Searches and Selection Criteria Six databases (Medline through PubMed, Embase, LILACS, Web of Science, Scopus, and SciELO), were systematically searched for studies investigating the association between periodontitis, asthma, COPD, and pneumonia. Reference lists from selected articles were also assessed to ensure literature saturation, along with databases containing texts from the gray literature (ProQuest). Randomized controlled trials, cross-sectional, case-control, and cohort studies conducted with human participants aged 18 years and older were included in the review. There were no language restrictions, and publications from January 2010 through June 2019 were included. Exclusion criteria were studies with less than 100 participants, without a clear description of the diagnostic criteria for periodontal or respiratory disease, or with data self-reported by participants. Key Study Factor A diagnosis of periodontal disease was the primary exposure of interest and was defined in studies using a variety of criteria: the Center of Disease Control/American Association of Periodontology (n = 15.38%), the community periodontal index (n = 15.38%), the United States Third National Health and Nutrition Examination Survey (n = 15.38%), the Armitage classification (n = 15.38%), and Gomes and Filho (2007, 2018) criterion (n = 23.10%). Periodontal disease was defined in one study as clinical attachment loss (CAL) >4 mm at 60% of measures sites (n = 7.69%), and another used the case definition of >4 teeth with >1 site, with pocket depths >4 mm, and CAL >3 mm (n = 7.69%), to define periodontal disease. Main Outcome Measure The primary outcome was evidence of respiratory diseases, namely, asthma, COPD, and pneumonia. Asthma was diagnosed according to the Global Initiative of Asthma; COPD was defined by the Global Initiative for Chronic Obstructive Lung Disease; pneumonia was classified by different diagnostic criteria for each study. Main Results Sixty-nine full-text articles were assessed for eligibility, of which 13 studies were included in qualitative analysis and 10 were included in quantitative analysis. Of the 13 studies identified, 8 were case-control, 3 were cohort, and 2 were cross-sectional design. Seven studies (53.85%) were conducted in Asia, 4 (30.77%) in South America, 1 (7.69%) in North America, and 1 (7.69%) in Europe. Quality assessment of the included studies using the Newcastle Ottawa scale was high (76.91%), collectively, studies had a mean score of 7.46, and no individual article was found to have low quality. Meta-analysis was performed using fixed and random effect models to compute odds ratios (ORs) and 95% confidence interval (CI), and adjusted for different covariables including age (92.31% of the studies), smoking habit (76.92%), body mass index (53.85%), sex (46.15%), schooling level (38.46%), and alcoholic beverage consumption (23.08%). Adjusted ORs were calculated for 3 studies investigating the association of asthma and periodontitis (Gomes-Filho et al., 2014; Khassawneh et al., 2019; Soledade-Marques et al., 2018). The adjusted OR for asthma and periodontitis was 3.54 (95% CI: 2.47-5.07) and I-2 as 0% (95% CI 0%-90%) indicating a strong association and low heterogeneity between studies. Adjusted ORs for 4 studies investigating the association of periodontitis with COPD (Barros et al., 2013; Harland et al., 2018; Ledic et al., 2013; Takeuchi et al., 2019) was 1.78 (95% CI: 1.04-3.05), I-2 = 37.9% (95% CI 0%-79%), indicating moderate association with moderate heterogeneity between studies. Analysis of three studies (Gomes-Filho et al., 2014; Iwasaki et al., 2018; Melo Neto et al., 2013) showed that pneumonia was also strongly associated with periodontitis, adjusted OR 3.21 (95% CI: 1.99-5.17) with low heterogeneity (I-2 = 0% [95% CI: 0%-90%]) between included studies. Egger's test confirmed the diffuse distribution of the studies, which resulted in publication bias. Conclusions In conclusion, this systematic review and meta-analysis highlights the association between periodontitis and asthma, COPD, and pneumonia. It validates previous studies and reviews highlighting the association between respiratory diseases and periodontal health; however, there is a need for further research to reduce the heterogeneity of study methodology, participant characteristics, and the diagnostic criteria for periodontal and respiratory diseases. Although oral health prevention and health promotion could be useful tools in reducing the burden of respiratory diseases on public health sectors, additional measures are required to address public health challenges
