I'd love to know what percentage of flights experience fume events to determine if it's a real problem I should worry about.

100% outside of those with electric compressors for bleed air. You can smell it with your nose on takeoff and landing.

For what it's worth, there is a very high number of harmful ultrafine particles in the air whenever a flight is taking off or landing, and also on the ground at the terminal. Most cases of exposure are just not acutely damaging to someone with normal lung function, but the damage absolutely accumulates with multiple flights. The heart in particular is no less sensitive to them than the lungs. The noted mask, if worn tightly, will block a good chunk of them.