We assume that it is unflavored zero-nic PG, as that would comply with the requirements:
1. Must coat and protect metal parts from oxidation, even in long-term storage.
2. Must not contain water as this might contribute to corrosion.
3. Should inhibit bacterial growth.
4. Must not be hazardous to inhale.
5. Should not contain nicotine or flavorings that the end user may not want to consume.
Note that the statement "It tastes bad", on first use, is not strictly true: the primer liquid has no taste - but there are various materials being vaporized on first use of an atomizer that do not taste very good, as it is cleaning out manufacturing residues. Such things might include coatings on the silica fibres within the heater coil or wick, residues on the metal of the coil, adhesive residues within the heating chamber, coatings on the steel sleeve, machine cutting oil and so on.
After the first few seconds these have gone, but it's a good idea to just puff out the first 6 or 10 puffs, just to be on the safe side. It's unlikely that such small amounts of anything could be toxic, but if something doesn't taste good then there is no reason to consume it.
Actually the issue is whether there are any toxic residues left over from manufacturing within the atomizer, not if the primer fluid is toxic, as a manufacturer would be unlikely to use anything other than a safe liquid. As such residues will be cleaned out within 30 seconds or so, it doesn't seem a big issue.
