Follow this guide, but with some minor adjustments stated below.
Don’t place any lasers. The bushes are optional.
Create a trigger.
Set the “activation scope” to “player”
Set “active on start” to “no”
Copy the trigger
Set the delay to “1”
Wire the zone to the trigger #1
(Player enters zone) → (Activate Trigger)
Wire the zone to the trigger #2
(Player enters zone) → (Activate Trigger)
Wire trigger #1 to trigger #2
(Triggered) → (Trigger)
Wire trigger #2 to trigger #1
(Triggered) → (Trigger)
Add a wire repeater
Set the delay to 0.1
Wire the zone to the wire repeater
(Player enters zone) → (Repeat wire pulse)
Wire the wire repeater to trigger #1
(Wire pulse repeated) → (Trigger)
Place a damager device.
Make it damage to whatever poison tick you want.
Set the “Player Knocked out by damager message” to “{Name} got knocked out by the fog!” (Or anything you want)
Wire trigger #2 to damager device
(Triggered) → (Damage)
Create another wire repeater
Wire zone to wire repeater #2
(Player leaves zone) → (Repeat wire pulse)
Wire wire repeater #2 to both of the triggers
(Wire pulse repeated) → (Deactivate trigger)
Repeat this for every zone you place down while following the guide.
You can use wire repeaters if you reach the maximum wire count for the triggers.