I agree with ZOGZOG that this problem has been present for years, H2, CO, and CO2 have been high all along ,and time comes to take action.
Note: May be Gases are generated by normal operation and aging, mostly H2 and CO with some CH4
presents of H2, CO :
-Possible Faults
Thermal fault...
I hope you can find the answer here in the attachment link
Aimnhttp://files.engineering.com/getfile.aspx?folder=e4a46f99-0c67-4cd7-861c-1c3a9c9d194e&file=Hazards_ATEX_directive.pdf