Abstract
Sustainable Development Goal (SDG) Indicator 6.2.1 requires household handwashing facilities to have soap and water, but there are no guidelines for handwashing water quality. In contrast, drinking water quality guidelines are defined: water must be "free from contamination" to be defined as "safely managed" (SDG Indicator 6.1.1). We modeled the hypothesized mechanism of infection due to contaminated handwashing water to inform risk-based guidelines for microbial quality of handwashing water. We defined two scenarios that should not occur: (1) if handwashing caused fecal contamination, indicated using Escherichia coli, on a person's hands to increase rather than decrease and (2) if hand-to-mouth contacts following handwashing caused an infection risk greater than an acceptable threshold. We found water containing <1000 E. coli colony-forming units (CFU) per 100 mL removes E. coli from hands with>99.9% probability. However, for the annual probability of infection to be <1:1000, handwashing water must contain <2 × 10-6 focus-forming units of rotavirus, <1 × 10-4 CFU of Vibrio cholerae, and <9 × 10-6 Cryptosporidium oocysts per 100 mL. Our model suggests that handwashing with nonpotable water will generally reduce fecal contamination on hands but may be unable to lower the annual probability of infection risks from hand-to-mouth contacts below 1:1000.
Highlights
Sustainable Development Goal (SDG) Target 6.2 calls for “adequate and equitable sanitation and hygiene for all” by 2030.1 The core hygiene indicator requires households to have handwashing facilities with soap and water, where an adequate handwashing facility is defined as “a device to contain, transport, or regulate the flow of water to facilitate handwashing with soap and water”.1 The target is not met when water is not available or when hands are dipped into stored water
We extended this model to create a quantitative microbial risk assessment (QMRA) model [QMRA model (Figure 1)] by incorporating microbial transfer from hands to saliva[11] and estimating the pathogen dose ingested accidentally due to hand-to-mouth contacts after handwashing and the corresponding probability of infection for the following reference pathogens: enteropathogenic and enterotoxigenic E. coli, Salmonella enterica, Shigella f lexneri, Vibrio cholerae, Cryptosporidium spp., Giardia spp., rotavirus, and norovirus
All Bayesian models converged after 50000 iterations as indicated by the fact that the upper 95% confidence limits (CL) of potential scale reduction factor (PSRF) values for each stochastic node were all below 1.15 with the exception of the α parameter of the V. cholerae dose−response model, which had a PSRF value of 1.27
Summary
Sustainable Development Goal (SDG) Target 6.2 calls for “adequate and equitable sanitation and hygiene for all” by 2030.1 The core hygiene indicator requires households to have handwashing facilities with soap and water, where an adequate handwashing facility is defined as “a device to contain, transport, or regulate the flow of water to facilitate handwashing with soap and water”.1 The target is not met when water is not available or when hands are dipped into stored water. Program for Water Supply, Sanitation, and Hygiene added a new category for safely managed drinking water, which stipulates that water must be “accessible on premises”, “available when needed”, and “free from contamination”.1. In contrast to drinking water, hygiene has no definition for safe management, and there are currently no guidelines for handwashing water quality. Such a guideline and definition would be especially useful for developing safe handwashing practices in locations where dry sanitation facilities are used or where potable water is scarce. Riskbased drinking water quality guidelines have been established;[2] the extent to which handwashing water quality influences disease transmission is unclear
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