A modern multi-story building with a large white cross on the facade, likely a healthcare facility, stands under a partly cloudy sky. Nearby, trees sway gently in the breeze, adding a touch of nature to the scene.

A modern hospital, likely a healthcare facility, with a cross sign

The Right Monochloramine Dosage for Legionella Control in Hospital Water Systems

Dr. Janet Stout’s Recommendation: Dr. Janet E. Stout – a leading Legionella researcher and president of Special Pathogens Laboratory – advocates maintaining a monochloramine residual in the range of roughly 2–3 parts per million (ppm) in hospital water systems. In a long-term evaluation of monochloramine in a 500-bed hospital, Dr. Stout’s team reported that facilities “with action limits at 2.0–3.0 ppm [monochloramine] should expect to see [effective Legionella control].”

This recommendation stems from field studies where an average residual around 2 ppm (with a typical range of 1–4 ppm) dramatically reduced Legionella contamination.

For example, Stout’s 2014 study at a Pittsburgh hospital (495 beds) maintained ~1–4 ppm monochloramine (avg. ~1.9 ppm) and saw Legionella positivity drop from ~50% of sites to <10% post-treatment.

In practice, Dr. Stout emphasizes keeping at least 2 ppm throughout the hot-water network, adjusting the feed as needed to ensure distal outlets (faucets, showers) sustain that level.

Below this threshold, Legionella may persist in a viable but non-culturable state, whereas above 2 ppm the bacteria are largely inactivated.

In short, Dr. Stout’s work suggests ~2–3 ppm monochloramine is the sweet spot for secondary disinfection in hospitals – potent enough to penetrate biofilms and kill Legionella, yet within safe drinking water limits.

Broader Scientific Consensus on Effective Dosages

Numerous studies and industry reports corroborate Dr. Stout’s recommendation, converging on roughly 2 ppm as a minimum effective residual and up to ~3–4 ppm as needed for safety margins. Key findings include:

Peer-Reviewed Studies: An international study of hospital water systems found that maintaining 2–3 mg/L (ppm) monochloramine was necessary to control Legionella. Over 36 months of continuous treatment, Legionella was virtually eliminated when 2–3 ppm was sustained. By contrast, lower levels led to persistence. Another case reported starting with ~3 ppm then dialing back to ~2.0–2.5 ppm; no Legionella grew for over a year except during a lapse when the chloramine feed stopped.
Industry and Case Reports: The Association of Water Technologies (AWT) and other water treatment experts note that “chlorine and monochloramine [are] effective against Legionella at residuals between 2–3 ppm.” Field case studies from multiple hospitals show that keeping ~2 ppm or slightly above at distal outlets achieves long-term Legionella control. A 3-year trial at an Italian hospital maintained 1.5–3.0 ppm monochloramine with only 1 of 84 samples positive after the initial startup period. In the U.S., real-world hospital implementations used 1–4 ppm with ~2 ppm average, yielding >80% reduction in Legionella positivity.
Mechanistic Rationale: Monochloramine’s biocidal efficacy, especially within biofilms, improves significantly above 1–2 ppm. At sub-1 ppm levels, Legionella may survive or only be weakened, but “above 2 ppm monochloramine can kill most of the Legionella bacteria,” achieving full inactivation. Monochloramine is more stable and better at penetrating biofilm than free chlorine, but it works more slowly, so maintaining a higher residual concentration for longer is key.

Case Studies and Guidelines in Hospital Settings

Hospitals are increasingly adopting on-site monochloramine generation for supplemental disinfection, with documented success in controlling Legionella:

UPMC Mercy Hospital (Pittsburgh, USA): This 495-bed hospital (studied by Dr. Stout et al.) installed a monochloramine system on its hot water recirculation. Over 2 years, it maintained roughly 1–4 mg/L (target ~2 mg/L) monochloramine. Legionella at outlets dropped from 53% of sites (baseline) to 9% after monochloramine was introduced. Within 2 months of continuous treatment, L. pneumophila was no longer detectable in any water samples. This case, published in Infection Control & Hospital Epidemiology, was the first U.S. evaluation of hospital-scale monochloramine treatment and demonstrated long-term efficacy without adverse effects on water quality.
Modena and Pisa Hospitals (Italy): At Modena’s university hospital, continuous dosing achieved complete Legionella eradication after the first 8 months once a 2–3 mg/L residual was reached. At the University Hospital of Pisa, a 3-year implementation of monochloramine (post thermal and chlorine flushing) held about 2 mg/L in the hot water; as a result, L. pneumophila counts dropped from ~7200 CFU/L to zero in routine samples.
Regulatory and Guidance Documents: While no U.S. federal law specifically mandates monochloramine in hospitals, public health authorities recognize it as an effective control option. The World Health Organization (WHO) notes that monochloramine is effective for potable water disinfection at ~3 mg/L. The U.S. EPA’s drinking water rules set a maximum residual disinfectant level (MRDL) of 4.0 mg/L for chloramines (as Cl₂), effectively capping how high hospitals can dose. In practice, 1–4 ppm is the normal dosage range. Industry guidelines such as ASHRAE Guideline 12-2020, AIHA 2015, and VA directives list monochloramine as a supplemental treatment, advising facilities to follow local regulations and maintain effective residuals throughout the system.

Best Practices and Conclusion

Both Dr. Stout’s recommendations and the wider scientific consensus point to 2–3 ppm monochloramine as an optimal target for controlling Legionella in hospital water.

To implement this, a facility typically installs an on-site chloramine generator feeding the hot water loop, aiming for ~2 mg/L at distal outlets (and adjusting within the 1–4 mg/L range as needed). Ongoing monitoring of residual levels and Legionella cultures is essential – the goal is to consistently sustain ≥2 ppm at taps and showers. If levels fall too low (e.g. <1 ppm), Legionella may rebound, so Water Safety Plans should define action thresholds (often 2 ppm) to trigger corrective steps. In tandem, water quality parameters like pH and ammonia are managed to prevent issues like nitrification (monochloramine performs best around pH 7.5–8.5).

Hospitals that have adopted monochloramine secondary disinfection report major reductions in Legionnaires’ disease risk, with some essentially eliminating hospital-acquired cases. Overall, the evidence base – including Dr. Janet Stout’s work – supports monochloramine as a proven strategy when maintained at appropriate dosages (~2–3 ppm).

Water safety authorities concur that this level minimizes Legionella in premise plumbing while keeping within potable water safety limits. As part of a comprehensive water management program (per ASHRAE 188 and CDC guidance), monochloramine treatment in the 2–3 ppm range is increasingly recognized as a best practice for protecting vulnerable hospital populations from Legionella.