Mainly its in the Structure, or the building, example we had a school where the fire was in the electrical room the entire school was an IDLH til it was vented, as for outside the structure, anywhere near an opening where smoke is flowing out could be considered too.
Right on, Buffalo Bill! IDLH is a measurement. This is a good question. We deal with IDLH levels in Haz Mat incidents and in structural fire incidents, we need to be concerned with carbon monoxide, hydrogen cyanide, carbon dioxide, oxygen enriched and deficient atmospheres, and others. When we deal with meth labs, add on hydrogen chloride, chlorine gas, phosgene, hydrocarbons, anhydrous ammonia, freon, sulfur, and others. We ain't fighting the old wood made structures any more. The plastics, vinyls and pre-fabbed materials being manufactured are providing us blacker, toxic and thicker smoke, creating much more of a hazard to us and to those trapped inside. It is such a great idea to do air monitoring now during overhaul and to keep our scba on and us operating on air until it's deemed a safe environment. I encourage all to look up those OSHA TELs and IDLH levels in the products of combustion we now face. John, KUDOS for this question! You got the gears churning.
Andy, I agree completely with encouraging everyone to look up the OSHA and NIOSH hazardous materials exposure limits and to wear SCBA . However, IDLH is not always a measurement that can be taken accurately in the real world. In structural firefighting, IDLH is any area that contains smoke, fire, heat, gases, or particulate that is produced by the fire. The reason for this is that there is no direct-reading instrument that can analyze all of the components that make structure fire byproducts hazardous. A typical 4-gas monitor that will work for hazmat and most confined spaces doesn't have the large number of sensors it would require to measure all of the hazards in structural fire smoke. Ditto for PIDs.
IDLH for hazardous materials, confined spaces, trenches, and the like typically have direct-reading instruments that can measure the hazards pretty specifically.
As far as the original question goes, the answer is that IDLH is considered to be any area that contains fire, smoke, heat, or the other gaseous hazards, or that may rapidly be exposed to those hazards. For a typical house fire, that means the entire house is IDLH.
For a high-rise fire, generally IDLH is considered to start 1 floor before the fire unless there's a combustible liquid or other hazard dropping fire downward. Most high-rise fires don't have products of combustion below the fire.
For strip malls, apartments, big box fires, etc. it might be situational, but the rule should be "if in doubt, wear your SCBA".
For outside fires involving manmade items like vehicles and dumpsters, anywhere in the immediate area is a potential IDLH, so wear your SCBA.
Great reply, Ben. You were a lot more clearer and more to the point. No better rule than "When in doubt, wear you SCBA!" This is what I meant about getting the gears churning. My apology to Ed. This was your great question/post. This site is a great resource.