Ozone level changed

The Ozone level changed trigger fires after the ozone (O3) reading on one or more air quality sensors changes. Ground-level ozone forms when sunlight reacts with pollutants from vehicles and industry. It is the primary ingredient in smog and tends to peak on hot, sunny afternoons. If you enjoy opening the windows on a warm day, ozone monitoring helps you decide whether the outdoor air is actually better than what’s already inside.

Imagine your HVAC switching to recirculation mode on a scorching summer afternoon because ozone levels shifted, keeping your family comfortable without pulling smoggy air indoors. Use this trigger to close windows, pause outdoor ventilation, or send a health reminder when ozone readings change. 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 Ozone 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 ozone sensor is in (like your backyard or living room). 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 Ozone 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, 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. 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.ozone_changed. A basic example looks like this:

trigger: air_quality.ozone_changed
target:
  entity_id: sensor.backyard_ozone
options:
  threshold:
    type: above
    value:
      number: 50
      unit_of_measurement: "ppb"

This fires whenever the backyard ozone sensor reading changes to a value above 50 ppb. To fire on any change, use type: any and leave out value.

Options in YAML

threshold:
  type: outside
  value_min:
    entity: input_number.ozone_alert_level
  value_max:
    number: 70
    unit_of_measurement: "ppb"

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.

Good to know

• Ozone levels are highest on warm, sunny days with little wind. Monitoring helps you decide when to keep windows closed and rely on indoor filtration.
• 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.
• To react only when ozone crosses a specific concentration in one direction, use Ozone 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: pause fresh-air intake on ozone change

On a scorching summer afternoon, ground-level ozone peaks just when you want fresh air the most. This automation switches the HVAC system to recirculation mode when the outdoor ozone reading changes to a value above 50 ppb, so you stop pulling smoggy air indoors.

• Trigger: Ozone level changed
  • Target: Outdoor ozone sensor
  • Threshold type: Above 50 ppb
• Action: Set thermostat preset mode

alias: "Recirculate on ozone change"
triggers:
  - trigger: air_quality.ozone_changed
    target:
      entity_id: sensor.backyard_ozone
    options:
      threshold:
        type: above
        value:
          number: 50
          unit_of_measurement: "ppb"
actions:
  - action: climate.set_preset_mode
    target:
      entity_id: climate.hvac
    data:
      preset_mode: "recirculate"

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:

• Ozone level crossed threshold: Triggers when one or more ozone levels cross a threshold.