From the paper:

Its cooling effect may contribute to the intensity of smoking (deeper inhalation and/or more prolonged breath holding) resulting in greater exposure to tobacco smoke toxins. The effect of menthol through increasing the permeability of cell membranes results in a greater absorption of smoked toxins [2,3] Ahijevych and Garrett[2] reported higher cotinine levels in mentholated cigarettes compared to nonmentholated cigarettes, suggesting greater nicotine absorption per cigarette. Clarke et al.[3] found that mentholated cigarette smoking was associated with higher serum cotinine and carbon monoxide levels per cigarette.