Foam based soles cushion runners feet in trainers. But these soles
contain sulfur hexafluoride, SF6. While methane, also a green house gas,
lasts for 12 years in the atmosphere, SF6 has virtually no natural decomposition
process and will stay in the atmosphere permanently. In June 2007 SF6
was banned by EU law but you may still own trainers containing SF6.
Source: Sunday Observer June 2007