Carbon dioxide level changed

The Carbon dioxide level changed trigger fires after the carbon dioxide (CO2) reading on one or more air quality sensors changes by a meaningful amount. Carbon dioxide builds up naturally in occupied rooms from breathing, cooking, and heating. A bedroom with the door closed overnight or a packed meeting room after lunch are classic spots where CO2 creeps up without anyone noticing. Rising CO2 is one of the clearest signs that a space needs fresh air.

Imagine your bedroom ventilation fan spinning up automatically in the middle of the night because CO2 climbed while you slept, so you wake up feeling refreshed instead of groggy. Use this trigger to automate ventilation, log indoor air quality trends, or remind household members to open a window when CO2 shifts noticeably.

Using this trigger from the user interface

If you prefer building automations visually, Home Assistant walks you through this trigger step by step. You pick what to watch, tweak a few options, and save. No YAML knowledge required.

To use this trigger in an automation:

1. Go to Settings > Automations & scenes.
2. Open an existing automation, or select Create automation > Create new automation.
3. In the When section, select Add trigger.
4. Select what you want to monitor. Under By target (see Targets), pick the area your CO2 sensor is in (like your living room or bedroom). You can also select a floor, a device, a specific entity, or a label.
5. From the triggers shown for that target, select Carbon dioxide level changed.
6. Under Threshold type, set how much the level has to change before the trigger fires.
7. Select Save.

Options in the UI

Using this trigger in YAML

If you work directly in YAML, or you want to know exactly what Home Assistant does under the hood, this section has the technical reference. It lists the field names you use in YAML, their types, and which ones are required.

In YAML, refer to this trigger as air_quality.co2_changed. A basic example looks like this:

trigger: air_quality.co2_changed
target:
  entity_id: sensor.bedroom_co2
options:
  threshold: 50

This fires whenever the bedroom CO2 sensor reading changes by at least 50 ppm.

Options in YAML

YAML sometimes provides additional options for more complex use cases that are not available through the UI.

Targets

This trigger supports targets. A target tells Home Assistant what the trigger should watch. You can point it at a single entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more], a device, an area, a floor, or a label, and Home Assistant watches every matching air_quality entity behind that target.

• Entity: one specific air_quality entity, such as air_quality.living_room.
• Device: every air_quality entity that belongs to a device.
• Area: every air_quality entity in a room or area.
• Floor: every air_quality entity on a floor.
• Label: every air_quality entity that shares a label.

You can also mix target types in one trigger. For example, combine a specific entity with an area to watch both at once.

Good to know

• Indoor CO2 levels typically range from about 400 ppm (well-ventilated) to over 1,000 ppm (stuffy room). A threshold of 50 to 100 ppm works well for most ventilation automations.
• The trigger fires on any change that meets the threshold, whether the level goes up or down.
• To react only when CO2 crosses a specific level in one direction, use Carbon dioxide level crossed threshold instead.

Try it yourself

Ready to test this? Go to Settings > Automations & scenes, create a new automation, and add this trigger. Save the automation, then change the state of the targeted entity to watch the trigger fire on your actual entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup.

Automation: ventilate when bedroom CO2 rises

Sleeping in a closed bedroom builds up CO2 overnight, leaving you groggy in the morning. This automation turns on the bedroom ventilation fan whenever the CO2 level shifts significantly, keeping the air fresh so you wake up feeling rested.

• Trigger: Carbon dioxide level changed
• Target: Bedroom CO2 sensor
• Threshold type: 100
• Action: Turn on fan

alias: "Ventilate bedroom on CO2 change"
triggers:
  - trigger: air_quality.co2_changed
    target:
      entity_id: sensor.bedroom_co2
    options:
      threshold: 100
actions:
  - action: fan.turn_on
    target:
      entity_id: fan.bedroom_ventilation

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with the trigger you’re using and what you expected to happen, or share on our subreddit /r/homeassistant.

Related triggers

These triggers work well alongside this one:

• Carbon dioxide level crossed threshold - Triggers after one or more carbon dioxide levels cross a threshold.