Smoke detectors identify the presence of smoke. When smoke is detected, the alarm sounds.
This is like an acute injury, like an ankle sprain.
You do something, there is an immediate consequence. 
An alarm sounds, and you need to do something.
You need to clear out the smoke, so you open your windows and turn on the fans.
Carbon monoxide alarms are different.
Because you can’t see or smell carbon monoxide, you don’t even know it’s there.
The alarm detect the presence of carbon monoxide over TIME.
It’s not just that instant. Carbon monoxide can slowly build up in the sensor, and then when it crosses the threshold marked by the alarm , it goes off.
This is like chronic pain.
Your problem builds and builds over TIME until one day the alarm sounds and you have pain.
The pain starts after you have had your problem for a while.
Is your alarm for smoke or carbon monoxide?