sub-slab depressurization 6

https://golf-wiki.win/index.php/Understanding_Radon_Crawl_Space_Cost:_What_St._Louis_Homeowners_Should_Expect

Sub-slab depressurization is a way to keep soil gas, especially radon, from seeping into a house through the concrete floor. A fan creates a slight vacuum under the slab, pulling those gases out through piping to the outside air before they can get inside. That gentle suction also lowers the pressure below the slab so gas won't find its way through cracks or joints.