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Objective: Low-middle-income countries (LMICs) face most of the burden of air pollution and increasing incidence and mortality from increased blood pressure (BP) and hypertension. Yet, there is a paucity of studies linking air pollution and BP conducted in LMICs. We aim to extend the spatial coverage of the evidence using data from countries with different income and air pollution levels. We investigated the effect of exposure to short and long-term ambient PM2.5 on BP and prevalent hypertension in 125,198 participants from 29 countries during May Measurement Month (MMM) BP screening campaigns in 2018 and 2019. Design and method: We conducted a cross-sectional study using satellite-derived data on monthly average PM2.5 and assigned exposure (averaged for 1month, 3months, 6 months, and 1 year) to participants for the month in which BP measurement was recorded based on the coordinates of the MMM screening site. We used generalized additive models with penalized splines for BP and generalized linear models for prevalent hypertension, accounting for potential confounders and country-level clustering effects. Results: Short and long-term exposure to ambient PM2.5 was associated with increased SBP and DBP at an individual level but not at the country level. We found the relationship non-linear, with significantly higher effects beyond 70ug/m3. An interquartile range increase (17.6ug/m3) in annual PM2.5 was associated with a higher prevalence of hypertension with an odds ratio of 1.12 [95% CI: 1.09, 1.15]. Stratified analysis showed that individuals with high BMI and those with hypertension, particularly among those untreated and participants with comorbidities, were more susceptible to the effect of air pollution on BP. Conclusions: More high-quality data are required to evaluate individual air pollution exposure (using robust and granular data) with cardiovascular risk factors such as BP and major adverse cardiovascular events. Meanwhile, health policies promoting air pollution reduction are needed, particularly for those at the highest risk.