Much like N2O I suspect there is a reason the CO2 is not food grade . Could be that it is just not as purified and the impurities will not really harm you in the very small quantities they are in but must be removed any way by fed law to be used in food .
Or you will get cancer some day in the future from ingesting them .
So why not just buy food grade ?
but then again you are going to get cancer from drinking the water , eating BBQ , breathing air , playing with your self , eating almost any food ........