Introduction:
How e-cigarette use relates to changes in smoking habits and respiratory symptoms in the population remains controversial. The aim was to prospectively study the association between e-cigarette use and changes in smoking habits and respiratory symptoms.

Material and Methods:
A prospective, population-based study of random samples of the population (age 16-69 years) was performed within The Obstructive Lung Disease in Northern Sweden (OLIN) study and West Sweden Asthma Study (WSAS). A validated postal questionnaire including identical questions was used in OLIN and WSAS at study entry in 2006-2008 and in a follow-up in 2016. In total, n=17325 participated on both occasions. Questions about respiratory symptoms and tobacco smoking were included in both surveys, while e-cigarette use was added in 2016.

Results:
In 2016, 1.6% used e-cigarettes and it was significantly more common in persistent tobacco smokers (10.6%), than in those who quit smoking (2.1%), started smoking (7.8%), or had relapsed into tobacco smoking during follow-up (6.4%), p<0.001. Among current smokers at study entry, tobacco smoking cessation was less common in e-cigarette users than e-cigarette non-users (14.2% vs. 47.6%, p<0.001) and there was no association with a reduction of number of tobacco cigarettes smoked per day. Those who were persistent smokers reported increasing respiratory symptoms, while the symptoms decreased among those who quit tobacco smoking, but there was no significant difference in respiratory symptoms between quitters with and without e-cigarette use.

Conclusions:
The use of e-cigarettes does not seem to further aid smoking cessation or improve public respiratory health.