Carbon monoxide level crossed threshold

The Carbon monoxide level crossed threshold trigger fires when a carbon monoxide (CO) reading on one or more air quality sensors crosses a specific level. Carbon monoxide is a colorless, odorless gas produced by incomplete combustion of fuels, and it is life-threatening at elevated concentrations. Most residential CO alarms activate around 35 to 70 ppm, but you deserve to know the moment levels start climbing, not just when an alarm goes off.

Imagine getting an urgent alert on your phone the second CO reaches a dangerous level in the garage, even in the middle of the night. Or having your exhaust fan kick on automatically when a sensor detects rising CO while you are away. This trigger gives you that early warning, so your home protects your family before a situation becomes critical.

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 monoxide level crossed threshold 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 monoxide sensor is in (like your garage or hallway). You can also select a device, a specific entity, or a label.
5. From the triggers shown for that target, select Carbon monoxide level crossed threshold.
6. Under Threshold type, configure the zone the reading must enter for the trigger to fire:
   • Select Above or Below and enter a value to fire when the reading crosses that level.
   • Select In range and enter a lower and upper bound to fire when the reading enters the range from outside.
   • Select Outside range and enter a lower and upper bound to fire when the reading leaves the range (crosses past either bound).
   • For each option, you can enter a fixed level, 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. Under Unit, select the concentration unit (ppb, ppm, mg/m³, or μg/m³) to use for the threshold comparison.
8. Under Trigger when (see Behavior), pick Each, First, or All to control how the trigger behaves when multiple entities are targeted.
9. Under For at least, set how long the reading must stay past the threshold before the trigger fires. Leave it at zero to fire immediately.
10. 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, 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.co_crossed_threshold. A basic example looks like this:

trigger: air_quality.co_crossed_threshold
target:
  entity_id: sensor.living_room_co
options:
  threshold:
    type: above
    value:
      number: 35
      unit_of_measurement: "ppm"

This fires when the living room CO sensor reading rises above 35 ppm.

Options in YAML

threshold:
  type: between
  value_min:
    number: 9
    unit_of_measurement: "ppm"
  value_max:
    entity: input_number.co_alert_level

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 monoxide sensor behind that target.

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

Behavior with multiple targets

When you target more than one entity (or select an area, floor, or label that contains several), the Trigger when option controls how the trigger responds:

• Each (default): the trigger fires every time any one of the targeted entities transitions. For example, if you monitor three motion sensors in the living room and someone walks past sensor 1, the automation fires. When they walk past sensor 2 a moment later, it fires again. Every individual event counts.
• First: the trigger fires only on the first transition in the targeted group, then waits until none of the targeted entities is in that state anymore before it fires again. For example, if you monitor the same three motion sensors, the automation fires when the first one picks up movement (someone entered the room). The other two firing afterward are ignored, so you get one notification per “someone walked in” event instead of three.
• All: the trigger fires only after the last targeted entity in the group has fired, meaning all of them are now in the expected state. For example, if you monitor the lights in the living room, bedroom, and hallway, the automation fires only once all three have turned off. This is useful for scenarios like “start the robot vacuum only after every light on the floor is off,” so you know the room is truly empty.

Good to know

• The threshold type sets the direction. With Above, the trigger fires when the level rises above the threshold. With Below, it fires when the level drops below it.
• Most residential CO alarms activate around 35 to 70 ppm depending on exposure time. A threshold in that range is a good starting point for safety automations.
• Pair this trigger with Carbon monoxide level changed if you want to react to every change of the level 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: send a safety alert when garage CO gets dangerous

Nobody wants to discover a CO problem too late. This automation sends an urgent notification straight to your phone the moment the garage sensor rises above 35 ppm, giving you time to open the door and check for running engines or faulty appliances.

• Trigger: Carbon monoxide level crossed threshold
  • Target: Garage CO sensor
  • Threshold type: Above 35 ppm
  • Trigger when: Each
• Action: Send a notification message
  • Target: My Device (notify.my_device)

alias: "Garage CO threshold alert"
triggers:
  - trigger: air_quality.co_crossed_threshold
    target:
      entity_id: sensor.garage_co
    options:
      threshold:
        type: above
        value:
          number: 35
          unit_of_measurement: "ppm"
actions:
  - action: notify.send_message
    target:
      entity_id: notify.my_device
    data:
      title: "Carbon monoxide warning"
      message: >
        CO in the garage rose above 35 ppm.
        Open the garage door and check
        for running engines or appliances.

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

Explore related triggers:

• Carbon monoxide level changed: Triggers when one or more carbon monoxide levels change.