I love how you cut off the sentence in order to mislead:
> While no epidemiological evidence supports that nicotine alone acts as a carcinogen in the formation of human cancer, research over the last decade has identified nicotine's carcinogenic potential in animal models and cell culture.
It then goes on to say:
> Nicotine has been noted to directly cause cancer through a number of different mechanisms such as the activation of MAP Kinases.[76] Indirectly, nicotine increases cholinergic signalling (and adrenergic signalling in the case of colon cancer[77]), thereby impeding apoptosis (programmed cell death), promoting tumor growth, and activating growth factors and cellular mitogenic factors such as 5-LOX, and EGF. Nicotine also promotes cancer growth by stimulating angiogenesis and neovascularization.[78][79] In one study, nicotine administered to mice with tumors caused increases in tumor size (twofold increase), metastasis (nine-fold increase), and tumor recurrence (threefold increase).
While it was dishonest to cut the sentence off in the middle, the lack of epidemiological evidence basically puts nicotine in the same camp as a ton of other substances that have "carcinogenic potential" but aren't considered a risk. Basically, it's plausible that nicotine is carcinogenic to some degree, but with the carcinogenicity of tobacco explained entirely by other factors, there's no real reason to believe that it's a significant risk, especially for bystanders.
> While no epidemiological evidence supports that nicotine alone acts as a carcinogen in the formation of human cancer, research over the last decade has identified nicotine's carcinogenic potential in animal models and cell culture.
It then goes on to say:
> Nicotine has been noted to directly cause cancer through a number of different mechanisms such as the activation of MAP Kinases.[76] Indirectly, nicotine increases cholinergic signalling (and adrenergic signalling in the case of colon cancer[77]), thereby impeding apoptosis (programmed cell death), promoting tumor growth, and activating growth factors and cellular mitogenic factors such as 5-LOX, and EGF. Nicotine also promotes cancer growth by stimulating angiogenesis and neovascularization.[78][79] In one study, nicotine administered to mice with tumors caused increases in tumor size (twofold increase), metastasis (nine-fold increase), and tumor recurrence (threefold increase).