Asthma is major public health issue, which is linked to genetic predisposition but also to environmental exposure. At-risk or protective thresholds are not available for any species, and the impact of exposure to multiple contaminants has been poorly studied up to now. Large collections of observational data, including specific quantification of contaminants and assessment of the exposome during early childhood are essential to help identify the causes of the disease. To that end, the ELFE cohort was initiated (French longitudinal childhood study including 18 329 children, with follow up from birth to adulthood) with multi-disciplinary objectives including assessment of environmental exposure. A first wave of sampling was performed using Electrostatic Dust collectors (EDC) in the dwellings of 3193 children included in the nested EBRA cohort at the time of their birth (2011). EDC were analyzed by quantitative PCR using a panel of 20 targets (molds, mites, bacteria, dogs, cats, and cockroaches) chosen for their allergic or protective effect for allergic diseases. The EMBRASE study, a coherent continuation of the EBRA study, plans to reiterate the sampling for the same children at age 5 (which is the expected age for initial asthma diagnosis). Using metabarcoding analysis, we will establish a complete list of microorganisms present in the bedrooms of asthmatic children (n=60) at birth and at age 5, versus a control group without asthma (n=60), to identify new microorganisms (including non-cultivable) which could influence asthma development or protection. Five species highlighted by qualitative or quantitative differences between the 2 groups will be selected for specific QPCR assay development, then the children exposure will be measured (25 QPCR targets) at birth and at 5 years. The aim will be 1/ to study links between environmental data collected at birth and at age 5, and health evolution profiles based on respiratory symptoms at 2 months, 1 year and 5 year, 2/ to study the effect of cumulative exposure, and 3/to identify agents or their associations which could influence asthma development (protective or deleterious effect). EMBRASE is a unique opportunity to provide considerable data to determine indoor contamination thresholds or microbial profiles associated with either protection or allergic risk. Thanks to all the available data at the end of the study, we will be able to select a small number of relevant targets that could help to predict risk or protection for asthma development. An inexpensive marker, based on a system with detection and quantification of a reduced number of species, could be performed routinely to assess indoor contamination, and promote preventive measures to protect children from asthma. This study could also help further research to determine potential candidates for immune-therapy against asthma. Other assets of the project are the precocity of the first sampling (2 months) and the longitudinal nature of the study.