Objectives: Use of tobacco-based narghiles may have potentially fatal consequences, such as carbon monoxide (CO) intoxication. However, there are only limited studies on the effects on human health and CO levels of tobacco-free narghile products. The primary aim of this study was to investigate the effect of tobacco-free narghile consumption on carboxyhemoglobin (COHb) levels and whether this would cause CO intoxication in active and passive narghile users. Design: Observational study. Methods: Four hundred ninety-nine individuals exposed to tobacco-free narghile smoke were divided into four groups; Group I: active narghile smokers, and non-cigarette smokers, Group II: active narghile smokers and active cigarette smokers, Group III: passive narghile smokers and non-cigarette smokers and Group IV: passive narghile smokers and active cigarette smokers. Fifty-nine healthy volunteers were enrolled as control groups (Group V, non-cigarette smoker controls, and Group VI, cigarette smoker controls). COHb levels were compared among the groups. Results: Mean pre-exposure % COHb levels for groups I-IV were 2.61 +/- 1.46, 3.00 +/- 1.37, 2.09 +/- 1.45 and 3.00 +/- 2.13, and post-exposure % COHb levels for groups were 3.45 +/- 1.92, 4.17 +/- 2.01, 2.60 +/- 1.66 and 4.50 +/- 2.32, respectively. Control groups (V-VI) % COHb levels were 0.90 +/- 0.82 and 1.60 +/- 0.85, respectively. COHb levels rose significantly after exposure to tobacco-free narghile smoke in all groups (p<0.001). COHb levels in all groups were higher than those in healthy individuals. Additionally, 12.8% of those exposed to tobacco-free narghile smoke had toxic COHb levels. Conclusion: The use of tobacco-free narghile products represents a potential danger, and COHb levels in active and passive smokers can reach intoxication levels.