I believe their new strength rating system has been put in place after the results of research and blood plasma trials. Unfortunately, I never ordered from them previous to the new system of strength ratings. However, I believe that 4.5% (which is the fluid I use to replace my analogues - very successfully may I add) is, by the usual e-fluid strength rating system, 45mg. However, their research suggests that the nicotine absorbed is similar to that of a strong and/or unfiltered cigarette (15mg analogue).
If you are
vaping for the pleasure of it, it is probably going to be too strong for you. If you are
vaping to replace smoking then I can't recommend it highly enough. With "high-strength" 18mg (by the traditional strength ratings) I tend to chain-
vape (I was never a chain smoker). However, with the ECOpure strong stuff, 6 or 7 draws will satisfy my cravings for about an hour - just like actual analogue cigs did for me.
Hope this helps.
EDIT: They also claim that because users of higher strength fluids use less, less often, the overall nicotine absorbed throughout the day is actually decreased when compared to users of weaker fluids. And for me, using less = better economy = longer battery use = winning! But that's just me. Many people just vape for the pleasure of it, i'm 99% sure that this fluid would not appeal to them.