There is emerging evidence, because it's not just.... The total amount of nicotine concentration needs to be considered and balanced, not so much with how much there is for the person who is going to vape it, but also with the risk to bystanders, children, or others who might get poisoned by it because they're not tolerant to nicotine. These are really the concentration effects. They are determined on that.
What we do know is that the more concentrated the nicotine is per millilitre in the e-juice, the less people vape and the less deeply they vape, but of course that is also modified by the voltage setting, which can be adjusted on the device.
I think the overall discussion will have to take a look at safety to others more than safety to the person vaping, because of the potential toxicity of nicotine to people who are not tolerant to it, especially children.