The objective was to evaluate the effectiveness and acceptability of locally tailored implementation of improved cookstoves/heaters in low- and middle-income countries. This interventional implementation study among 649 adults and children living in rural communities in Uganda, Vietnam and Kyrgyzstan, was performed after situational analyses and awareness programmes. Outcomes included household air pollution (PM2.5 and CO), self-reported respiratory symptoms (with CCQ and MRC-breathlessness scale), chest infections, school absence and intervention acceptability. Measurements were conducted at baseline, 2 and 6-12 months after implementing improved cookstoves/heaters. Mean PM2.5 values decrease by 31% (to 95.1 µg/m3) in Uganda (95%CI 71.5-126.6), by 32% (to 31.1 µg/m3) in Vietnam (95%CI 24.5-39.5) and by 65% (to 32.4 µg/m3) in Kyrgyzstan (95%CI 25.7-40.8), but all remain above the WHO guidelines. CO-levels remain below the WHO guidelines. After intervention, symptoms and infections diminish significantly in Uganda and Kyrgyzstan, and to a smaller extent in Vietnam. Quantitative assessment indicates high acceptance of the new cookstoves/heaters. In conclusion, locally tailored implementation of improved cookstoves/heaters is acceptable and has considerable effects on respiratory symptoms and indoor pollution, yet mean PM2.5 levels remain above WHO recommendations.