## 5 Conclusions
#### Limitations and Future Directions  
As evidenced in the above findings, Appalachian-type settings, especially those in developed nations like the United States, are currently well-placed to allow for reliable estimations of current UFP levels and the appropriate measures that need to be taken to control exposure. However, the measurement and tracking of UFPs remain a major challenge, largely due to a lack of technical standardization and clear guidelines. The lack of clear guidelines tends to hamper policy development, highlighting the need for future research to create comprehensively new ambient UFP particle estimations and, in turn, better standards for regulation. As part of this initiative, we collaborated with the Mechanical Engineering Department to work on analyzing how engine design, loading conditions, truck speed, and uphill or downhill movement affect ultrafine particle concentrations across varying topographies using multivariate analyses. Evidence of the role of topography, as noted earlier, is still limited despite the inherent health risks associated with UFP exposure in low-lying areas. Using one single location (Morgantown, WV) with less than 2 weeks of sampling, as was done here, will not ultimately be sufficient. Concentrations of ultrafine particles in complex terrains and inclines can be significantly more variable and have potentially higher exposure. Should weather conditions develop to trap UFPs, an issue arises that must be taken into consideration when assessing regional exposure risk and the potential for public health issues with these particles. We were able to choose sunny, light wind and dry conditions for our short sampling times. Fog-inducing valley stagnations are often apparent at the two sampling sites. Morgantown is a topographic twin located just upstream from Donora, PA, site of one of the historic examples of valley stagnation. Appalachia hosts a bounty of similar sites with similar opportunities for similar environmental calamities and poses significant potential for understanding those risks. Also, we tested only one sampling integration period of 15 min. While we do not believe this is an important consideration, future testing should include variable integration times to ascertain the validity of that assumption. This multidisciplinary approach will help researchers further understand the role of traffic conditions such as uphill and downhill truck movement and truck characteristics such as engine size, speed, and loading condition in UFP emissions, as well as their distribution in different topographical settings. We are suggesting the conduction of a health effect study to assess the impact of these particles on public health using the method we have described herein with the goal of establishing a dose-exposure limit. More research into UFP emissions in different environments is essential in accurately determining levels of exposure and potential health risks. More importantly, such research will provide valuable insights for developing strategies to mitigate UFP pollution in areas with diverse topographical features. Conceptualization, N.O.I., O.A.T. and M.M. ; methodology, N.O.I. ; software, B.R. ; validation, N.O.I. and B.R. ; formal analysis, N.O.I. ; investigation, N.O.I. ; resources, M.M. ; data curation, N.O.I. ; writing--original draft preparation, N.O.I. ; writing--review and editing, M.M., T.K. and A.C.N. ; visualization, M.M. ; supervision, A.C.N., S.W., T.K., M.C. and M.M. ; project administration, R.M. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The data, in particular the video data, presented in this study are available on request from the corresponding author. The data are not publicly available due touncertainty of displaying license plates without them being blurred which would have to be requested with longer response delays if truck count checks are desired.