Plastics, the increase in background radiation, pesticides, and or a side effect of extra calories are all possibilities. Daily allergy medicines might also be a factor as those reduce immune response slightly.
That link suggests nothing of the sort. That aside, I’ve been tracking background radiation for years with my own sensors (for other reasons). I can’t replicate this claim there either, as expected.
That doesn't talk about an increase in radiation. If there was an issue, we would see it in flight crew. There is also a difference with altitude and soil composition, and that would show up in the data.