Remember, it's the
amount of what's in liquid or vapor, not necessarily just what's there. For example, tiny
trace amounts of formaldehyde were found in one study of exhaled vapor. Thing is,
all humans emit trace amounts of formaldehyde, smoker or non-smoker, as it is. Like nicotine, not harmful in the amounts we use for vaping, or smoking for that matter (unless you have heart issues or perhaps if pregnant, but that's case-by-case w/your own doctor to determine). But nicotine in high doses would definitely be dangerous, as would straight caffeine in high doses. Even water can be overdosed. So when someone says there's X in the liquid/vapor, ask how much, and find out how that compares to other sources of that particle in daily life. That will put things in perspective and explain why vaping, and even snus, is >99% less harmful than smoking.
/rant