This is absolutely true. My life was spent in the radar business which is all about ionizing radiation so I know quite a bit about this. Of all career fields airline pilots receive more ionizIng radiation annually than all other relevant career fields, e.g., nuclear power plant workers, dental technicians, etc. I have taken a Geiger counter up with me countless times and the higher in altitude you go, the more ionizing radiation you get! Trip across the pond? Figure it's like an extra chest x-ray. Our NASA missions to Mars are challenged right now about having our astronauts survive cosmic radiation over such a long mission. Earth's atmosphere does wonders to protect us from chromosomal mutation.
Dan