Posted By Dave Merchant
IG-55 is a 50-50 argon-nitrogen mix.
IG-541 is 52% nitrogen, 40% argon, 8% carbon dioxide.
FM-200 is a hydrofluorocarbon called heptafluoropropane.

All three are roughly the same density as air and work by reducing the oxygen concentration in the space below the ignition viability limit of ~14%, typically they result in levels between 12% and 14% when used correctly. A healthy adult human can happily tolerate down to 10% for short periods (the time taken to leave the premises), although for longer exposure it can lead to the usual symptoms of hypoxia (headache, dizziness etc.) especially if performing exercise. The constituent gases in each of the extinguishants are completely non-toxic, so the only possible health risk is from the reduction in oxygen, and whilst this may possibly have more influence on people with existing cardiorespiratory disease it's extremely unlikely to cause significant risk to anyone healthy enough to be in work in the first place. You wouldn't want to spend all day in there, but I've worked in 12% for days and the only outcome was a headache - it's about the level of oxygen you'd be breathing if you went to Tibet.

There's always a potential problem if the system hasn't been installed properly and the levels of oxygen fall lower than expected, but that's a design and maintenance issue not an inherent danger of the gas.