Effect of monochloramine disinfection of municipal drinking water on risk of nosocomial Legionnaires' disease

Lancet. 1999 Jan 23;353(9149):272-7. doi: 10.1016/S0140-6736(98)06394-6.


Background: Many Legionella infections are acquired through inhalation or aspiration of drinking water. Although about 25% of municipalities in the USA use monochloramine for disinfection of drinking water, the effect of monochloramine on the occurrence of Legionnaires' disease has never been studied.

Methods: We used a case-control study to compare disinfection methods for drinking water supplied to 32 hospitals that had had outbreaks of Legionnaires' disease with the disinfection method for water supplied to 48 control-hospitals, with control for selected hospital characteristics and water treatment factors.

Findings: Hospitals supplied with drinking water containing free chlorine as a residual disinfectant were more likely to have a reported outbreak of Legionnaires' disease than those that used water with monochloramine as a residual disinfectant (odds ratio 10.2 [95% CI 1.4-460]). This result suggests that 90% of outbreaks associated with drinking water might not have occurred if monochloramine had been used instead of free chlorine for residual disinfection (attributable proportion 0.90 [0.29-1.00]).

Interpretation: The protective effect of monochloramine against legionella should be confirmed by other studies. Chloramination of drinking water may be a cost-effective method for control of Legionnaires' disease at the municipal level or in individual hospitals, and widespread implementation could prevent thousands of cases.

MeSH terms

  • Case-Control Studies
  • Chloramines*
  • Chlorine
  • Cross Infection / epidemiology
  • Cross Infection / prevention & control*
  • Disease Outbreaks / prevention & control
  • Disinfection / methods*
  • Humans
  • Legionnaires' Disease / epidemiology
  • Legionnaires' Disease / prevention & control*
  • Regression Analysis
  • Risk Factors
  • United States / epidemiology
  • Water Microbiology*
  • Water Supply / standards*


  • Chloramines
  • Chlorine
  • chloramine