I did some guerrilla street usability testing, of animated science education video fragments.[1] About the size of objects - sort of like Powers of Ten. During red lights at busy street crossings.
Even with a very small test population, I saw people distressed by surprising things.
Mention of millimeters reminded an elderly Brit of unpleasant childhood experiences when learning metric.
A college student expressed distress over a character breaking the head off an (enlarged to arm-sized) T4 bacteriophage.
I had someone run away, across the street, saying "how could you show me something so disgusting!", following a quiet background sound effect, a child's hacking cough, accompanying the word "virus". When I later stopped the video immediately after it to get feedback, most people were "what cough?".
Many people were variously distressed by the mention or discussion of viruses and bacteria. Which I didn't expect, but in retrospect seems unsurprising - for many, they have strong and negative associations. They get very bad press. Think "fun story about viruses!" having the emotional flavor of "fun story about genocide!".
So at this point, I'm unclear on what can reasonably be expected to not distress people.
"How about this nice weather we've been having?"... "Houston has been bringing back painful memories of my family losing its home and business to flooding. :(" Somewhat tongue in cheek, but if you are aiming for do no harm...?
[1] There are some bits of the test videos in the "How to remember sizes" section of http://www.clarifyscience.info/part/Atoms .