<p>Size-fractionated particulate matter analysis is key to understanding how particle size influences health risks. However, data on metals distribution and respiratory deposition across PM sizes in Southeast Asia is still scarce. In this study, we evaluated the levels, size distribution, and health-related impacts of metals in particulate matter (PM) from Ho Chi Minh City (HCMC), the most densely populated urban in Southern Vietnam. Sampling was performed monthly with a five-stage nano-sampler (PM_&lt; 0.5, PM_0.5−1, PM_1−2.5, PM_2.5−10, and PM_&gt; 10). Concentrations of 8 metals were analyzed by ICP-MS, and the levels were observed in the following order: Al (1741 ng m^− 3) &gt; Fe (958 ng m^− 3) &gt; Zn (731 ng m^− 3) &gt; Pb ( 22.6 ng m^− 3) &gt; Mn (21.3 ng m^− 3) &gt; Cu (19.4 ng m^− 3) &gt; Ni ( 4.86 ng m^− 3) &gt; Cd (0.52 ng m^− 3). Crustal metals (Al and Fe) predominantly resided in the coarse particulate fraction, whereas anthropogenic metals (e.g. Pb, Zn, and Cd) were enriched in fine particles. Deposition modeling indicated that crustal metals were mostly retained in the head region (62–72%), while anthropogenic metals were deposited more in deeper lung regions (7–12%). The health risk assessment showed that the Hazard Index (HI) was markedly higher in children than in adults for both PM_2.5 and PM₁₀. Specifically, the HI for children was 1.95 for PM_2.5 and 3.54 for PM₁₀, whereas for adults, these values were 0.49 and 0.88, respectively. Regarding cancer risk, Total Cancer Risk (TCR) for children reached 1.31 × 10^− 5 for PM_2.5 and 1.75 × 10^− 5 for PM₁₀, exceeding the low-risk threshold. In contrast, the TCR for adults was considerably lower at 3.26 × 10^− 6 and 3.0 × 10^− 6. These findings indicated the importance of size-resolved PM studies to better understand exposure characteristics and potential health risks posed by toxic elemental contaminants.</p>