The Emission Characteristics of VOCs and Environmental Health Risk Assessment in the Plywood Manufacturing Industry: A Case Study in Shandong Province

Abstract

The current emission characteristics of volatile organic compounds (VOCs) in the plywood manufacturing industry are not yet clearly understood, and their impact on occupational health warrants attention. This study examines VOC concentrations in adhesive-coating and hot-pressing workshops, aiming to discern the emission characteristics and evaluate the health risks to workers. The calculated VOC emission factors range from 1.5 to 3.6 g/m³ for plywood, and an average total VOC concentration of 954.17 μg/m³ is observed. Hot pressing (336.63 μg/m³) and adhesive coating (276.24 μg/m³) substantially contribute to organized and unorganized emissions, respectively. Oxygenated VOCs (OVOCs) (50.79%) predominate, followed by alkanes (16.22%) and halohydrocarbons (15.81%). Formaldehyde, acetone, and acetaldehyde are most prevalent in organized emissions, while dichloromethane, formaldehyde, and methyl methacrylate are dominant in unorganized emissions. Ozone formation potential (OFP) values range from 905.04 to 1822.35 μg/m³, with notable contributions from formaldehyde, methyl methacrylate, and acetaldehyde. Health risk assessments using the total lifetime cancer risk (T-LCR) values suggest potential cancer risks for identified VOCs, particularly formaldehyde in the hot-pressing process. These findings will contribute valuable insights for regional-scale VOC pollution control and offer guidance for minimizing environmental impact and improving occupational health and safety within the plywood manufacturing industry.

1. Introduction

The plywood manufacturing industry plays a vital role in China’s wood-based panels sector, which has experienced significant growth in recent years driven by rapid socio-economic development and urbanization [1]. However, this growth has also brought about various environmental challenges, particularly in terms of air pollution [2]. Volatile organic compounds (VOCs), considered crucial precursors in the formation of secondary air pollutants, have emerged as a critical concern [3,4]. Notably, certain VOC species, including formaldehyde, benzene, and chloroform, have been identified as hazardous air pollutants (HAPs) and are classified as carcinogens [5,6]. Consequently, comprehending the emission characteristics and associated health risks of VOCs in specific industries is paramount for devising effective pollution control measures and safeguarding public health.

Previous studies have primarily focused on VOC emissions from industries such as petroleum, chemicals, printing, and industrial coatings [4,7,8,9,10,11]. However, the understanding of VOC emissions within the plywood manufacturing industry remains limited. Given that China is the largest global producer of wood-based panels, with the plywood manufacturing sector accounting for over 50% of total production [1], understanding the VOC emission characteristics in this industry is of utmost importance. The total production capacity of plywood manufacturing industry in Shandong Province accounts for over 25% of the national total capacity in this sector [12]. Hence, selecting Shandong Province as a representative region, this study aims to comprehensively assess VOC emission characteristics and conduct environmental health risk assessments within the plywood manufacturing industry of this area.

VOC samples were collected from workshops in two typical plywood manufacturing factories in Shandong. An analysis of a wide range of VOC species, encompassing alkanes, alkenes, alkynes, aromatics, halohydrocarbons, oxygenated VOCs (OVOCs), and organic sulfur compounds (VOSCs), was performed to quantify their concentration levels and chemical compositions emitted from various processes. Additionally, specific VOC source profiles for the plywood manufacturing industry in Shandong Province were established. Localized VOC emission factors were calculated to provide a more accurate estimation of VOC emissions in the region. Subsequently, the health risks associated with VOCs were assessed using the health risk assessment methods recommended by the United States Environmental Protection Agency (US EPA). Furthermore, by conducting the ozone formation potential (OFP) calculations, the key active species contributing to ozone formation were identified. The outcomes of this study will offer guidance for the improvement of current waste gas collection and purification technology, providing valuable insights for regional-scale VOCs pollution control and industry sustainable production, and improving occupational health and safety in the plywood manufacturing industry.

2. Materials and Methods

2.1. Sample Collection

As plywood manufacturing technology has matured and standardized, with consistent production processes and raw material types across enterprises, this study selects two representative companies of different production scales from Shandong Province for investigation. Figure 1 illustrates the typical manufacturing processes in the plywood industry, with detailed descriptions of the production process provided in the supporting information. Notably, the adhesive-coating and hot-pressing processes are critical process nodes that significantly contribute to the emission of VOCs.

During the sample collection process, the sampling points were categorized into unorganized emission sampling points and organized emission sampling points. The sampling points for this study have also been marked on Figure 1. The unorganized emission sampling points included the adhesive-coating and hot-pressing workshops, while the organized emission sampling points were positioned at the inlet and outlet of the exhaust ducts during the adhesive-coating and hot-pressing processes. The detailed sample collection and detection methods are provided in the supporting information. The target compounds in this study encompassed non-oxygenated organic compounds such as alkanes, alkenes, aromatics, and halohydrocarbons, as well as oxygenated organic compounds including aldehydes, ketones, and ethers. These compounds were selected to provide a representative characterization. Specific components are detailed in Table S1.

2.2. Data Analysis

2.2.1. Calculation of Emission Factors

By considering the total annual consumption of raw and auxiliary materials containing VOCs, as well as their corresponding VOC percentages, collective efficiencies, and measured removal efficiencies through specific treatment technologies, a mass balance approach was implemented to estimate VOC emissions. In this study, two calculation methods were utilized to determine the emission factors. The first method involved calculating the mass of VOCs emitted per unit of adhesive consumption, using pollutant emissions and the annual adhesive usage, as outlined in Equation (1). The second method involve calculating the mass of VOCs emitted per unit of plywood production, utilizing pollutant emissions and the annual plywood output, as described by Equation (2). These equations provide a quantitative framework for evaluating VOC emissions, considering both the consumption of adhesive materials and the production of plywood, thus enabling a comprehensive assessment of VOC release within the investigated processes.

$$E{F}_m=E_{out}/M_{in}$$

(1)

where EF_m represents the VOC emission factor per unit of VOC-containing adhesives, kg/kg; E_out is the annual VOC emissions, kg/a; and M_in is the quantity of adhesive used per year, kg/a.

$$E{F}_w=E_{out}/W_{out}$$

(2)

where EF_w corresponds to the VOC emission factor per unit of plywood production, g/m³ plywood; E_out is the annual VOC emissions g/a; and W_out is the annual plywood production, m³/a.

2.2.2. VOC Source Profile Calculation Method

In this study, source profiles of VOC composition were calculated for both organized and unorganized emissions. The data from the same process were averaged and normalized to obtain the source profiles for each process. The emission quantities of each VOC species in different processes were summed, and then the concentrations of each species were normalized to derive the source profiles for organized emissions, unorganized emissions, and the plywood manufacturing industry as a whole. The calculation methods for the VOC source profiles are presented in Equations (3) and (4).

$$C_i=\sum _{k=1}^n{C}_{ik}\times Q_k$$

(3)

$$\eta =C_i/C_0\times 100\%$$

(4)

where C_i denotes the emission concentration of a specific VOC component, μg/m³; C_ik represents the concentration of species i in the process k, μg/m³; Q_ik represents the air flow rate in the process k, m³/h; η represents the percentage contribution of VOC concentration; and C₀ represents the summation of concentrations of all VOC components, μg/m³.

2.2.3. Estimation of Reactivity of OFP-Based VOC Emission Inventory

The OFP is calculated as the product of the emission concentration and its corresponding maximum incremental reactivity (MIR) value reported from the literature [13,14]. The OFP of an individual VOC species is calculated using Equation (5)

$$OF{P}_i=\sum _j{E}_{ij}\times MI{R}_i$$

(5)

where OFP_i is the total ozone formation potential of species i, μg/m³; E_ij is the emission of the species i for source j, μg/m³; and MIR_i is the maximum increment reactivity of species i.

2.2.4. VOCs Health Risk Assessment

For organized VOC emissions, these harmful gases have been collected into the air purification device for treatment, and after purification, they are discharged through the exhaust pipe. The main hazard to workers’ health is the unorganized emission of VOC gas in the workshop. Workers can ingest harmful gases through the respiratory system and thus endanger their health. Therefore, we further assessed the health risks of VOC components from unorganized emissions. In this study, the toxic (non-cancer) and carcinogenic effects of VOCs were assessed through the hazard index (HI) and lifetime cancer risk (LCR) for long-term exposure [15]. The HI is calculated by summing the hazard quotient (HQ_i) for individual VOC species i, representing the non-cancer risk. HQ_i can be determined using Equations (6) and (7).

$$H{Q}_i=E{C}_i/Rf{C}_i$$

(6)

$$E{C}_i=(M{C}_i\times ET\times EF\times ED)/(AT\times 365 \mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}\times 24 \mathrm{h})$$

(7)

where RfC_i refers to the reference concentration for the compound i (mg/m³), indicating the daily inhalation exposure level with no significant health risk throughout a person’s lifetime (Table S2); EC_i represents the inhalation exposure concentration of VOCs, μg/m³; MC_i represents the mass concentration of VOCs measured in workshops, μg/m³; ET and EF denote the daily exposure time (assumed as 8 h per day) and exposure frequency (assumed as 240 days per year), respectively; ED represents the exposure duration in years (assumed as 30 years based on statutory retirement ages in China: 50 for female workers and 55 for male workers); and AT indicates the average life expectancy (assumed as 70 years).

The total lifetime cancer risk (T-LCR) of VOCs is the sum of LCR_i for individual VOC species i, which is calculated using Equation (8) based on EC_i and the upper-bound of inhalation cancer risk (IUR_i).

$$LC{R}_i=E{C}_i\times IU{R}_i\times 1000\times (\mathsf{\mu }\mathrm{g}/\mathrm{m}\mathrm{g})$$

(8)

The IUR data for each VOC species are obtained from reputable sources such as the IRIS, OEHHA, or IARC (Table S2).

3. Results and Discussion

3.1. VOC Emission Factors

The basic production information for both enterprises obtained through investigation is provided in Table S3. The calculations show that the emission factor for enterprise A is 3.6 g/m³ for plywood, and for enterprise B, it is 1.5 g/m³ for plywood. Various factors influence VOC emission factors, including process flow, variations in raw and auxiliary materials, and the efficiency of purification facilities. Although both enterprises use urea–formaldehyde resin as their adhesive, the composition analysis of the adhesive materials reveals variations in VOC content between the two enterprises. This discrepancy in VOC content may be a primary factor contributing to the observed differences in emission factors.

It is worth noting that studies specifically focusing on emission factors in the plywood manufacturing industry are limited. Existing research primarily encompasses the wood-based panels industry as a whole, without discerning between different panel types such as plywood, fiberboard, and particleboard. The Ministry of Ecology and Environment’s “Technical guidelines for the source emission inventories of atmospheric volatile organic compounds (trial)” provides a recommended emission factor of 0.5 g/m³ for wood-based panels [16]. Additionally, a study conducted by Lv et al. in 2020 reported an emission factor of 0.9 g/m³ for plywood in Shandong, which deviates from our study’s findings [17]. Another study conducted by Zhao et al. in Chengdu, utilizing on-site sampling and analysis, reported a local emission factor of 1.5 g/m³ for wood-based panels [18]. This finding closely aligns with the emission factors observed in the two target enterprises of our study.

These contrasting results highlight the need for further research and refinement of emission factors specific to the plywood manufacturing industry. Future studies should strive to investigate regional variations in order to improve the accuracy and applicability of emission factors in this industry.

3.2. Fugitive Emission Characteristics of VOCs

3.2.1. General Characteristics of VOCs

Figure 2 illustrates the basic chemical composition of VOC emissions from distinct processes in two representative plywood manufacturing enterprises. The findings provide valuable insights into the prevalent VOC components and their relative contributions within the plywood manufacturing industry. Overall, OVOCs emerge as the most significant constituent, constituting 50.79% of the total VOC emissions. Following OVOCs, alkanes, halohydrocarbons, and aromatics contribute substantially, accounting for 16.22%, 15.81%, and 15.12% of the VOC emissions, respectively. Alkenes and alkynes exhibit generally low concentrations, and there are no emissions of organic sulfide.

The extensive usage of urea–formaldehyde resins, primarily consisting of formaldehyde, as adhesives in the plywood manufacturing process results in the substantial release of OVOCs during the adhesive-coating process. Additionally, the high-temperature hot-pressing process at 200 °C triggers significant OVOC formation through chemical reactions. Consequently, the VOCs emitted during the hot-pressing process primarily comprise OVOCs.

Notably, variations in the main VOC components are observed between the two enterprises regarding emissions from the unorganized adhesive-coating process. Enterprise A predominantly releases halohydrocarbons (52.98%), while enterprise B primarily emits OVOCs (46.52%). Despite both enterprises employing urea–formaldehyde resin as the adhesive, disparities in the ratio and performance of raw materials contribute to the variations in VOC emissions composition within the same process across different enterprises.

Furthermore, enterprise A demonstrates a notably lower proportion of halohydrocarbons in the organized emissions of VOCs from the adhesive-coating process. This observation suggests that the UV photo-oxidation technology employed by this enterprise exhibits high efficiency in the removal of halohydrocarbons.

3.2.2. Characteristic Components

Figure 3 illustrates the top ten characteristic VOC components emitted by each process in the two enterprises. Predominantly, OVOCs, such as formaldehyde, acetaldehyde, and acetone, constitute the characteristic components of the VOCs emitted from organized processes. Formaldehyde, in particular, exhibits the highest mass percentage of organized emissions in both enterprises. The significant presence of organized OVOC emissions suggests extensive secondary reactions within the organic waste gas treatment process, indicating complex chemical transformations.

Differences in the types and corresponding quantities of VOCs emitted through unorganized processes are observed between the enterprises. In the unorganized emissions from the adhesive-coating process, enterprise A primarily emits dichloromethane as the characteristic pollutant, whereas formaldehyde dominates in the unorganized emissions from the hot-pressing process. Conversely, methyl methacrylate is the primary component in the unorganized adhesive-coating process emissions of enterprise B, with dichloromethane as the second most emitted compound, corresponding to emissions from its hot-pressing process. Dichloromethane and methyl methacrylate are the main components of solvents, indicating that in addition to formaldehyde, we should pay attention to the importance of VOC emissions in solvents in the future.

Based on the analysis results of these characteristic pollutants, both companies should consider adjustments and improvements to their existing VOC treatment facilities. The development of adhesives and solvents with lower VOC content is the key to cleaner production and sustainability in the plywood manufacturing industry. Furthermore, further research should focus on determining the optimal combination of temperature and time for the adhesive-coating and hot-pressing techniques to minimize VOC emissions.

3.3. VOCs Souce Profiles of the Plywood Manufacturing Industry

Figure 4a provides a comprehensive overview of the characteristic composition spectrum of organized VOC emissions in each process. The adhesive-coating process predominantly emits OVOCs and alkanes. Formaldehyde (32.81%), acetone (8.80%), and n-undecane (8.26%) emerge as the most predominant species in this particular process. Conversely, in the hot-pressing process, the emissions are primarily composed of OVOCs, with formaldehyde (35.74%), acetone (9.84%), and acetaldehyde (9.71%) constituting the most abundant species within this process.

The characteristic composition spectrum of unorganized VOC emissions in each process is presented in Figure 4b. OVOCs and halohydrocarbons dominate the unorganized emissions across all processes. In the adhesive-coating process, the top three VOC species in terms of unorganized emissions are dichloromethane (38.16%), methyl methacrylate (13.30%), and formaldehyde (5.05%). In the hot-pressing process, the leading VOC species are formaldehyde (28.28%), methyl methacrylate (10.86%), and dichloromethane (7.18%).

Based on the aforementioned analysis, Figure 4c presents the characteristic composition spectrum of organized and unorganized VOC emissions in the plywood manufacturing industry. Organized VOC emissions are predominantly characterized by OVOCs, with formaldehyde, acetone, and acetaldehyde ranking as the top three species in terms of concentration. Conversely, unorganized VOC emissions are primarily composed of halohydrocarbons and OVOCs, with dichloromethane, formaldehyde, and methyl methacrylate being the dominant species.

Table S4 displays the source profile of the top 20 VOC components in the organized and unorganized emissions within the plywood manufacturing industry. The mass percentages of these top 20 components in the organized and unorganized VOC emissions amount to 96.50% and 89.90%, respectively. Research conducted by Lv et al. highlights that formaldehyde, acetone, and dichloromethane are the primary species in organized VOC emissions from the plywood manufacturing industry. Additionally, dichloromethane emerges as the major species in unorganized emissions, accompanied by notable quantities of acetone and m/p-xylene [17]. Consistent with the aforementioned findings, our study reveals that formaldehyde, acetone, and acetaldehyde are the main species in the organized VOC emissions, while dichloromethane predominates in the unorganized emissions.

3.4. Ozone Formation Potential (OFP) of VOCs

Figure 5a illustrates the OFP values of total VOCs and their chemical composition emitted from each process in the plywood manufacturing industry. The OFP values for the emitted VOCs range from 905.04 to 1822.35 μg/m³ for each process. Specifically, the hot-pressing process demonstrates the highest OFP value in terms of organized emissions, while the adhesive-coating process exhibits the lowest OFP value in organized emissions. Among the different processes, OVOCs (72.28~83.37%) and aromatics (10.22~21.61%) contribute significantly to the overall OFP. Additionally, the proportion of alkenes and halohydrocarbons in the OFP remains below 1% for all processes.

Figure 5b highlights the top three VOC species that contribute to ozone generation in each process. For the adhesive-coating process, formaldehyde (544.87 μg/m³) exhibits the highest reactivity among the species emitted in organized emissions, while methyl methacrylate (573.67 μg/m³) demonstrates the highest OFP in unorganized emissions. In the case of the hot-pressing process, formaldehyde (1138.01 μg/m³) significantly dominates the OFP in organized emissions compared to other species. Additionally, formaldehyde also ranks as the highest OFP species in unorganized emissions, with methyl methacrylate (443.44 μg/m³) contributing significantly to the overall ozone-forming activity. Notably, formaldehyde, methyl methacrylate, and acetaldehyde emerge as significant contributors to the overall OFP, highlighting the need for targeted control measures concerning these substances.

3.5. Health Risk Assessment of VOCs

3.5.1. Non-Cancer Toxic Risk of VOCs

In this study, the health risk assessment method recommended by the US EPA was employed to calculate the non-cancer toxic risks associated with 16 VOC components. These components encompass two alkanes, one alkene, six aromatics, two halohydrocarbons, and five OVOCs (Table S2). For a given VOC species, a hazard quotient (HQ) value greater than 1.0 signifies the potential for causing non-cancerous toxic harm to human health. Conversely, a specific VOC with an HQ value lower than 1.0 indicates a relatively low health risk in terms of toxicity [15,19].

Figure 6 displays the hazard index (HI) values of the total VOCs and the HQ values for individual VOCs from each process in the plywood manufacturing industry. The unorganized emissions from the adhesive-coating process and the hot-pressing process exhibit HI values of 0.49 and 0.82, respectively. These results suggest a relatively low non-cancerous toxic risk associated with VOCs for the workers exposed to these environments. Furthermore, formaldehyde and naphthalene exhibit HQ values ranging from 0.1 to 1, indicating the need to consider their potential non-cancer health hazards. Conversely, the HQ values for the remaining VOCs are all below 0.1, indicating a low non-cancerous toxic health risk associated with these specific VOC species.

3.5.2. Cancer Toxic Risk of VOCs

The lifetime cancer risk (LCR) of VOCs was assessed in this study, incorporating five specific VOCs, comprising two aromatics, one halohydrocarbon, and two OVOCs (Table S2). The categorization of cancer health risks associated with these VOCs is determined based on the following thresholds [15,20,21]: LCR values exceeding 1.0 × 10⁻⁴ are considered definite risks, values ranging between 1.0 × 10⁻⁵ and 1.0 × 10⁻⁴ are classified as probable risks, and values falling between 1.0 × 10⁻⁶ and 1.0 × 10⁻⁵ are regarded as possible risks, while values below 1.0 × 10⁻⁶ are deemed negligible risks.

Figure 7 presents the total lifetime cancer risk (T-LCR) values of VOCs from each process in the plywood manufacturing industry. The T-LCR values of the emissions from the adhesive-coating process and the hot-pressing process fall within the range of 1.0 × 10⁻⁵ and 1.0 × 10⁻⁴, suggesting probable cancer health risks for the identified VOC species. Specifically, the LCR values for formaldehyde emitted from both processes, as well as for styrene emitted from the unorganized adhesive-coating process, range between 1.0 × 10⁻⁵ and 1.0 × 10⁻⁴, indicating a probable cancer health risk associated with these compounds. Acetaldehyde emitted from the unorganized hot-pressing process emerges as the only VOC species with an LCR value ranging from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵, suggesting a possible cancer health risk. The LCR values of benzene and dichloromethane are lower than 1.0 × 10⁻⁶, indicating negligible cancer health risks for workers exposed to those compounds.

The results of the cancer risk assessment indicate that the cancer risk of formaldehyde in the plywood manufacturing industry is significantly higher than that of other VOC species. Therefore, greater attention should be given to formaldehyde in the process of controlling VOCs to decrease the cancer risks for workers in the plywood manufacturing industry.

4. Conclusions

In conclusion, this study comprehensively investigated the emission characteristics of VOCs and their potential health risks to workers in the plywood manufacturing industry in Shandong Province. The calculated VOC emission factors range from 1.5 to 3.6 g/m³ for plywood, and the average total VOC concentration is 954.17 μg/m³. The hot-pressing process and adhesive-coating process significantly contributed to organized and unorganized emissions, respectively, with concentrations of 336.63 μg/m³ and 276.24 μg/m³. OVOCs were identified as the predominant component of VOCs, accounting for 50.79% of the total mass concentration. VOC source profiles specific to the plywood manufacturing industry were established, with organized emissions predominantly composed of OVOCs, including formaldehyde, acetone, and acetaldehyde as the most abundant species. Unorganized emissions were characterized by halohydrocarbons and OVOCs, with dichloromethane, formaldehyde, and methyl methacrylate as the top three species. The OFP values of the emitted VOCs ranged from 905.04 μg/m³ to 1822.35 μg/m³ for each process. Notably, formaldehyde, methyl methacrylate, and acetaldehyde were significant contributors to the overall OFP, warranting targeted control measures for these substances. Moreover, the T-LCR values of the emissions from the adhesive-coating process and the hot-pressing process suggested probable cancer health risks for the identified VOC species. Formaldehyde emitted from the unorganized hot-pressing process exhibited the highest HQ value of 0.45 and LCR value of 5.72 × 10⁻⁵, underscoring its significant role as a species threatening workers’ health in the plywood manufacturing industry. These findings will provide valuable insights for regional-scale VOCs pollution control, offering guidance for minimizing environmental impact and improving occupational health and safety in the plywood manufacturing industry.

Looking ahead, further research is warranted to address several key areas. Firstly, ongoing monitoring and analysis of VOC emissions in the plywood manufacturing industry are essential to track any changes over time and assess the effectiveness of implemented control measures. Additionally, studies focusing on the development and implementation of advanced VOCs treatment technologies, such as catalytic oxidation and adsorption, could offer sustainable solutions for reducing emissions and improving air quality. Furthermore, comprehensive risk management strategies should be devised and implemented to safeguard the health and safety of workers in the plywood manufacturing industry. This includes the establishment of stringent occupational exposure limits, regular health monitoring programs, and the provision of appropriate personal protective equipment. Moreover, the collaboration between industry stakeholders, government agencies, and research institutions is crucial to facilitate knowledge exchange, policy development, and the adoption of best practices in VOCs pollution control. By working together, significant progress can be achieved in mitigating environmental pollution and promoting sustainable development in the plywood manufacturing industry and beyond.