Each cigar contains about 9mg of nicotine, but only 1mg enters the body. It is not the primary cause of smoke-related problems, however the tar that create a thick buildup and kills the lung tissue.
Generally, a 50–100 mg can be already harmful to the body. You only need to take 1-4 mg a day to get the benefits of nicotine.
30-60mg is a high concentration of nicotine already. It is neurotoxic, which means it is lethal to humans. It's affecting mostly the nervous system, nicotine decreases the appetite, stimulates intestinal motility, increases salivation, increases both the heart rate and blood pressure, and may even cause nausea and vomiting. It only takes one milligram of nicotine per one cigarette stick to stimulate the brain. This would be enough to cause cigarette smoking-induced nicotine addiction. So it's very harmful to humans especially to smokers.
One cigarette contains about 1 mg of absorbed nicotine. 50-100 mg of nicotine is considered harmful. You also have to worry about the additives in cigarettes...including the same stuff they use to embalm bodies. In other words, every cigarette is harmful.