If you have something significant to say to your users, you need to get their attention—fast. One of the best ways to get a user's attention quickly is to scare them.
Sorry, but it's true.
Just be forewarned—using fear-based messaging to make an impression is a strong tactic. It works, but if you abuse it (and the thing you have to tell a user really isn't that important) then you've insulted their trust, and the relationship between you and that user will never be the same.
So, tread lightly.
It works something like this: the amygdala is the part of your brain that is responsive to stimuli that enable human survival. It's the part of the brain that controls the "fight or flight" response to threats or anxiety, and when it is activated it tends to take over pretty much everything.
The volume of information that is on the internet today is entirely too much for any one human mind to process. So, to cut above the noise, you can utilize a cognitive bias that humans are hard-wired to respond to (literally "target" the amygdala) by using certain words like "never" "danger" "caution" or "fire"—all words which typically preclude a way to avoid something that could kill us.
However, don't go constructing headlines that purport to be life-saving but are really just camouflaged listicles designed to sell ads—think the entire content strategy of "The Daily Mail." That's how fake news happens.
In many ways, this strategy is like "the force" from Star Wars. It's got two sides: one side is a force for good, and the other side is dark as f*ck and has been ruined by evil people using it to sell things.
Actually, if we're being honest about the Star Wars franchise, that's just everything made after "The Empire Strikes Back" in a nutshell.