The first long-term study of the risks and effects of e-cigarettes was released in the American Journal of Preventive Medicine, on Monday.

Performed by the University of California San Francisco, the research set out to determine the associations between e-cigarette use and respiratory disease, controlling for people that use combustible tobacco, like cigarettes. 

The results found that e-cigarette use can be an independent risk factor for respiratory disease in addition to using combustible tobacco smoking. Dual usage of both e-cigarettes and regular cigarettes, is much riskier than using either product on its own, the report adds, with higher odds of developing respiratory diseases.

Researchers gathered data from longitudinal studies (studying the same person for a long period of time) between 2013 and 2016. The research also analyzed the subject’s lifestyles, demographics, and other clinical variables, asking people if they already suffer from a respiratory condition.

Over the past few months, e-cigarettes and vaping have been facing major scrutiny after a number of deaths and illnesses have been linked to some of the products.