Ozone level crossed threshold

The Ozone level crossed threshold trigger fires when the ozone (O3) reading on one or more air quality sensors crosses a specific level. Ground-level ozone forms when sunlight reacts with pollutants from vehicles and industry, and it tends to peak on hot, sunny afternoons. High ozone levels irritate the lungs and are especially risky during outdoor exercise.

Imagine getting a notification before your afternoon run telling you ozone is too high to exercise outside today. Or having your ventilation system close its fresh-air intake automatically when ozone spikes, so your indoor air stays clean without you thinking about it. This trigger watches the sky for you and lets your home take action the instant conditions become unhealthy.

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 Ozone 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 ozone sensor is in (like your backyard or living room). You can also select a device, a specific entity, or a label.
5. From the triggers shown for that target, select Ozone 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, 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. 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.ozone_crossed_threshold. A basic example looks like this:

trigger: air_quality.ozone_crossed_threshold
target:
  entity_id: sensor.backyard_ozone
options:
  threshold:
    type: above
    value:
      number: 100
      unit_of_measurement: "μg/m³"

This fires when the backyard ozone sensor reading rises above 100 μg/m³.

Options in YAML

threshold:
  type: between
  value_min:
    number: 120
    unit_of_measurement: "μg/m³"
  value_max:
    entity: input_number.ozone_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 ozone sensor behind that target.

• Entity: one specific ozone sensor, such as sensor.living_room_ozone.
• Device: every ozone sensor that belongs to a device.
• Area: every ozone sensor in a room or area.
• Floor: every ozone sensor on a floor.
• Label: every ozone 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.
• Ground-level ozone peaks on hot, sunny afternoons. Automations that close fresh-air intakes or notify household members before outdoor activities are common use cases.
• Pair this trigger with Ozone 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: remind yourself to exercise indoors on high-ozone days

On hot afternoons, ground-level ozone spikes without any visible sign. This automation sends a notification when your backyard ozone sensor rises above 100 μg/m³, giving you a friendly heads-up to take your workout inside.

• Trigger: Ozone level crossed threshold
  • Target: Backyard ozone sensor
  • Threshold type: Above 100 μg/m³
  • Trigger when: Each
• Action: Send a notification message
  • Target: My Device (notify.my_device)

alias: "Ozone exercise advisory"
triggers:
  - trigger: air_quality.ozone_crossed_threshold
    target:
      entity_id: sensor.backyard_ozone
    options:
      threshold:
        type: above
        value:
          number: 100
          unit_of_measurement: "μg/m³"
actions:
  - action: notify.send_message
    target:
      entity_id: notify.my_device
    data:
      title: "High ozone outside"
      message: >
        Ozone rose above 100 μg/m³. Consider
        exercising indoors today.

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:

• Ozone level changed: Triggers when one or more ozone levels change.