Conditions leading to unrepaired
DNA
double‐stranded breaks are potent inducers of genetic instability. Systemic conditions may lead to fluctuation of hydrogen ions in the cellular microenvironment, and we show that small variations in extracellular
pH
, termed suboptimal
pH
e, can decrease the efficiency of
DNA
repair in the absence of intracellular
pH
variation. Recovery from bleomycin‐induced
DNA
double‐stranded breaks in fibroblasts proceeded less efficiently at suboptimal
pH
e values ranging from 7.2 to 6.9, as shown by the persistence of repair foci, reduction of H4K16 acetylation, and chromosomal instability, while senescence or apoptosis remained undetected. By allowing escape from these protective mechanisms, suboptimal
pH
e may therefore enhance the genotoxicity of double‐stranded breaks, leading to genetic instability.