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. 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 rises. Use the threshold type to filter which changes matter to your automation.

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 Carbon dioxide level changed 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 carbon dioxide sensor is in (like your bedroom or office). You can also select a device, a specific entity, or a label. When you target multiple entities (via area, label, or multiple entity selections), the trigger fires whenever any of them changes.
5. From the triggers shown for that target, select Carbon dioxide level changed.
6. Under Threshold type, configure what kind of change fires the trigger:
   • Select Any change to fire on any change, regardless of direction or new value.
   • Select Above or Below and enter a value to fire only when the new reading is above or below that value.
   • Select In range and enter a lower and upper bound to fire only when the new reading falls inside the range.
   • Select Outside range and enter a lower and upper bound to fire only when the new reading is outside the range.
   • For each option, you can enter a fixed level in ppm, pick a sensor entity or a number helper entity as the threshold.
     • If you don’t have a number helper, you can create one by selecting Create a new number helper.
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. For how an automation is built in YAML, refer to Automations in YAML.

In YAML, put the settings that are specific to this trigger under options. Keep target, if the trigger uses one, and general settings such as id, alias, enabled, and variables at the top level, next to trigger.

When this trigger starts an automation, it passes data about what happened to the automation. You can use this trigger data in the templates of your conditions and actions. To see which data is available, refer to Trigger data in templates.

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:
    type: above
    value:
      number: 1000

This fires whenever the bedroom CO2 sensor reading changes to a value above 1000 ppm. To fire on any change, use type: any and leave out value.

Options in YAML

Targets of the trigger

This trigger requires a target. The target is the object that Home Assistant will watch. You can select 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 as a target, and Home Assistant will watch every matching carbon dioxide sensor behind that target.

• Entity: one specific carbon dioxide sensor, such as sensor.living_room_co2.
• Device: every carbon dioxide sensor that belongs to a device.
• Area: every carbon dioxide sensor in a room or area.
• Floor: every carbon dioxide sensor on a floor.
• Label: every carbon dioxide sensor that shares a label.

You can also select different target types in one trigger. For example, you can add a specific entity and an area as targets in the same trigger to monitor both of them at once.

To learn which entities Home Assistant includes for a device, area, floor, or label, and how it handles several entities, refer to Automation targets.

Trigger variables

When the trigger reacts, the trigger variable identifies the entity that made it react. This is useful when a target contains multiple entities, such as a device, area, floor, or label.

The following trigger-specific variables are available:

• trigger.entity_id: Entity ID of the entity that made the trigger react.
• trigger.from_state: Previous state object of that entity.
• trigger.to_state: New state object of that entity.
• trigger.for: Timedelta object of how long the trigger criteria were met, if applicable.

For example, use {{ trigger.to_state.name }} in an action to include the name of the entity that made the trigger react in a notification.

The trigger variable describes the entity that made the trigger react. It doesn’t contain all the entities of the target.

Good to know

• Indoor CO2 levels typically range from about 400 ppm (well-ventilated) to over 1,000 ppm (stuffy room). A threshold around 800 to 1,000 ppm works well for most ventilation automations.
• The threshold type controls which changes fire the trigger. Use Above or Below to fire only when the new level is above or below a value, In range to fire only when it’s inside a range, and Outside range to fire only when it’s outside a range. Use Any change to fire on every change.
• The threshold is in ppm. A sensor that reports CO2 in another unit never matches. If you use an entity as the threshold, it must also report ppm.
• 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, and then make the trigger react. For example, change the state of the targeted 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], turn on a light that the trigger monitors, or change its brightness. To check whether the automation ran, open its traces.

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup. In the visual editor, select the UI options listed in the example when they are available, or paste the YAML code.

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 changes to a value above 1000 ppm, keeping the air fresh so you wake up feeling rested.

• Trigger: Carbon dioxide level changed
• Target: Bedroom CO2 sensor
• Threshold type: Above 1000 ppm
• Action: Turn on fan

alias: "Ventilate bedroom on CO2 change"
triggers:
  - trigger: air_quality.co2_changed
    target:
      entity_id: sensor.bedroom_co2
    options:
      threshold:
        type: above
        value:
          number: 1000
actions:
  - action: fan.turn_on
    target:
      entity_id: fan.bedroom_ventilation

Still stuck?

To find out why the trigger didn’t react, make the change that it should react to, and then open the traces of the automation. If the automation didn’t run, there’s usually no trace, but some triggers record a Did not trigger trace. For details, refer to Testing automations. For common problems and their solutions, refer to Troubleshooting automations.

If that doesn’t help, 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

Explore related triggers:

• Carbon dioxide level crossed threshold: Triggers when one or more carbon dioxide levels cross a threshold.