A large number of materials can be considered a poison (bring harm) to a human at a certain level of consumption over time. However, when used as directed by the safety information provided, the reasonable consumption in of Nicotine in an e-cigarette doesn't make poisoning effect.
Most of the risks of using Nicotine associated with a temporary change of heart rate and blood pressure. For people in normal average health condition, there are no known long-term health side effects. Nicotine availability to the blood is very quick, so taking too much Nicotine by vaping is associated with visible neurological effects (fast pulse, the spinning head, weakness). Tobacco users mostly rely on increased CO2 in blood effects (not enough air feeling) to control cigarette consumption. Since e-cigarette doesn't contain CO2/CO - this trained protection doesn't work and it takes time for new vapers to use Nicotine associated mild effects to control consumption.
Overdose is very individual for each person, but since effects are coming fast, most people can avoid overdose after mild nicotine effects.
A lethal momentary amount is believed to be between starting from 30mg, but during vaping >90% of e-liquids exhaled and from the rest >80% is oxidized in the air before reaching the blood. Normal consumption is around 2.5ml/day. So daily consumption would be less than 1mg. Nicotine becomes 2 times less potent in the blood after 20 minutes, so at the single moment, the nicotine content will be at least hundreds of times less than minimum lethal dose.
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