## 3 Results
### Observational Studies  
#### 3.2.1 Data Synthesis  
A total of 31 studies were observational (Table 1): 10 were cross-sectional surveys using questionnaires (7 were self-administered, 2 were in-person/audio computer-assisted, and 1 was an interview) [41; 45; 46; 47; 48; 49; 50; 53; 57; 58; 112; 113]; 6 studies included a cross-sectional observation with periodontal clinical examination (mainly BOP and other periodontal indicators) [30; 33; 36; 40; 43; 44]; 5 studies focused on the previously mentioned parameters, but were related to implants [28; 29; 32; 34; 46]; 5 studies included both a questionnaire and a clinical examination [38; 42; 56; 57], and in 1 of these, the examination consisted of an oral HPV test [39]; 3 studies analyzed caries variables [43; 51; 54]; 1 study examined oral mucosal lesions, and the last evaluated breath odor [37].  
Figure 1: Prisma Flow Chart.  
All participants were adults (age > 18 years), except for five studies that included younger patients [39; 47; 48; 54; 114]. Age varied among the studies conducted using surveys, where children, early adolescents, young adults, middle adults, and older adults were questioned, and those using cross-section studies with clinical examinations, which were performed mainly on young adults and middle adults. Four studies included participants with systemic diseases [35; 47; 48; 50], while all the others included only healthy participants. Additionally, in 13 studies, participants with recent dental treatments or pharmacological therapies were excluded (from 1 to 6 months before the clinical assessment).  
Ten surveys included questions on socio-demographic status, self-perceived oral health and/or oral health practice [31; 39; 42; 47; 49; 50; 52; 53; 57; 114]. Of these, six studies analyzed data from national surveys on smoking habits, with a sample size ranging from 4618 [51] to 456,343 subjects [45]. Another study was a national questionnaire survey about self-reported oral lesions that differentiated the type of smoking and even the differences among subjects infected by COVID-19 [48]. Different characteristics of ECS were described in two papers, which were both derived from the same questionnaire performed by the NIH in the USA between 2016 and 2018, called PATH (Population Assessment of Tobacco and Health). The first [112] focused on self-perceived oral health in the first wave of the PATH survey, while the second one [40] focused on new gingivitis cases before the third wave. A third study used the PATH survey from the 2013-2014 database and focused on adolescents' oral health and smoking status. Another study recalled ECS from the NHANES (National Health and Nutrition Examination Survey) performed in 2015-2016 and collected oral cells for HPV testing [39]. One study from a national Japanese survey considered HTP and periodontitis using two questions: if periodontal disease is present, and if it has been treated [58]. Several studies described data regarding the \"smoking session\", including duration and daily frequency. The effect of ENDS on periodontal parameters (i.e., BOP, PI, PPD, CAL) was examined in nine cross-sectional studies, while peri-implant parameters (i.e., BOP, PPD) were examinated in five studies. From these 14 studies, 9 studies further analyzed salivary inflammatory biomarkers or receptors of crevicular fluid. The sample size ranged from 57 [61] to 160 subjects [31].