Shale oil contains polycyclic aromatic hydrocarbons which are carcinogenic. It has been described that raw shale oil has a mild carcinogenic potential which is comparable to some intermediate refinery products, while upgraded shale oil has lower carcinogenic potential as most of the polycyclic aromatics are believed to broken down by hydrogenation.[34] The World Health Organization classifies shale oil as Group 1 carcinogens to humans.[35]