Two blinded trials, in Iowa and Melbourne, gave households a real or a sham filter and found the same rate of stomach illness in both groups. A trial in adults over 55 found a borderline reduction. On a well-run utility, a filter is not shown to improve health. Where water is unsafe, it is a different question.
| Source | What it found (quoted) and its limits | Evidence |
|---|---|---|
| Colford JM Jr, Hilton JF, Wright CC, et al. The Sonoma Water Evaluation Trial: a randomized drinking water intervention trial to reduce gastrointestinal illness in older adults. Am J Public Health 99(11):1988-1995. (2009) · new RCT, blinded crossover (active 1-um filter + UV vs sham countertop unit) · Sonoma County, California, USA; Russian River supply meeting US microbial standards; adults >=55 · n = 714 households/988 enrolled; 557 households/770 analysed |
IRR (active vs sham) 'were 0.88 (95% confidence interval [CI] = 0.77, 1.00)' (GEE) and '0.85 (95% CI = 0.76, 0.94)' (GLMM) HCGI episodes per year; '0.84 (95% CI = 0.78, 0.90) HCGI days per person per year' Limits: 157 households received mislabeled devices and were dropped; older population only; GEE estimate borderline |
Evidence: strong Free to read · source · added 2026-10-30 |
| Colford JM Jr, Wade TJ, Sandhu SK, et al. A randomized, controlled trial of in-home drinking water intervention to reduce gastrointestinal illness. Am J Epidemiol. (2005) · new RCT, triple-blinded, crossover (active vs sham POU device) · Iowa, USA; well-run municipal utility, microbiologically challenged source water · n = 227 households/646 persons active; 229 households/650 persons sham |
'the relative rate of highly credible gastrointestinal illness (sham vs. active) for the entire trial was 0.98 (95% confidence interval: 0.86, 1.10)'; 'episodes in cycle A among the active and sham device groups were 707 and 672'; cycle B '516 and 476' Limits: Abstract only (via Europe PMC); single community; device designed for microbes, not chemical endpoints |
Evidence: strong Paywalled · source · added 2026-10-30 |
| Hellard ME, Sinclair MI, Forbes AB, Fairley CK. A randomized, blinded, controlled trial investigating the gastrointestinal health effects of drinking water quality. Environ Health Perspect. (2001) · new RCT, blinded (real vs sham water treatment units) · Melbourne, Australia; municipal supply · n = 600 families |
'Six hundred families were randomly allocated to receive either real or sham water treatment units'; 68 weeks; 2,669 HCG cases, '0.80 cases per person-year'; real:sham rate ratio 0.99 (95% CI 0.85-1.15; p = 0.85) Limits: Abstract only (PMC full text blocked by captcha, EHP robots-disallowed); high-quality source water, may not generalize |
Evidence: strong Free to read · source · added 2026-10-30 |
| Payment P, Siemiatycki J, Richardson L, Renaud G, Franco E, Prevost M. A prospective epidemiological study of gastrointestinal health effects due to the consumption of drinking water. Int J Environ Health Res 7(1):5-31. (1997) · new Randomized, unblinded, 4-arm (tap, tap-valve, bottled plant water, purified bottled water) · Canada; tap water meeting current standards; Sep 1993-Dec 1994 · n = 1400 families (4 x 350) |
'excess of gastrointestinal illness associated with tap water was 14% in the tap group and 19% in the tap-valve group'; 'Children 2-5 years old ... excess of 17% in the tap group and 40% in the tap-valve group' Limits: Abstract only; unblinded (self-reported outcome); comparator is purified bottled water, not a home filter; single utility |
Evidence: moderate Paywalled · source · added 2026-10-30 |
Evidence grade: strong = trials, large field studies or official datasets; moderate = smaller or lab studies with real water; weak = single small studies, makers' specs or secondary reports. "Free to read" means the source itself is free to read online.