The discussion section of the paper really hammers home what we’ve learned so far from studies of carbonyls like formaldehyde in e-cig vapor. Methodological problems are common, and they usually come down to the same thing: dry puffs.
Most vapers know what dry puffs (or dry hits) are because they happen from time to time, especially when you’re first starting out. If you try to vape but there isn’t enough e-liquid in the wick, the liquid that is present can overheat and this leads to an unpleasant burning taste. There are lots of articles about this on vaping websites, and the authors point out that it was first described in the scientific literature in 2013.
As the replication of the NEJM study showed, vapers readily identify these conditions, and dry puffs are likely to be responsible for the most concerning results for carbonyls in vapor. Many of the issues with how studies are conducted discussed in the Farsalinos review revolve around reducing the risk of dry puffs.
The authors’ primary recommendations for future researchers are:
- The puffing regime. The volumes, puff times, and intervals between puffs vary a lot in the research, but certain combinations can lead to dry puffs. For example, too short an interval doesn’t give the coil time to cool down between puffs, and long puffs make it more difficult for the atomizer’s wick to keep up. The authors recommend 40-70 mL puffs, 2-4 seconds in length, and with 30 seconds between puffs
- Power settings and atomizers. The key issue for most studies on later-generation vaping products is that some researchers seem to think that any setting is fine for any atomizer. As all vapers know, this isn’t the case. A previous Gillman study showed this convincingly by testing five atomizers at different settings, and showed that newer-generation atomizers – with better wicking – produce only very low amounts of carbonyls even at high power settings
- PG/VG ratios and viscosity. The viscosity of the liquid impacts how well it wicks, and problems with wicking can lead to dry puffs. Intuitively, this would mean high-VG e-liquids are more likely to cause issues. There is some evidence to back this up, but the authors note that more research is needed
- Temperature. You’d probably expect the temperature of the coil to be linked to the risk of dry puffs and carbonyls, but it’s not certain, and more research is needed. Two studies looked directly at the temperature during vaping, and found that emissions increased sharply at around 300 and 350 °C (570 and 660 °F), respectively. The other heated e-liquid outside of an e-cigarette and found that levels increased at 150 °C / 300 °F. The authors point out that it’s “currently unclear” if there is a specific temperature associated with the carbonyl increase during realistic use conditions.
Of course, there is one really simple solution to all of these problems: get vapers to test your protocol before conducting a study. Since dry puffs are identified by taste and this is subjective, the only way to get a reliable answer is to involve real-world vapers. Unfortunately, only four studies of the 32 actually got vapers involved.
There are some other issues that need to be addressed, too. One example is flavorings. A study found that carbonyl levels increased a massive 10,000 times in flavored e-liquids compared to unflavored e-liquids, and dry puffs don’t seem to have been the cause. However, this result doesn’t seem reliable, because many flavored e-liquids were tested in the other studies reviewed, without comparable results. The authors argue that study needs to be replicated to confirm or refute the result.
Another issue is how results are reported. Saying how much formaldehyde is in a puff seems like a good idea, but that isn’t always the case. Higher power settings produce more vapor per puff, so even if the same percentage of the liquid is converted to carbonyls during vaping, it would lead to bigger amounts in each puff. The authors argue it would be more accurate to report results per mL inhaled or per gram of e-liquid consumed.
Overall, there are many potential issues when testing for formaldehyde and other carbonyls in vapor, and researchers shouldn’t just wade in without considering things carefully. The easy solution – getting a vaper to test your method – should become the norm, rather than the exception.