Alfatoxin and benzene are carconogenic, and this is the major concern for low, chronic doses. They are regulated to the few parts per billion level in food. However, according to http://toxnet.nlm.nih.gov/cpdb/pdfs/ChemicalTable.pdf, aflatoxin is three to five orders of magnitude more carcinogenic, as evident from the lower doses needed to induce tumors in rodents.
Why are the permissible thresholds similar?