the dose of C14 is so low, it's almost equivalent to 2 hrs standing under the sun. Because it's so safe. it's being approved by FDA for pregnant women and children.
Have a read from Prof Marshall's early article about C14 dosage.
http://jnm.snmjournals.org/content/34/5/821.longThe effective dose equivalent (global mean) to an average person from natural sources was recently reported by UNSCEAR to be 2.4 mSv/yr. Approximately 800 C14-urea "microdose" breath tests (37 kBq per tests) would generate the same effective dose equivalent.