Do traditional measures of water quality in swimming pools and spas correspond with beneficial oxidation reduction potential?

Public Health Rep. Mar-Apr 2009;124(2):255-61. doi: 10.1177/003335490912400213.


Objectives: Oxidation reduction potential (ORP) is a more direct measure of water quality in swimming pools and spas than free chlorine. However, ORP is not considered in some state pool codes, including Minnesota's. This study examined whether compliance with the Minnesota Pool Code assured an ORP > or = 650 millivolts (mV), a value defined in the literature as adequate to kill viral and bacterial pathogens within seconds. We also examined predictors of ORP.

Methods: Water samples from public swimming pools and spas in Hennepin County, Minnesota, were collected during routine health inspections from May through August 2004 and assessed for compliance with the state pool code. ORP values were also recorded. A Chi-square test was used to evaluate the association between code compliance and ORP. Analysis of covariance (ANCOVA) and logistic regression models were used to determine predictors of ORP.

Results: The study included 132 pools and 30 spas. Compliance with the Minnesota Pool Code did not assure an ORP > or = 650 mV (p < 0.01). Outdoor pools had significantly lower ORP values than indoor pools (p < 0.001). ANCOVA and logistic regression models showed that ORP decreased with increasing cyanuric acid, increasing pH, and decreasing free chlorine.

Conclusions: Compliance with the Minnesota Pool Code did not coincide with adequate ORP values, particularly for outdoor pools and spas. Therefore, it may be appropriate for states to include a minimum ORP standard of > or = 650 mV in their swimming pool regulations. Doing so would likely benefit the health of swimmers.

MeSH terms

  • Analysis of Variance
  • Chlorine / analysis
  • Environmental Monitoring / legislation & jurisprudence
  • Fresh Water / analysis
  • Fresh Water / chemistry
  • Fresh Water / microbiology
  • Government Regulation
  • Guideline Adherence / statistics & numerical data*
  • Health Resorts / legislation & jurisprudence
  • Health Resorts / standards*
  • Humans
  • Maximum Allowable Concentration
  • Minnesota
  • Oxidation-Reduction*
  • Public Health Administration*
  • Safety / legislation & jurisprudence*
  • Safety / standards
  • Swimming Pools / legislation & jurisprudence
  • Swimming Pools / standards*
  • Triazines / analysis
  • Water / analysis*
  • Water / chemistry
  • Water Microbiology*


  • Triazines
  • Water
  • Chlorine
  • cyanuric acid