An estimated 1.5 billion people globally lacked access to basic sanitation services in 2022, including 570 million with limited or shared services, 545 million with unimproved sanitation, and 419 million who practiced open defecation (UNICEF & WHO, 2023). The United Nations Sustainable Development Goal (SDG) 6.2 aims to achieve access to adequate and equitable sanitation and hygiene for all and end open defecation by 2030 (UN, 2023a). To achieve SDG 6.2, the World Health Organization (WHO) and United Nations Children's Fund (UNICEF) Joint Monitoring Programme (JMP) led the development of global indicators and standardized core questions for the consistent classification of sanitation services for national and global monitoring (UNICEF & WHO, 2018). In 2022, only 17 countries were on track to reach SDG 6.2, 84 were making slow progress, and 24 were off the mark (UNICEF & WHO, 2023). By the JMP definition, basic sanitation includes improved sanitation facilities — which hygienically separate human waste from human contact — that are not shared among multiple households (WHO, 2023). Shared sanitation facilities that are otherwise of an improved sanitation type are classified as “limited” sanitation (WHO, 2023).

Historically, the JMP has debated endorsing shared sanitation as a type of basic or improved sanitation due to concerns over potential decreased security, cleanliness and hygiene, and accessibility (Nelson et al., 2014; UNICEF & WHO, 2010). According to the JMP, the overarching goal of basic or safely managed sanitation for all requires the implementation of private, household sanitation facilities (UNICEF & WHO, 2023). However, the provision of individual household sanitation facilities can be challenging, particularly in informal settlements, due to high population density, space constraints, high cost involved, and the quantity of water required to operate most sanitation systems efficiently (Schouten and Mathenge, 2010; Lebu et al., 2024).

Informal settlements are characterized by 1) a lack of tenure security, 2) neighborhoods cut off from basic services and city infrastructure, and 3) housing that does not comply with building regulations and is often located in environmentally hazardous areas (UN-Habitat, 2015). Slums — the most excluded form of informal settlements — experience high rates of poverty, overcrowding, and are often located in the most hazardous urban land (UN, 2021b; UN-Habitat, 2015). Many residents of informal settlements live on rented or squatted areas without owning the land, which leaves them powerless to make any decisions regarding development (Awunyo-Akaba et al., 2016; Manga et al., 2020). In such conditions, high-quality toilets that are shared by multiple households may be the only viable option for improving sanitation services (Evans et al., 2017). In many informal settlements, such shared sanitation facilities have greatly improved sanitation access by replacing open defecation (Schelbert et al., 2020; VerKuilen et al., 2023).

Significant variability exists across shared sanitation contexts in terms of the number of users, sanitation facility locations, and operation and maintenance responsibilities (Mazeau et al., 2014; Simiyu et al., 2017, Simiyu et al., 2020). Some studies have suggested adjustments to measuring the adequacy of shared sanitation, rather than just focusing on technologies. Adequacy should address the number of households sharing a facility, cleanliness, and user acceptance (Mazeau et al., 2013; Tidwell et al., 2020). Additionally, the Human Right to Water and Sanitation (HRTWS) calls for water and sanitation services that meet criteria related to availability, physical accessibility, quality, safety, affordability, and acceptability (UN, 2014). Under the HRTWS framework, there must be enough sanitation facilities to meet the needs of all people and avoid long wait times, facilities must ensure hygiene and be designed in a physically accessible way, the location of facilities must ensure the physical security of users, and toilets must be regularly cleaned and have facilities for menstrual hygiene management (UN, 2014). The HRTWS framework also incorporates aspects of dignity, privacy, and user acceptance; sanitation facilities will only be perceived as acceptable if their design and conditions of use are sensitive to people's cultures and priorities (UN, 2014). Measuring user acceptance and satisfaction with shared sanitation facilities, however, poses challenges, as satisfaction is a complex concept that reflects personal and cultural experiences and expectations (Nelson et al., 2014; Roma et al., 2010). A previous systematic review did not support the inclusion of shared sanitation as improved sanitation in the existing global sanitation monitoring standards (Heijnen et al., 2014). However, further research is needed on the conditions under which shared sanitation can offer a safe and acceptable alternative to individual household toilets (Heijnen et al., 2014).

While the JMP does not include shared sanitation as basic sanitation, the JMP does report estimates of shared sanitation use, including country-wide prevalence estimates as well as prevalence in urban and rural areas (UNICEF & WHO, 2023). JMP's estimates of shared sanitation prevalence in urban areas may underestimate prevalence in informal settlements, which tend to be more crowded and have less space for individual household latrines than the overall urban area (Ren et al., 2020). In addition, data on the prevalence of shared sanitation within informal settlement areas is limited. To promote shared sanitation as a viable option in informal settlements, it is necessary to understand how common shared sanitation facilities are in this setting, as well as user preferences, experiences, and barriers to shared sanitation use.

This systematic review aims to describe the contexts under which shared sanitation may be an acceptable sanitation option in informal settlements. Our objectives were to document evidence on 1) the prevalence and number of households sharing sanitation in informal, and 2) user experiences and barriers to successful implementation of shared sanitation. Using evidence from objectives 1 and 2, we also aim to 3) provide recommendations for improvement of shared sanitation services and propose additional indicators regarding shared sanitation to complement current JMP indicators.