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Although CO levels less than 10ppm are traditionally considered to define non-smoking, various studies have found that the figure that marks the cut-off point is around 45ppm.26,29,30 Therefore, any subject who shows CO levels higher than 5ppm should be considered a smoker, providing that the examination has been correctly carried out and all cases of false positive co-oximetry have been ruled out (Table 4)

Famele M, Ferranti C, Abenavoli C, Palleschi L, Mancinelli R, Draisci R (2015) The chemical components of electronic cigarette cartridges and refill fluids: review of analytical methods

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