Database: toxicology
Question: Which element is the least numerous in non-carcinogenic molecules?
SQL: SELECT T.element FROM (SELECT T1.element, COUNT(DISTINCT T1.molecule_id) FROM atom AS T1 INNER JOIN molecule AS T2 ON T1.molecule_id = T2.molecule_id WHERE T2.label = '-' GROUP BY T1.element ORDER BY COUNT(DISTINCT T1.molecule_id) ASC LIMIT 1) t