The rate of changes from significant weather events is up (e.g. number of storms and the amount of the change in pressure).
CO2 concentrations are changing quite enough at ground level, though that gets to even a ventilation issue in the house or workplace. Working at home I've put a CO2 meter in the room where I work and I can see the fluctuations in that room over the course of a day with me working in it. Overall the world has gone from about 320 ppm in 1960 to 420 ppm in 2021. My reading for "in this room" is currently 687. Normally its in the 530 to 640 range across a day, but if I do any baking (gas stove) it can go up significantly and takes a few days (cold weather - don't have windows open) for it to return back down to pre-baking levels.
Significant daily fluctuations of pressure and/or humidity.
It's not the increase (or decrease), but rather the rate of change.
Once the weather stabilizes, headaches tend to go away.
(I don't think an absolute increase in atmospheric pressures has caused anyone a headache ever. Relative changes over a short enough time might do so, though.)