BACKGROUND: Despite the established body of evidence documenting the impacts of particulate matter smaller than 2.5 µm (PM2.5) on mortality among native populations, limited evidence is available for United States (US) diplomats and their accompanying family members, who undergo frequent relocations and dramatic changes in exposure to air pollution. US diplomats are provided air purifiers for use in their homes, and it is suspected that diplomats’ personal exposure to PM2.5, and related mortality estimated based on ambient PM2.5, may be far lower that published literature suggests. The research described in this thesis characterizes US diplomats’ PM2.5 personal exposure while working in a highly polluted city, including an assessment of personal exposure before and after adding high-capacity air purifiers in diplomats’ residences, and investigates whether US diplomats may have increased PM2.5-related mortality in comparison to Americans who have lived exclusively in the US. METHODS: The thesis consisted of four parts. First, a literature review was conducted to identify the current evidence and research gaps regarding the health of people who relocate to an area with a different ambient air pollution level. Second, PM2.5 personal exposure monitoring was conducted among the US diplomatic corps in a city with high ambient PM2.5, Kathmandu, Nepal, to explore the dynamics of personal-level PM2.5 exposure, the influence of various microenvironments and impact of added mitigation measures implemented in the diplomats’ residences. A natural experiment was conducted comparing diplomats’ PM2.5 exposure and microenvironments associated with PM2.5 exposure before (September 2019 to February 2020) and during (March to June 2020) the period when COVID-19 lockdown orders were in place. Third, an intervention study was conducted comparing diplomats’ PM2.5 exposure before and after adding high-capacity air purifiers to diplomats’ homes in Kathmandu. Fourth, using a set of pre-defined international assignment scenarios, PM2.5 related mortality among US diplomatic corps and accompanying family members was quantified by a life table modeling method. Findings from the intervention study were used in the model to estimate the impact of a high level of mitigation on mortality. RESULTS: Among 15 relocation studies identified with heterogeneous study designs, most studies suggested consistent short- or long-term adverse health effects on biological function or mortality. However, many of these empirical studies of change in exposure have design weaknesses that limit the robustness of interpretation. A natural experiment in Kathmandu during the COVID-19 pandemic revealed that ambient PM2.5 in Kathmandu was approximately 40% lower during COVID-19 lockdown in 2020 than in the same period of the previous three years and within-person comparisons suggest PM2.5 personal exposure was 50.0% to 76.7% lower during the lockdown than before it. The intervention study indicated that when high-capacity air purifiers were added to US diplomats’ homes, the ratio of PM2.5 personal exposure to ambient PM2.5 decreased from 0.32 pre-intervention to 0.16 post-intervention. The health modeling results suggested that life expectancy may decrease due to ambient PM2.5 exposure in a standard 20-year diplomatic career by up to 84 days. The use of high-capacity air purifiers in polluted cities was projected to reduce the days of life lost by up to 60% for diplomats. Alternating assignments in high and low ambient PM2.5 cities and the use of high-capacity air purifiers may reduce the PM2.5 exposure-related DLL. The modeling results were highly sensitive to lag assumptions and further research on outcome-specific delayed impacts of PM2.5 exposure is needed to improve the model. CONCLUSION: My research provides insights into PM2.5 personal exposure levels variable to time, micro-environment, activity patterns, and mitigation used at home among internationally assigned US diplomats in cities with high ambient PM2.5 and the anticipated PM2.5 related impacts on mortality. In the high pollution environment of Kathmandu, air purifiers with home sealing provided substantial protection against ambient PM2.5 in the home environment. Application of the health model to the US diplomatic corps indicates the magnitude of the impact of varying PM2.5 exposure levels on diplomats’ life expectancy and the possible benefits of the mitigation measures used in diplomats’ residences and in the workplace in many highly polluted cities