#1: If you live in any major urban area, you are inhaling dust, secondhand smoke and exhaust fumes from restaurants and vehicles on a daily basis. If you're concerned about the health of your lungs, all of these are worse than anything Toppik or any concealer would do to you.
#2: Wear a painter's mask or hold your nose with one hand and sprinkle with the other when you use the concealer.
#3: The fibers are weighed down by gravity. They end up on your sink or on your toilet base. They don't just hang out in the air. You would have to inhale the fibers while they are being sprinkled on your head at the same time. See #2 above and then clean your bathroom so the fibers don't sit around on surfaces.
#4: You can get cancer from pretty much anything. Two-thirds of cancers are actually random genetic mistakes in your body
with no discernible cause.
#5: So much quackery and pseudo-science exists on the internet right now, especially on forums and hair loss sites. Example: I once read someone claim Minoxidil gives you brain cancer. Yet there are men who have used Minoxidil since it was FDA approved in the 80's and are still walking around today.
#6: Try not to take anything as truth unless it is a peer-reviewed, unbiased scientific study with a control group.
You have two options:
A.) Live in fear, avoid fibers and have a bald spot or less-full looking hair
B.) Work up a solution to not breathe in the fibers if you're scared of them and go forward