Background: USA implemented a national Biomonitoring program and as result found that 88% of the population is affected by secondhand smoking and then started a monitoring program to control smoking. In Mongolia, due to the capacity of the biomonitoring tests, only children’s lead level was determined. The study showed that blood lead level in children aged 0.5-14 years was 0.4-16.5 μg/dl in average. However, there is no reference level and therefore reference level from other countries are used. Objectives: To study the correlation between the environmental effect and toxic metal level in children’s’ blood Methods and materials: 103 children aged 0-5 years, consisting of 10 male and female children of each age. 6 kindergartens and 14 households were chosen for their internal air quality monitoring; measured the level of toxic metals and dust by cross-sectional study based on laboratory analysis. Results: 41.7% (44) are male and 58.3% (59) are female of the total participants. 16.5% (17) are 1 year old, 18.4 (19) are 2 years old, 23.3% (24) are 3 years old, 19.4% (20) are 4 years old, 22.3% (23) are 5 years old. The result showed the children’s’ blood level was 4.12±1.35 μg/dl. The lead level in the internal air of households close to steam boilers were measured and it was 0.000095 mg/m2. Conclusion 60.1% of the children had blood lead levels 3 times higher than the reference level. However, the lead level of internal air was lower than the reference level. No effect of Internal air quality on blood lead level was observed and weight and height were inversely correlated.