There are good points all around this discussion, and a lack of organization. Let's try to clear this up a bit.
Different e-cigarette juices contain different carriers. Some specifically exclude chemicals which produce formaldehyde or, particularly, acetaldehyde, largely because acetaldehyde is known to cause popcorn lung in chronic, high exposure. Most high-quality formulations list their contents in full; and the content of lower-quality formulations is often known, but not readily-listed. High-quality formulations often don't contain chemicals producing acetaldehyde, and use propylene glycol as a carrier; lower-quality formulations also often omit those compounds, but frequently do not.
Different e-cigarettes have different temperatures and control mechanisms as well. They may prevent overheating, or they may reach high temperatures, or they may be designed for brief activation intervals with no temperature controls. Fast-reaction circuits necessarily draw high current, and will overheat without temperature management; thus cheap, fast-reaction circuits intended for brief activation will most often overheat and cause reactions, converting benign substances such as propylene glycol into dangerous substances such as formaldehyde.
Finally, gaseous vapors produce visual distortion when diluted. If you suck in 2cc of suspended smoke or vaporized PPG and then blow it out into the air, it will expand to a liter or more and demonstrate itself as a gray cloud. The real measures are temperature and mass of substance; the substance changes its standard volume at pressure and becomes diluted when diffusing through atmosphere, and so these are poor measurements.
Thus it is wholly-possible to engineer a substantially-safe e-cigarette, if examining specific concerns of e-cigarettes (conversion of chemicals to dangerous chemicals; high-temperature vapor irritating the throat and lungs; basic chemical content). This requires engineering of the compound itself and the delivery device.