A study in Chennai examines how well household reverse osmosis (RO) systems make water microbiologically safe, comparing treated water with source water. Researchers collect 262 water samples from 216 households, covering both water entering RO units and water after treatment. The study reports that even after RO treatment, 31% of post-RO samples still test positive for E. coli, indicating ongoing fecal contamination risk. The findings suggest that RO systems may not consistently achieve effective disinfection unless they are properly maintained and operated. By including both source and treated samples, the study evaluates whether contamination is present before treatment and whether treatment reduces microbial presence. The study’s conclusions focus on the maintenance requirement for RO units to function effectively, rather than attributing results to a single cause. It also underscores that reliance on RO alone may not guarantee safety if system performance declines or if contamination persists through plumbing, storage, or operation practices.