This study examines the size-segregated particulate matter (PM) concentrations in blacksmith workshops (WSs) in West Java, Indonesia, using the Ambient Nano Sampler (ANS). PM was categorized into six size fractions: > 10 µ m, 10-2.5 µm, 2.5-1 µm, 1-0.5 µm, 0.5-0.1 µm, and < 0.1 µm (ultrafine particles, UFPs). The results showed that WSs with intensive welding activities had the highest ultrafine PM concentrations. WS-A recorded the highest total suspended particle (TSP) concentration (3,504.58 µg/m3), with UFPs contributing 998.27 µg/m3 (37% of PM2.5), while WS-B had the lowest TSP concentration (1,978.16 µg/m3). The ratio of PM < 0.1 to PM2.5 ranged from 0.26 to 0.42, demonstrating the dominance of UFPs in fine PM fractions. Outdoor UFP levels (4.64 µg/m3) were significantly lower than indoor concentrations, confirming that blacksmithing activities are a major emission source. Heavy metal analysis revealed iron (Fe) as the dominant element (up to 284.775 µg/m3 in UFPs), with Cr, Pb, and Mn also detected, highlighting occupational exposure risks. These findings indicate that blacksmith WSs generate significantly higher UFP and metal emissions than ambient environments, posing potential health hazards for workers. This study underscores the need for effective air quality management in small-scale metal industries. © The Author(s) 2025.