There have been some (somewhat limited) studies that show (certain) cancer risks increase with flying hours:
The risk for all cancers, malignant melanoma, prostate cancer, basal cell carcinoma of skin, and basal cell carcinoma of trunk increased with an increase in number of employment years, cumulative air hours, total cumulative radiation dose, and cumulative radiation dose sustained up to age of 40 years.
Is there anything that can be modified within aircraft design to mitigate or lower this risk? (to either both passengers and pilots, or at least just the pilots (but what about cabin crew).
An (admittedly terrible) example might be to have some sort of protective layer within the aircraft body, perhaps lead.