A funny thing happened on the way to 12mg/ml...
I was a heavy smoker. I consumed about 2 1/4 PAD of strong 'pipe tobacco' stuff-your-own cigarettes. Evidence suggests that the equivalent in traditional cigarettes would have been about 3 PAD of Marlboro 100's.
Based on heavy consumption, I chose 36ml/mg ejuice to start my vaping experience. I credit that level for an unequivocal and instant stopping of smoking on a dime. For economy, it wasn't long before I took up DIY for eliquids. There were some effects I found undesirable at that level. The main one was dry skin on my face, forehead and ears. Drinking high quantities of water didn't and still does not appeal much to me per circumstances that come aling with aging (66). I do take in a reasonable amount of liquid. The dry skin, along with perhaps a little light-headedness caused me to think that cutting my nicotine level seemed to be in order. I don't recall how long I lingered at 30mg/ml, but it wasn't too long before I settled in at 24mg/ml. I stayed there for two years or so.
At some point, I began to notice significant light-headedness and it seemed to be related to the level of nicotine consumption. I decided to cut to 18mg/ml. That alleviated the problem...for a while. I suppose I spent the better part of a year at 18mg/ml. Supplies of nicotine base may have changed in that period.
Not long ago, the light-headedness began to return and it seemed substantial. "Oh well," I thought and decided to move to 12mg/ml. Certainly that didn't provide the same satisfaction that I was used to getting in terms of throat hit. I suspect my total time vaping may have increased some as well.
The diminution of light-headedness didn't seem to happen as it had when I previously cut nicotine levels. I shortly found myself cutting my 12mg/ml mixes to about 8ml. The light-headedness continued. Could I be satisfied with 6mg/ml or less..?
By chance, I came across some commercially produced 18mg/ml ejuice I had purchased not long ago. Due to having some floral taste to it, I had left it to vent uncapped and it was away from where I might frequently notice it. I forgot about it for a while until I inadvertently came across it. I decided to give it a try. I was thinking that since I'm getting light-headed at ~8mg, 'this stuff is going to really take me for a ride.'
I was pleased to find that was not the case at all. Thank goodness the throat hit was back and I'm experiencing no unusual light-headedness. In checking two other commercial 18mg/ml ejuices, I found no effects from one and some light-headedness from another. I don't plan on doing extensive vape testing of those that seem to produce such a side-effect, though verification could be valuable. Given an appropriate nicotine base, I could imagine returning to 24mg/ml.
I not saying that there aren't other factors that could be at play, such as flavorings and bases, but it seems to me signs point towards particular nicotine sources as a cause of light-headed sensations.
In addition to the many comparative qualities of nicotine properties found in this thread, there may others due consideration. Light-headedness and similar effects may be a category to evaluate. All nicotine may not be the same in this regard. Cumulative effects may be a concern. Did I mention arthritis..?
A fresh nicotine base sample and VG has been obtained from a well respected source and no ill effects have been noticed at 18mg/ml. Perhaps I'll try some 24mg/ml.
BTW I do use Google and it did not do that thing about translate lol
