\({}^{-3}\) air, is classified as an E2 product. In 2023, the European Commission established a new class E0.5 with a maximum emission limit for formaldehyde in wooden articles and furniture at 0.062 mg\(\cdot\)m\({}^{-3}\) air (half of the emission limit of class E1), which will be introduced in a revised version of EN 13986:2004+A1:2015 and will become compulsory from July 2026 [35]. The Environmental Protection Agency of the United States of America (USA) issued limitations on formaldehyde emissions in different WBPs in accordance with the California Air Resources Board, based on a large chamber method ASTM E 1333 [36]. After 2018, formaldehyde emissions in WBP produced and/or circulating in the USA must not exceed 0.05 ppm (0.061 mg\(\cdot\)m\({}^{-3}\) air) for hardwood plywood panels produced with a veneer or composite core, 0.11 ppm (0.135 mg\(\cdot\)m\({}^{-3}\) air) for MDF, 0.13 ppm (0.160 mg\(\cdot\)m\({}^{-3}\) air) for thin MDF and 0.09 ppm (0.111 mg\(\cdot\)m\({}^{-3}\) air) for PB [37]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Resin System** & **Applications** & **Advantages** & **Disadvantages** \\ \hline \multirow{4}{*}{UF} & PB & Inexpensive; Fast curing; & Low resistance to water; \\  & MDF & Adaptable to curing conditions; & Emission of formaldehyde at \\  & Plywood & Water soluble; Hardness; & slow rates. \\ \hline \multirow{4}{*}{MF \& MUF} & Exterior or semi-exterior & Moisture tolerant; Durable; & Expensive; \\  & plywood & Less susceptible to release & Slow curing. \\  & PB & formaldehyde. & \\ \hline \multirow{4}{*}{PF} & Construction plywood & Resistant to water; & Expensive; \\  & OSB & Durable; Fast curing; & Dark colored when cured; \\  & & Low formaldehyde release. & Reduced mechanical properties. \\ \hline \multirow{4}{*}{pMDI} & PB & Moisture tolerant; & Expensive; \\  & OSB & Fast curing; Durable; & Adheres to metal; \\ \cline{1-1}  & MDF & VOCs and formaldehyde-free. & Potentially carcinogenic \\ \cline{1-1}  & & & (when uncured). \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of applications, advantages, and disadvantages of the most used synthetic thermosetting resins. Adapted from [28; 29; 30; 31; 32]. In 2021, China updated its regulation for the classification of indoor wood-based panels and their products in accordance with its formaldehyde emissions in GB/T 39600-2021 [38]. The Chinese E1 corresponds to the European E1 (0.124 mg\(\cdot\)m\({}^{-3}\) air), while the Chinese E0 refers to panels with formaldehyde emissions higher than 0.025 mg\(\cdot\)m\({}^{-3}\) air but not exceeding 0.050 mg\(\cdot\)m\({}^{-3}\) air. Grade ENF is used to classify panels with formaldehyde emissions equal to or lower than 0.025 mg\(\cdot\)m\({}^{-3}\) air [39].