Although the overall prevalence of washing hands before eating, after using the toilet and with soap at least once per day, as well as daily tooth brushing, was generally high among LMICs, irrespective of age, gender or BMI (all prevalence >90%), hygiene practices were still poor in several LMICs. For example, 38.7% of students in Tuvalu never or rarely washed their hands before eating, 27.5% of students in Timor-Leste never or rarely washed their hands after using the toilet, 58.7% of students in Honduras never or rarely washed their hands with soap, and 37.8% of students in Egypt brushed their teeth less than once per day. From 2003 to 2015, washing hands with soap showed a significant increasing trend, whereas washing hands before eating showed a decreasing trend.

Currently, studies of hygiene practices among young adolescents mostly focus on oral hygiene [18-20] and at the country level [19,21-23], and the global extent and prevalence of hygiene practices (especially hand hygiene) among adolescents is poorly described. In 2015, McKittrick et al [21] reported that the prevalence of infrequent tooth brushing and hand washing among 33 174 students aged 13-15 years in 15 Latin American and Caribbean countries that participated in the GSHS ranged from 2% to 9%. A study that focused on Iran found that 67.21% of children and adolescents reported daily tooth brushing, and prevalence for washing hands before eating, after using the toilet and with soap ranged from 50.32% to 85.61% [22]. Toothbrushing frequency is similarly high among young adolescents in LIMCs and high-income countries [20], however, a meta-analysis including 42 studies suggested that frequency of handwashing with soap was about 30% higher in high-income countries comparing to LIMCs [24]. Similar to a previous study [20], our study found that estimates differed greatly among countries. The prevalence of hygiene practices varies worldwide, depending on many variables including economic status, urbanisation and parents’ education levels. Therefore, it is of utmost importance to develop health and other youth-centric services, as well as disease prevention and intervention programmes, that are tailored to different LMICs.

Despite the fact that the overall prevalence of hand washing was overall generally high, several LMICs (eg, Tuvalu, Timor-Leste, and Kiribati) showed a high prevalence of infrequent hand washing. In those LMICs, dirty latrines, a lack of toilet paper, overcrowding and the availability and accessibility of water and sanitation facilities in schools are all challenges faced by school staff trying to teach fundamental health behaviours to children [8]. Moreover, hand washing may require infrastructural, cultural and behavioural changes, which take time to develop and require substantial resources [25,26]. Children and adolescents are at risk of multiple infectious diseases when basic hygiene and hand washing habits are inadequate [4,6,27]. For example, Shigella, one of the common pathogens associated with childhood diarrhoea, led to 569 737 deaths of children and adolescents worldwide in 2015 [1,27,28], and there is no vaccine to prevent it [28]. However, the spread of shigellosis from an infected person to others can be stopped by frequent and careful hand washing with soap [27]. A recent meta-analysis [29] of nine community-based trials in LMICs (15 303 participants) found that promoting hand washing prevented 36% of diarrhoea cases. Frequent and careful hand washing is important among all age groups, and supervised handwashing of all children and adolescents should be followed in day care centers, schools and homes, especially in those LMICs with the highest prevalence of infrequent hand washing.

Dental caries is increasing in developing countries, and if untreated it can affect children’s quality of life [22]. In 2015, Kassebaum et al. [30] reported that around 621 million children suffered from untreated caries in deciduous teeth. Caries can alter children’s eating and sleeping habits, dietary intake and metabolic processes, and might affect school attendance, growth and weight gain [31]. Twice-daily tooth brushing with fluoride-containing toothpaste should be encouraged. Long-term exposure to an optimal level (1000 to 1500 ppm) of fluoride results in a substantially lower incidence and prevalence of tooth decay across all ages [11]. An increased frequency of daily tooth brushing was also associated with a decreased risk of tooth plaque, gingivitis and caries [32]. Thus, tooth brushing is an effective way to prevent oral diseases.

In this study, the minority (<10%) of participating students reported never brushing their teeth, which is consistent with previous studies of adolescent oral hygiene practices in LMICs [20,21]. However, a serious oral hygiene problem was also observed, less than 70% of participating students reported brushing their teeth more than once a day, which could reduce its ability to prevent oral diseases. Moreover, in eight LMICs in the Eastern Mediterranean region, over 20% of students reported brushing their teeth less than once per day. This high prevalence of infrequent brushing might be explained by the use of chewing sticks in Arab cultures, leading to a misinterpretation of the question about ‘brushing or cleaning’ teeth [20]. Based on 32 countries, Maes et al. [18] found that poor family affluence was clearly related with a low prevalence of tooth brushing. Children and adolescents in LMICs, compared to those in high-income countries, may have limited access to a variety of options for oral health promotion (eg, community water fluoridation, routine dental sealants) [33,34]. It has been documented that children and adolescents who have early-established oral health practices are more likely than others to maintain these healthy behaviours in adulthood [35,36], minimising their risks of reduced quality of life through pain and tooth loss [37], and reducing the burden of chronic diseases. Therefore, it is especially important that children and adolescents in LMICs develop good oral hygiene practices to prevent oral diseases early in life.

Global reductions in disease burden, improvements in living conditions, dietary transition and lifestyle changes make the sustainable development targets related to health in LMICs increasingly complex. The world has a larger cohort of adolescents and young people today (just under 2 billion, aged 10-24 years) than ever before, of whom 88% live in low-income and middle-income countries [38]. It is clear that improving adolescent health at the hygiene level is an essential and cost-effective investment worldwide. However, the state of knowledge of adolescent health outside high-income countries is restricted, and the information needed to develop effective interventions is commonly unavailable [39]. Currently, school oral health interventions are mostly implemented in primary schools, which is in line with the Health Promoting School concept [25]. As opportunities for school-based oral health interventions can be limited in LMICs, the establishment of prevention-oriented community health programmes is also important. For hand hygiene, the WHO suggests that everyone over 6 months of age washes their hands frequently and practices good personal hygiene during food handling and preparation activities, and notes that persons with diarrhoea, especially children, should wash their hands after using the toilet [27]. Toothbrushing is considered a prerequisite for maintaining good oral health, but some study also suggested that excessive hygiene might be harmful. For example, toothbrushing also has the potential to have an impact on tooth wear, particularly with regard to dental erosion [40]. In addition, our findings highlight the importance of understanding sustainable development goals (SDG) related to malaria, access to safe water, sanitation and hygiene.

The main strength of our study is its large and nationally representative sample of adolescents, with assessment of hygiene patterns in most countries using standardised and well-validated questionnaires [41,42]. However, several limitations should also be considered. First, the GSHS is a self-report survey administered in school settings across countries, which can be subject to recall bias and problems of understanding of the questions. Additionally, different cultural factors in LMICs can results in different patterns of hygienic practices, which can in turn affect self-reporting about prevalence of hygienic practices, a further potential bias in data across countries. In Arab cultures, ‘tooth brushing or cleaning’ may introduce ambiguity about chewing sticks being a form of tooth cleaning [20]. Second, we observed substantial heterogeneity in the prevalence of hygienic practices across regions, which were not fully explained by major study characteristics. Therefore, overall and regional estimates must be interpreted cautiously. Third, estimates are representative at the country level, but we lack additional variables to perform subanalyses by setting (urban vs rural), social economic status or health literacy education. Fourth, GSHS data was collected between a fairly long period of time (2003-2015) and direct comparison between countries should be made with caution. However, most of the surveys (54 of 75) in our study were conducted between a narrow time interval (2009-15).

The findings of this population-based study suggest that although hygiene practices are generally high in most LMICs, they remain poor in several LMICs. Increasing trends of poor hygiene practices was also observed, which emphasises the need for hygiene and health education targeting young adolescents in LMICs.