What is hookah?
A hookah is a single or multi-stemmed instrument for vaporizing and smoking flavored tobacco called shisha in which the vapor or smoke is passed through a water basin-often glass-based-before inhalation.Health risks of smoking hookah include exposure to toxic chemicals that are not filtered out by the water[4] and risk of infectious disease when hookahs are shared.The origin of the waterpipe is from the time of the Safavid dynasty in Persia (Iran),from where it eventually spread to the east into India during that time.The hookah or Argyleh also soon reached Egypt and the Levant during the Ottoman dynasty from neighbouring Safavid Iran, where it became very popular and where the mechanism was later perfected. The word hookah is a derivative of "huqqa", an Arabic term.[Outside its native region, smoking the hookah has gained popularity in North America,South America, Europe, Australia, Southeast Asia, Tanzania, and South Africa, largely due to immigrants from the Levant, where it is especially popular, who introduce it to younger people.
Operation
The jar at the bottom of the hookah is filled with water sufficient to submerge a few centimeters of the body tube, which is sealed tightly to it. Deeper water will only increase the inhalation force needed to use it. Tobacco or tobacco-free molasses are placed inside the bowl at the top of the hookah. Often the bowl is covered with perforated tin foil or a metal screen and coal placed on top. The foil or screen separates the coal and the tobacco, which minimizes inhalation of coal ash with the smoke and reduces the temperature the tobacco is exposed to, in order to prevent burning the tobacco directly.
When one inhales through the hose, air is pulled through the charcoal and into the bowl holding the tobacco. The hot air, heated by the charcoal vaporizes the tobacco without burning it.[verification needed] The vapor is passed down through the body tube that extends into the water in the jar. It bubbles up through the water, losing heat, and fills the top part of the jar, to which the hose is attached. When a smoker inhales from the hose, smoke passes into the lungs, and the change in pressure in the jar pulls more air through the charcoal, continuing the process.
If the hookah has been lit and smoked but has not been inhaled for an extended period, the smoke inside the water jar may be regarded as "stale" and undesirable. Stale smoke may be exhausted through the purge valve, if present. This one-way valve is opened by the positive pressure created from gently blowing into the hose. It will not function on a multiple-hose hookah unless all other hoses are plugged. Sometimes one-way valves are put in the hose sockets to avoid the need to manually plug hoses.
Article From https://en.wikipedia.org/wiki/Hookah#Operation
