Figure 1.

General characteristics of four generations of electronic cigarettes (ECs) and atomizing units. The boxes in the column on the right are terms used to describe the three generations of ECs [4]. These terms are based on the external appearance of the EC (cig-a-like and clearomizer) and on whether it is modified (Mod). Each box gives the generation number and the main features of the battery for each generation. The boxes on the left describe the atomizing units found in ECs of each generation. Each box is titled with the overall group classification name (e.g., “3-Piece EC”) followed by a description of the battery, atomizing unit, and fluid reservoir. Blue box = not included in this study; light brown boxes = included in the study; grey box = an emerging class of ECs not included in this study.