Calculate the percentage of molecules containing carcinogenic compounds that element is hydrogen.
hydrogen refers to element = 'h'; label = '+' mean molecules are carcinogenic; percentage = DIVIDE(SUM(label = '+' and element = 'h'), COUNT(molecule_id)) * 100.0
SELECT CAST(COUNT(CASE WHEN T1.element = 'h' 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 WHERE T2.label = '+'