I tried searching but didn't find anything. What is your location's policy on gas leak inside a building. Do you evacuate immediatly, or do you investigate first and only evacuate if LEL is over 10%???
If you can smell gas, in all likelyhood you are still well below the flammable limits even the 10% LEL.
If you are within the flammable limits, 4% (40,000ppmV)for methane, in all likelyhood you would not be standingup as you would be either axphyxiated or blown up.
If within 10% LEL, you would be well over the TLV being 1000ppm, so yes get out of the building. There may be a short term (15 minutes) higher exposure limit but I don't know what it is.
So the advices are those from the above responders.
And the instant the first person gets clear of the building, have him/her call the Fire Department. They are the experts.
The ONLY way I would not evacuate a gassy building is if I had a currently calibrated Air Monitor and was hunting down the leak's source. If the monitor alarms at the source - 10 or 20% LEL - then I would call an evacuation.
Without an air monitor [which the Fire Department will have], you are just guessing. You will probably guess right, like chicopee said above. But you are still guessing with other folks lives. Do you really want to be guessing in a Explosion/Fire/Injury & Death situation? I'm a little arrogant, but not that arrogant.
I am not sticking around to check the levels..... I am running - I am a chicken!!!
Call in the PROS -that is their expertise. But as a part of your safety plan - call the gas company - get and post their recommendations - show a couple of mechanically inclined people where and how to shut down the exterior valve.
I agree with what has been said above about evacuating and calling the FD. I also believe, if I am not mistaken, that if you do investigate yourself that that falls under emergency response and you need to have 2 in - 2 out and an Inicident Command strutcure etc... The FD has all that in place, but you would have to train and implement all that yourself. It would be easier and safer to just evacuate and have the FD deem it safe.
The people killed in Connecticut by a industrial gas explosion DID smell the gas fumes in the enclosed sheetmetal building surrounding a combustion turbine generator being built, DID NOT evacuate the building, and DID continue welding the last pipe joint.
They DID get killed.
1) One hazard nobody has mentioned: Natural gas odorizers in sufficient levels can "overload" your senses and prevent you from noticing the smell. If you smell it, then the odor goes away but the next guy coming into the space asks you about "an odor" (or "does it smell funny in here?") then you need to re-sniff your atmosphere with a four-gas sensor. Quickly.
2) Sniffers run continuously do run out down batteries - but they beep loudly for a while before they shut down. Doesn't mean that anybody heard the beeps.