4.c.i. Model

'WHO estimation of health impacts from environmental risks is based on comparative risk assessment (CRA) methods, which are used extensively in burden of disease assessments (Ezzati et al., 2002). This approach estimates the proportional reduction in disease or death that would occur if exposures were reduced to an alternative baseline level bearing a minimum risk (also referred to as theoretical minimum risk), while other conditions remain unchanged. The CRA methodology combines data on exposure, disease burden and the exposure-response relationship to estimate the burden of disease associated with that exposure (Ezzati et al., 2002). For each risk factor (unsafe water, sanitation, or hygiene), the population attributable fraction (PAF) is estimated by comparing current exposure distributions to a counterfactual distribution, for each exposure level, sex and age group:

$P A F = \frac{\sum_{i = 1}^{n} p_{i} ({R R}_{i} - 1)}{\sum_{i = 1}^{n} p_{i} ({R R}_{i} - 1) + 1}$

Where pi and RR_i are the proportion of the exposed population and the relative risk at exposure level i, respectively, and n is the total number of exposure levels. The joint burden of exposure to unsafe water, sanitation and hygiene was estimated by the following formula (6):

$P A F = 1 - \prod_{r = 1}^{R} (1 - {P A F}_{r})$

Where r is the individual risk factor, and R the total of risk factors accounted for in the cluster. Additional details on the methods of estimation are available from various publications (1,7).

This methodology has been used extensively to calculate the health gains from improvements in water supply, as well as sanitation and hygiene and had been published in various documents (Clasen et al., 2014; Prüss-Ustün et al., 2014; Prüss-Ustün et al., 2019)

The following four types of data are required to produce estimates for indicator 3.9.2:

Data type

Source

Population

Country level population figures

UN Population Division. https://population.un.org/wpp/

Exposure

The necessary water indicators include

safely managed drinking water services;
basic drinking water services;
population using surface water, unimproved drinking water sources, or limited drinking water services;
population practising household water treatment with filtration, chlorination, or solar disinfection.

The necessary sanitation indicators include

basic sanitation services with sewer connections;
basic sanitation services without sewer connections;
open defecation, unimproved sanitation facilities, or limited sanitation services

One hygiene indicator is used:

population practising handwashing with soap and water after potential faecal contacts.

Many of these data are available in the global database maintained by the WHO/UNICEF Joint Monitoring Programme for Water Supply, Sanitation and Hygiene, and several are SDG indicators. Where countries lack data for one or more indicators, missing values are imputed using multi-level logistic modelling (Wolf et al, 2013; Prüss-Ustün et al., 2014; Prüss-Ustün et al., 2019)

www.washdata.org

Diseaase burden

The total number of deaths and DALYs caused by diarrhoeal disease per year.

WHO Global Health Observatory (GHO) https://www.who.int/data/gho/data/themes/mortality-and-global-health-estimates/ghe-leading-causes-of-death

Exposure-response relationship

The relative risk, which links exposure with disease.

The calculation uses the exposure-response relationship for drinking water and diarrhoea calculated as part of the most recent systematic review of water and sanitation intervention studies and impacts on diarrhoea

(Wolf, J, 2022, under review).

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Rætta metadátur: 3.9.2