And the problem is it s a carcinogen.

The dry deposition was in the form of gaseous pollutants and soot particles containing heavy metals and polycyclic aromatic hydrocarbons, including benzoapyrene, which is a known carcinogen.

And because these materials are so prolific, because they're found in so many places, there's one other place you'll find this material, styrene, which is made from benzene, a known carcinogen.

In the first F4 report, the Panel noted that, for the types of cancers that Iran proposed to study, particularly solid tumour cancers, there was generally a latency period of 15 to 20 years between the time of the initial exposure to a given carcinogen and the first clinical evidence of cancer.