Well, it isn't healthy.
It will not provide you vitamins, or cure you from cold.
But it isn't very unhealthy. What that means is that there are health risks you must take into account and decide whether it worths it. Smoking hookah can also be very beneficial in terms of the way you feel. For some people, those benefits overly exceed these on cigarettes, which is one of the reasons they quit smoking, which is a good thing, because cigarettes are more harmful than hookah, but they can also be less harmful than hookah. It depends on how much you smoke. The more you smoke the more the risk increases. People have tendency to abuse cigarettes more than hookah, partially because smoking cigarettes is easier, cheaper and more practical.
So, generally there are much more harmful chemicals in generic tobacco, but we will discuss the top 3 most hazardous of them. Videlicet:
- Carbon monoxide (and dioxide when oxygenation)
Let's start off with the nicotine.
Not all hookah is prepared with a tobacco that contains nicotine. It could still be tobacco, but wiped out tobacco, soaked in water even boiled, until its full purification. I would still not give my bets on that though as it may still contain some levels of nicotine, which immediately turns it into an unhealthier product. You can also smoke shisha gels, shisha fruits, steam stones and Hookah Creams - all of these do not contain nicotine unless explicitly added.
Although there is insufficient evidence to classify nicotine as a carcinogen, there is an ongoing debate about whether it functions as a tumor promoter. Almost all hookah tobacco (like Al. Fakher, Gazi and whatever) contain approximately .05% nicotine. If it says .5%, that is probably a typo. Nakhla shisha is the only brand that boast .5% approx.
these percentages will stay the same regardless of whether you load a 20g bowl or a 100g bowl. That means if you buy 50grammes you will be smoking 0.025mg of nicotine around 5 times. Note that this is the general content of nicotine not the actual amount, being absorbed from the organism. The smoker inhales smoke, by which the nicotine absorbs. Hookah is something to be shared with a company, so very often it is being shared from 2 to even up to 10 people, distributing the total nicotine intake, but eventually exposing all to the secondhand smoke. How much of the substance you would get is determined by variety of different factors such as how long you smoke, how much smoke you inhale, how often you inhale. Some people think that because of the huge amount of smoke the hookah produces it is by any chances loading you with much higher levels of nicotine, which is not true, because 1. 50% of the smoke being produced is actually a vegetable glycerine and 2. Because the nicotine level will not magically increase, specially for your bigger clouds. Bigger clouds only means overtoasting the tobacco, which ultimately leads to a reducing time of use. At least pleasurable time. Almost all the times bigger clouds means shorter hookah session lifespan. One hookah session can vary but people generalize it to between 45 minutes and one hour. So there is still some nicotine you will absorb, which makes it unhealthy. Lots of people smoke cigarettes for 45 minutes too, smoking much cigarettes that contain much higher density of poison.
Nicotine leads to addiction. Adding good flavors only make it more charming and more addictive.
The tar is a known carcinogen.
Sometimes, this substance is even more dangerous, because unlike the nicotine, which absorbs in your body fast, the tar does not and it produces dense accumulation over time, that kills the pulmonary tissue. It may also cause cancer. (Support Reference)
The process of purifying tobacco removes some of the tar, but not completely.
Cigarettes take advantage of some 3 centimeter filter of "foam" while the hookah gives you a few hundreds of water filtration, which "wash" the smoke itself.
And it works a-okay. As the time goes and you smoke, you can notice that the water becomes more yellowish. Some people wrongly assume that's all tar, but it is proven that it contains so much more, including molasses and solid particles /heavy metals. The water does not only filter, but it also cools down the smoke and when the smoke is cooled down, it is lighter and when it is lighter it is easier for your lungs to aspirate it.
So a good practice is to put ice in the water phial. You can also try one of those water enhancers. Manufactures claim that it empowers the water with special molecular level filtration abilities. I haven't encounter scientific studies remotely related to this product yet, but from using it I can not tell.
Carbon monoxide and dioxide
This is what is being produced from the process of burning. It is being released by the charcoal. When smoking cigarettes you are exposed to a direct burning of a tobacco set on fire, which gives you a lot of carbon mono and some dioxide. When you smoke hookah, the shisha does not burn. It bakes. On a temperature of anything around 100C. But, I shall tell you that, the carbon monoxide released from angry lighter coal might exceed that from cigarettes and cigs, mainly because of the duration of your drag. Carbon monoxide might be the least of your problems. It causes dizziness, caused by the blood vessels (arterial) tightening, whereat the heart rate increases to compensate the blood delivery to your brain.
Nicotine is in very low levels and density - there is a slight but existing risk of developing cancer and increase of cholesterol levels and even greater risk of addiction. Smoking inside somewhere may lead to a nicotine exposure equal to or greater than that if you were smoking, it depends on the place, its ventilation, air flow and how much people smoke.
The tar is in low levels and density, the more you smoke, the higher levels of it you accumulate. Smoking way too frequent leads to more tar being accumulated, which may damage everything between your mouth and lungs and cause cancer.
Carbon dioxide isn't risky - it WILL violate human internal processes.
So it is unhealthy, obviously. Not so unhealthy to lead to death or cause irreversible consequences as such haven't been documented, to my knowledge.
Very broad NCBI article on nicotine's effects:
In case you need, you can find more support for all that, all over the internet. Trust only trusted articles though, because in this topic, there are a lot of disagreements, from all kind of people: concerned parents, politicians, kids, "informed" bloggers, gov agencies with an unclear amploa, critics, conspiracy diggers etc and etc