The aim of the present study is to compare serum nicotine levels of different e-Cigarette strength with usual cigarettes. Serum levels of carcinogenic and toxic substances will be also compared.

The study is randomised cross-over trial designed to compare: 1) serum nicotine levels of four different e-Cigarette strength ; 2) Serum levels of carcinogenic and toxic substances will be also compared. Participants will be requested to abstain from smoking and alcohol from 20:00 on the night before each study day and from food and caffeine for at least 1 h before the session. On arrival at the study centre, carbon monoxide (CO) will be measured in participants' expired breath. If CO will be less than 15 parts per million (ppm), the assigned study treatment will be allocated; however, if CO was > 15 ppm or they will report smoking in the previous 12 h, participants will be rescheduled wherever possible to a subsequent session.

On the first study day, participants will be randomised to use one of five ordered conditions (each separated by 3 day): ENDDs (fourth generation) containing nicotine 2,4%; ENDDs (second generation) containing nicotine 7,4 mg; ENDDs containing nicotine 9 mg; ENDDs nicotine free; their usual cigarette. Additionally participants will be trained to use an electronic cigarette. The following tests will be completed at the each visits: Aldehydes; ROM/TOS; Total thiols; Nicotine; Hydroxypyrene; Cotinine; 1,3-butadiene; Acrylonitrile. After, they will be cannulated and serial venous blood samples for Aldehydes; ROM/TOS; Total thiols; Nicotine will be collected at 0.5, 2, 5, 15, 30 and 60 min after being instructed to take 10 puffs. Additionally, levels of carbon monoxide in exhaled breath (eCO) will be measured 0.5, 2, 5, 15, 30 and 60 min after being instructed to take 10 puffs. At 60 min, after being instructed to take 10 puffs, urine sample for Hydroxypyrene; Cotinine; 1,3-butadiene; Acrylonitrile will be collected. Participants then will leave the study centre with instructions to continue their usual daily activities and to use the study product regularly (for at least 10 hrs) and freely throughout the day. At bedtime and after the puffing chronic phase, when participants will use the study product regularly for at least 10 hrs, urine sample for Hydroxypyrene; Cotinine; 1,3-butadiene; Acrylonitrile will be collected. Participants will be allowed to smoke as they will wish once these measures will be collected and during the 3-day washout period between each study day.

Inclusion Criteria:

• 20 regular smokers (smoking ≥ 15 factory-made cigarettes per day for at least the past 10 years and not currently attempting to quit smoking or wishing to do so in the next 30 days, CO > 10 parts per million ) will be recruited from the local Hospital staff in Catania, Italy.

Exclusion Criteria:

• people who reported recent myocardial infarction, angina pectoris or other serious medical conditions (diabetes mellitus, severe allergies, poorly controlled asthma or other airways disease, poorly controlled psychiatric disorders or current chemical dependence other than nicotine) and
• pregnancy,
• breastfeeding,
• blood pressure > 180 mm Hg systolic and/or 100 mm Hg diastolic,
• weight < 45 or > 120 kg, or
• current use of any other smoking cessation medications