Water-pipes (hookah, narghile, shisha, goza, hubble bubble and in South Africa hubbly-bubbly) have been used to smoke tobacco for more than 400 years. Traditionally in the Middle East water-pipe smoking has been the habit of older men, but it is increasing in popularity worldwide, especially among students, other young people and children. 1 The popularity of water-pipe smoking appears to be based on its social nature and the assumption that its effects are less harmful than those of cigarette

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... moking. 2 The increase in water-pipe smoking can partly be attributed to the popularity of the sweetened and variously flavoured tobaccos that are used in the water-pipe. A water-pipe is essentially a glass bottle with hoses and a mouthpiece through which to smoke. 3 The head contains the tobacco, and since the tobacco is too moist to burn on its own, charcoal is placed on top of the tobacco-filled head. The body is fixed to the neck of the bottle, and its central conduit is submerged in the bottle. The bottle is half-filled with water and placed on the ground. The hose is attached to an aperture in the side of the pipe (Fig. 1) . When the charcoal is lit, the smoker inhales through the hose. This creates a vacuum above the water, drawing air through the body and over the tobacco and charcoal. The smoke bubbles through the water, where it is diluted and cooled, and then inhaled by the smoker. Carbon monoxide (CO) is produced by the incomplete combustion of carbon-containing material, notably tobacco, biomass fuel and fossil fuel. The carboxyhaemoglobin (COHb) level in the blood (half-life 2 -6 hours) after inhalation of CO will depend on concentrations of inspired CO, duration of exposure, pulmonary ventilation and the COHb level before the inhalation. 4 COHb is a sensitive specific physiological marker of atmospheric CO exposure from both indoor and outdoor sources. In water-pipe smokers expired CO was reported to increase by 300% after an hour of smoking, while in cigarette smokers it only increased by 60%. 5 During a single water-pipe smoking session a smoker may produce a 24-hour urinary cotinine level that is equivalent to smoking 10 cigarettes a day (95% confidence interval (CI) 7 -13 cigarettes a day). 6 Contrary to popular belief, noxious substances such as nicotine, tar and heavy metals (chromium, arsenic, lead) are found in the smoke of water-pipes. 7 Although water-pipe smoking is so widespread, its adverse health effects are not well documented. Data on CO levels in the blood after a controlled smoking session comparing water-pipe smoking with cigarette smoking are limited. We aimed to measure COHb levels in the blood before and after water-pipe smoking and cigarette smoking under controlled conditions. Materials and methods Only self-confessed smokers of cigarettes and water-pipes aged 18 years and older were recruited from students and hospital workers. There were no regular pipe smokers among Water-pipe smoking is growing in popularity, especially among young people, because of the social nature of the smoking session and the assumption that the effects are less harmful than those of cigarette smoking. It has however been shown that a single water-pipe smoking session produces a 24-hour urinary cotinine level equivalent to smoking 10 cigarettes per day. Aim. We aimed to measure carboxyhaemoglobin (COHb) blood levels before and after water-pipe and cigarette smoking sessions. Method. Self-confessed smokers older than 18 years (N=30) volunteered to smoke a water-pipe or a cigarette and have their blood COHb levels measured under controlled conditions. Results. Mean baseline COHb levels were 2.9% for the 15 cigarette smokers and 1.0% for the 15 water-pipe smokers. Levels increased by a mean of 481.7% in water-pipe smokers as opposed to 39.9% in cigarette smokers. Conclusion. The study demonstrated that water-pipe smokers had significantly higher increases in blood COHb levels than cigarette smokers during a single smoking session.