Cigarette smoking is a high risk factor for respiratory disease. Smoking leads to increased susceptibility to lung infection, and also leads to sustained lung injury which causes chronic respiratory disease of the lungs.
However, hookah smoking, which also involves the intake of tobacco is perceived as relatively benign. This involves first passing tobacco smoke through water in a pipe where it is diluted and cooled. Many people, including medical practitioners believe that toxic components of tobacco are dissolved in the water, making hookah smoking less harmful. While it has always been popular in the middle east, hookah smoking is gaining in popularity across the rest of the world.
Researchers decided to compare respiratory symptoms indicative of lung problems in both cigarette smokers and hookah smokers. They studied 673 participants from Iran and estimated wheezing, chest tightness, cough and sputum production in each of these as a test of respiratory problems. All of these symptoms except sputum production were significantly higher in hookah smokers as compared to non-smokers. Moreover, no significant difference was seen between water-pipe smokers and cigarette smokers.
In a slightly different analysis, which might be relevant to cigarette smokers, they found that the method of inhalation also determined the degree of respiratory problems. Smokers who inhaled deeply had a much higher degree of respiratory problems than those who inhaled with normal quiet breathing.
Despite popular perceptions of hookah smoking, it does have high levels of nicotine and carbon monoxide, in fact greater than cigarette smoke. While there is a chance that people with more respiratory problems are less likely to smoke cigarettes, the above facts make a strong case for caution to be exercised while smoking hookahs.
You can read about this research here.