Database: toxicology
Question: Calculate the percentage of molecules containing carcinogenic compounds that element is hydrogen.
SQL: SELECT CAST(COUNT(CASE WHEN T1.element = 'h' AND T2.label = '+' THEN T2.molecule_id ELSE NULL END) AS REAL) * 100 / COUNT(T2.molecule_id) FROM atom AS T1 INNER JOIN molecule AS T2 ON T1.molecule_id = T2.molecule_id