## 1 Introduction  
Directive 2017/164, published on 31 January 2017, proposed new indicative occupational exposure limit values (IOELV) with the aim of increasing health protection in mines. This directive called for a significant reduction in nitrogen monoxide (NO) levels from 25 ppm to 2 ppm, and nitrogen dioxide (NO\({}_{2}\)) levels from 3 ppm to 0.5 ppm. However, concerns have arisen about the feasibility of implementing such stringent limits, especially in the context of tunnelling and underground mining activities [1]. The directive allows for a transitional period until August 2023 [2], during which measures need to be taken to comply with the proposed reductions. On the other hand, the progression of emissions standards for non-road machinery throughout the past two decades highlights a substantial and ongoing commitment to the reduction of NOx emissions. The transition from less stringent Stage I (1999) and Stage II (implemented from 2001 to 2004) regulations to the more rigorous Stage III (enforced from 2006 to 2013) and Stage IV (introduced in 2014) standards vividly demonstrates a continuous commitment to emissions reduction. Notably, the permissible level of NOx emissions for engines with a net power range of 130 to 560 kW evolved as follows: 9.2 g/kWh in Stage I, reduced to 6 g/kWh by the conclusion of Stage II, and further lowered to 2 g/kWh at the end of Stage III. Stage IV engines, however, adhere to an exceptionally stringent NOx emission limit of just 0.4 g/kWh. Importantly, Stage V, established in 2019, maintains the same NOx emission limit as Stage IV. This remarkable decrease in permitted emissions clearly demonstrates the significant breakthroughs made in emissions control technology and procedures. The legislative development from Stage I/II to Stage III/IV represents a determined attempt to reduce NOx emissions from machinery, with each level reducing emission thresholds and fostering technical advancement to set the groundwork for a cleaner and more sustainable future. In response to these concerns about NOx Directive 2017/164 and Stage V application, plant manufacturers are exploring alternatives to traditional diesel power, such as electric or hydrogen fuel-powered motors, as well as adopting emission control systems like diesel particulate filters (DPFs) and selective catalytic reduction (SCR) [2; 3]. While efforts are being made to comply with the new limits, there is a lack of published studies on nitrogen oxides (NOx) in tunnel construction or mining activities. A study conducted in a tunnel that was constructed using the New Austrian tunnelling method (NATM) indicated that significant efforts are required before 21 August 2023, to meet the directive requirements [3]. Similarly, an investigation in an underground tungsten mine using the room and pillar mining methods reported NO\({}_{2}\) and CO values that exceeded the directive limits [4]. Also, a magnesite and dolomite underground mine, exploited using the post pillar and sublevel open stopping methods, showed NO\({}_{2}\) values surpassing the directive [4]. Similar levels were also shown in a Swedish iron ore mine [5]. Furthermore, the Health and Safety Executive (HSE) reported average NO values of up to 3 ppm and NO\({}_{2}\) values of up to 1.9 ppm in a sewer drill and blast tunnel [6]. In addition to studies assessing NOx concentrations in tunnel construction and mining activities, there have been previous studies that estimate the emissions of nitrogen oxides (NOx) in tunnel motorway construction [7; 8]. The global tunnel-boring machine market is experiencing significant growth, but there are limited data on NOx emissions during tunnel construction with tunnel-boring machines (TBMs). TBMs are preferred for their avoidance of explosives, which are a major source of NOx emissions in tunnelling construction activities [9]. In recent decades, mounting concern has surrounded NOx exposure due to its significant health implications.