The water leftover from the extraction process, known as tailings, contains arsenic, mercury, napthenic acids and polycyclic aromatic hydrocarbons (PAHs).

BBC: Canada's 'dirty oil' challenge

The researchers looked at the level of chemicals linked with cancer, polycyclic aromatic hydrocarbons (PAH), in 12 patients after smoking.

BBC: Smoking

Fracking fluids are believed to contain, in addition to water, benzene, ethylbenzene, formaldehyde, methanol, naphthalene, polycyclic aromatic hydrocarbons, toluene, xylene, boric acid, hydrochloric acid, isopropanol, and diesel fuel.

FORBES: European Gas Giant Backs French Fear Of Fracking