pH can go as low as 6.5 at tumor microenvironment (tumor type GBM). See my paper for references within https://www.researchgate.net/publication/233742776_Blebbishields_the_Emergency_Program_for_Cancer_Stem_Cells_Sphere_Formation_and_Tumorigenesis_after_Apoptosis
Article Blebbishields, the Emergency Program for Cancer Stem Cells: ...