If you're not a trained firefighter, whose job description involves fire suppression, you're not covered under the presumptive cancers. As such, there's no reason why you should be anywhere near the IDLH.
When I worked in NYC EMS, wen we were at a Fire75 or MCI21 (structure fire standby), we brought our equipment close to the building, but out of the collapse zone. I left NYC in 2007. At that time, FDNY was providing it's EMT's and medics with structural PPE, excluding SCBA (IIRC). They were required to don the gear on MVA's, but I think it was necessary anywhere else. Their Haz-Tac crews had SCBA's and such, because they were Hazmat Ops.
I've worked in a few different single role EMS systems, where the fire side were only first responders or non-transport BL at the most. At no time were we required to enter the IDLH for ay reason.
At my current place of employment, we're similar to the OP's system, and run mutual aid with them frequently. All of our transport crews carry full PPE and SCBA at all times. We typically respond in our station uniforms (or PT gear with coveralls and safety shoes as the case may be), and get dressed (don PPE) if necessary. What's considered necessary varies from person to person, and BC to BC. Whether we get dressed at the station, or stage and then get dressed, the same exact amount of time passes by, so it makes no difference if we wait until we get dressed at the fire. If anything, if there's a patient U/A, we (txp crew) delayed care a few moments by getting dressed at the firehouse.
Some BC's expect the first due medic unit to help with exterior fireground tasks (if no patients U/A), or even take in a back-up line inside if the second due engine is delayed. Even if we're just helping to throw ladders and pop doors, we typically use our PPE, less SCBA. If we're assigned to rehab on a high-rise fire, we're two floors below the fire floor, so we'll be working with full PPE and SCBA (but not on air). Inside of the fire building, no one-s doing any ALS; the most that you're going to do for someone is assist/carry them out. The only people with spare air packs are the RIT crew, and that pack is for the mayday caller. No real pt care will occur until the patient is outside of the building, and I don't need structural PPE to do curbside pt care. For a car fire or pin job, we'll always be responding with an engine, and all of our engines have at least one medic, so that medic will be in PPE and addressing the patient in the car, with maybe a BLS provider assisting them. How much ALS can you really do inside of a car, that requires two medics? In our system, it's typical for the engine medic to get inside the car with the pt, and the right bucket FF will stand by with a charged hose line. A Heavy Rescue Squad will be there with four people doing the cut job. There's no need for te txp crew, in our system, to get dressed for a pin job or car fire.
In understaffed/underfunded systems, particularly ones that use "single pull," (leave your gear in the middle of the bay and jump on whatever unit gets a call, due to lack of staff), it may be necessary to perform both suppression and medical treatment on the same call. In that case, I say have PPE with you, and just get dressed when it's necessary to do so. The car's on fire? Throw on your gear and get to work. Bodies hanging out of a window, get dressed and go to work. In either case, when getting dressed first (maybe for no reason), and getting dressed at the scene, the time to the patient is virtually the same. In the dead of summer, I find it difficult to concentrate on medical skills if I'm hot and dehydrated, and restricted by my PPE that's been on for a while. If I can work without my coat on, for the sake of argument, then maybe I didn't need the PPE in the first place. In fact, our high heat advisories tell us to remove our PPE ASAP during an incident to prevent hyperthermia, dehydration, and saturating our gear in sweat, which pre-disposes us to steam burns in a structure fire.
