How to detect bad smells?

[urinetid]

Hey everyone!

I need to detect bad smells or smoke with an ESP32 s3
Specifically, I need a sensor that can detect:
– sewage smell coming back from a septic tank (sometimes its smelly for 10-20 mins)
– wood‑smoke / burning smell when someone is starting a fire and it gets smoky. (this is also a temporarily issue for 20-30mins)

I have a DIY heat recovery ventilation system and i was thinking to put a sensor on the roof where i have the fresh air intake. But then my esp32 is like 5 meters away from the sensor. Worst case i could put a slave esp32 on the roof as well so its close to the sensor, but best would be to avoid this.

but anyhow, as a starting point, which sensors are good for my usecase? What i want is this simple rule: if air is smelly, turn off the ventilation for 15 mins, then retest.

thanks in advance for any reply.