sentences:  Third generation e-cigarettes tend to be larger, with a more powerful battery and two heating elements which allow users to carry the power.Nicotine delivery has typically increased with successive generations of e-cigarette, and it has been suggested that repeated use of second and third generation devices can result in sustained venous blood levels of nicotine which are comparable with those expected in smokers.Electronic cigarettes are a promising harm reduction technology because they deliver nicotine without the dangerous chemicals in tobacco smoke, while remaining attractive to smokers.