[en] Methane and carbon dioxide are naturally emitted by soils as products of different, mainly biological, processes. These emissions are usually low, but vary significantly in time and space. There is still very limited quantitative data regarding the natural biogenic flux of soil gas in France. However, it is important to know the CH₄ and CO₂ flux levels usually observed in sites that are considered natural and unpolluted, in order to distinguish these natural biogenic emissions from possible anomalies. These may be of anthropogenic (post-mine, caused by contamination, landfills, city gas leak, etc.) or natural origin (mainly geological origin). From the experimental data acquired by Ineris over 20 years, we analyzed naturally occurring biogenic CO₂ and CH₄ emissions. On one hand, it confirmed that there is no significant flux of CH₄ from soils that are normally drained and, on the other hand, that there is a generalized presence of a natural, largely measurable, flux of CO₂ in almost all normally drained soils. The natural flux of CO₂ undergoes generalized and very pronounced seasonal variations. On average, emission was clearly strongest in summer and weakest in winter. It is intermediate in spring and fall. Seasonal variations are on average stronger than the influence of other specific parameters of the studied sites (climate zone, land use, geology, pedology, etc.). Ranges of normal values, average values, low limits, high limits, and the most frequent values for specific configurations were established from statistically processed data. For example, the most probable CO₂ flux ranges on natural sites are respectively from 0 to 6 cm³ min^-1 m^-2 in winter and 3 to 20 cm³ min^-1 m^-2 in summer. The higher thresholds that mark an abnormally high flux, corresponding to a natural or anthropogenic anomaly, were respectively evaluated at 12 cm³ min^-1 m^-2, for winter times 30 cm³ min^-1 m^-2, for summer times. Practical recommendations were also made to lead to professional standards for in situ measurement of gaseous emissions from soil. The study's results can be used as reference data for natural emissions, needed to evaluate non-biogenic phenomena, particularly the 'mine gas' hazard. They can also be used outside of the mining and post-mining context: impact linked to industrial pollution of soils, impact of underground storage, landfills, etc. (authors)