Despite China’s commitment to reach a CO2 emission peak by 2030 and achieve carbon neutrality by 2060, coal remains the dominant energy resource in the country. A large amount of dust is inevitably generated during coal mining processes (Zhang et al. 2021; Paluchamy et al. 2021), which poses health risks to workers. Ventilation can dilute and remove dust particles in mine tunnels, but it cannot cover every corner effectively in practice (Liu et al. 2019; Gui et al. 2020; Wang et al. 2020). The dust concentrations in some local underground environments are still high, ranging from 1200 to 8000 mg/m3 (Li et al. 2018; Yin et al. 2019), which significantly exceeds the maximum allowable concentration stipulated in the “Coal Mine Safety Regulations of China (2022)”. Prolonged exposure to dust can cause chronic inflammation and pulmonary fibrosis in the lungs, resulting in pneumoconiosis. It is closely associated with respirable dust, which has an aerodynamic diameter of less than 7.07 µm and can penetrate and accumulate in the lung tissues (Zhao 2023). Coal workers’ pneumoconiosis accounts for over 50% of annual officially reported occupational cases in China (Chen et al. 2022). Respiratory dust can cause irreversible damage to human lungs. Hence, it is imperative to find effective dust removal methods to protect workers from occupational hazards. A key step in this process is to study the exposure characteristics of respirable dust for mine workers.
Numerous studies have investigated dust distribution patterns in underground tunnels. Chen et al. (2022) simulated the spatial and temporal evolution of dust and suggested that the dust concentration in the footway and transport route generally exhibited a rapid increase and a gradual decrease from the heading face. Wang et al. (2015) performed a numerical simulation and found that the dust concentration declined sharply from the heading face and then slowly decreased under a high dust production rate. Yao et al. (2020) conducted a field measurement and observed that from the heading face to the exit of the tunneling roadway, the dust concentration fluctuated around a constant value and gradually diminished. These studies indicated that dust and respirable dust were not uniformly distributed in the roadway. Therefore, the research on the dust exposure of mine workers should focus on the workers’ surrounding space that covers their respiration.
To limit the research zone for workers’ dust respiration, the breathing zone was commonly defined as the area within a radius of 0.30 m (or 10 inches) of the human nose and mouth (Ojima et al. 2012). Guffey et al. (2001) sampled different parts of the human body model in a wind tunnel and discovered that gas distribution near the human body had significant spatial differences. Liden et al. (2010) suggested that for workers exposed to large particles, the radius of their breathing zone should be reduced to 0.10 cm. Ojima’s study (2012) recommended that the definition of the breathing zone of workers handling organic solvents should be decreased to 0.05–0.10 m. These studies indicated that a smaller breathing zone had a gas or particle concentration closer to the human inhalation concentration. Consequently, the breathing zone was redefined in the present work as the hemispherical space around the human face, with a radius of 0.20 m and the center between the nostrils.
Many scholars have investigated particle distribution in the breathing zone. Spitzer et al. (2010) used the Phase Doppler Anemometry to measure particles in the breathing zone of a thermal manikin and examined the effect of human movement on the entry of particulate matter into the breathing zone. Alshitawi et al. (2019) designed a personalized air curtain to maintain the air quality in the human breathing zone and determined its optimal airflow velocity through numerical simulation. Lu et al. (2021) analyzed the dust concentration in the potential breathing zone of workers at three breathing height levels to identify the dust safety zone of workers at different stations in the longwall mining face. Cai et al. (2019) simulated coal dust distribution under different airflow velocities to identify the best airflow velocity for lower dust concentrations in the footway breathing zone. They reported more high dust concentration areas of breathing heights on the coal transportation route than on the footway. Xie et al. (2021) simulated the airflow field composition at the heights of 1.60 and 3.00 m after tunnel blasting, suggesting that the 3.00 m personnel breathing zone ahead of the driving face experienced more severe dust pollution. Zhou et al. (2022) simulated the dust distribution of the fully mechanized mining face under different inclination angles. They established the functional relationship between the dust concentration in the sidewalk breathing zone and the distance from working surface. These studies provided extensive data for the breathing zone research. However, the respirable dust variations in the breathing zone influenced by the external dust-containing airflow and corresponding parameters were seldom discussed.
The dust concentration in the breathing zone is easily affected by the outside environment in the complex underground tunnels. The individual sampling method can obtain the instantaneous average dust concentration in the breathing zone but loses detailed information about the concentration’s real-time distribution and variations, especially when human respiration processes are considered. In contrast, the numerical simulation methods are reliable in obtaining detailed data on such a small scale and have been widely applied in many studies. Zhou et al. (2023) used numerical simulation to study airflow-dust-gas migration law and determined the best dust and gas exhaust air volume. With the numerical simulation of spraying dedusting around the miner, Guo et al. (2020) determined the fog droplet parameters that would result in the best dust suppression effect. The law of dust migration in tunneling and shotcrete operation at the same time was obtained by Chen et al. (2022) using numerical simulation. Nie et al. (2022) simulated the interaction mechanism of airflow, gas, and dust, and obtained the optimal purification distance of the air duct. In this work, considering the gas-solid two-phase flow, a coupled computational fluid dynamics (CFD) and discrete phase model (DPM) method was used to simulate the airflow and dust distribution in the breathing zone for underground workers. The effects of the large-scale external environmental parameters were examined. This study will provide fundamental research data and theoretical guidance for preventing occupational diseases for underground workers.