oh well that goes without saying! same if u try to blow dust off and accidentally get a bit of spit on the disc. dont laugh its very easy, just a tine bit of moisture coming out ur mouth will do it. and thats 10x worse than a bit of dust. i would leave it if its not that bad. too risky blowing it off and getting the disc with a tiny bit of spit.
i done it once on a dvd-r and the spot that it occupied was COMPLETELY unburned.
thats why discs have all those signs on them eg. dont get dust on it, dont touch it, dont leave in sunlight, dont get water on it etc etc. after a disc is burned ive never found anything like that to even phase the operation of the disc. but do that stuff before its burned and u got problems.