## 4 Discussion  
The revised oxygen mask designs through the virtual fit analysis method proposed in the present study showed a better fit to KAF pilots in terms of accommodation percentage than the existing designs. In the present study, the RFMs of the KAF pilots were identified for the oxygen mask sizing system based on two key facial dimensions (sellion to promentale length and lip width). An initial boundary of the oxygen mask designed using the RFM may not accommodate a high percentage of KAF pilots if the RFM does not properly represent other facial dimensions (e.g., nasal root breadth) or the detailed facial characteristics of the other pilots [20, 21]. The initial mask boundary could be improved in terms of accommodation percentage by iterative design adjustment. For example, Figure 11 shows that the mask boundary finally selected for the small-narrow size by the iterative design and virtual fit analysis process improved 3.4-11.7% of accommodation percentage compared to the initial mask boundary prepared by the corresponding RFM. The present study introduced a novel method to analyze the fit through a virtual fit simulation with 3D body scan images to find an optimal design of a product for better fit and comfort to users. Different from the previous studies on virtual fit analysis for design customization [15, 21, 22, 23, 24, 25, 26, 27, 28], this study has achieved a design optimization based on a quantitative measure such as horizontal distances between the mask boundary and the face surfaces. A placement strategy of the mask to a particular location of the face based on the information of the wearing positions of the mask over the face was introduced. The wearing positions of the mask over the faces were empirically found using measurements of the distances between the face and mask at the nose and the chin. The mask placement strategy of the present study would be preferred for a realistic virtual fit of the mask to the face to an automatic alignment method such as the iterative closest point (ICP) algorithm that can align the mask to the impractical location of the face. The proposed virtual fit analysis method using 3D body scan images would be of use to develop an ergonomic design of a wearable product for proper fit and comfort to the user population. The photos of mask-wearing pilots, instead of 3D scans, were used in the present study due to the limited access of our research team to military facilities and the lack of portability of the 3D scanning system used in the present study. Our research team obtained approval of access to military facilities to scan KAF pilot faces, but failed to obtain approval of follow-up access to scan pilot faces worn with oxygen masks due to heightened military situations. Avoidable errors could occur in identifying mask-wearing positions due to the manual alignment of a mask CAD model and a face scan to the corresponding mask-wearing face image on the photo. Figure 10: Comparison of the accommodation percentages of the existing and revised oxygen mask designs by mask size category (SN: small narrow, MN: medium narrow, MW: medium wide, and LW: large wide; \({}^{\dagger}\) significant at \(\alpha=0.05\)). The virtual fit analysis method proposed in the present study was found effective by reducing the time and cost of prototyping and usability evaluation in the oxygen mask design process by providing the ergonomic design of oxygen mask boundary with efficiency. After preparing an oxygen mask design, a physical prototype of the mask design needs to be manufactured using an expensive molding process for usability testing under different mask operating situations (e.g., low atmospheric pressure or high gravity acceleration situation) and evaluations of other military conformity requirements (e.g., durability, toxicity, and compatibility with existing components such as microphone and valve).