BACKGROUND AND AIM: Air pollution contains a mixture of different pollutants from multiple sources. However, the interaction of these pollutants with other environmental exposures, as well as their harmful effects on children under five in tropical countries, is not well known. This study aims to characterize the external exposome (ambient and indoor exposures) and its contribution to clinical respiratory and early biological effects in children. METHODS: A cohort study is conducted on children under five in two cities in Colombia. Enrolled children are followed monthly (phone call) post-enrollment with upper and lower Acute Respiratory Infections (ARI) examinations and will be followed at months 6 and 12 (in person) to evaluate asthma development, asthma control, and genotoxic damage. The asthma diagnosis is pediatric pulmonologist-based and a standardized protocol is used. Exposure, effect, and susceptibility biomarkers are measured on buccal cells and blood samples. For environmental exposures, PM2.5 was sampled in two moments of the year to chemical characterization. Additionally, the standardized questionnaire developed for HELIX is used and geographic information, dispersion models, and Land Use Regression models for PM2.5 and NO2 will be used. RESULTS: these are preliminary data. Of 135 children 47.4% are female and 49.6% are 4 years old. Exposure to pets 45.9%, cockroaches 41.5%, mold 47.6%, parental tobacco smoke in pregnancy and early childhood 14.3%, parental electronic cigarettes 4.44%, noise 20.7%, and never eaten vegetables or fruits 9.6% have been found. Prevalence of asthma is 29.4% with TRACK<80 in 83.3% of children with previous asthma. Comet assay damage indicator median 55 (IQR: 29 – 90.5). CONCLUSIONS: This is the first exposome study on the region. There is a high prevalence of asthma with high exposure to unfavorable environmental conditions. KEYWORDS: Exposome, Air Pollution, Children, Asthma, Respiratory Infection Disease, Early Biological Effect, DNA Damage, Polycyclic Aromatic Hydrocarbons