Nitrogen dioxide level changed

The Nitrogen dioxide level changed trigger fires after the nitrogen dioxide (NO2) reading on one or more air quality sensors changes. Nitrogen dioxide is a reddish-brown gas with a sharp odor, produced mainly by traffic, power plants, and gas stoves. It irritates the airways and contributes to smog and acid rain. Indoors, your gas stove is often the biggest source. Every time you fire up a burner, NO2 levels in the kitchen rise.

Imagine your range hood turning on automatically when you start cooking, clearing combustion byproducts before they spread through the house. Use this trigger to start ventilation, close windows facing a busy road, or send a health alert. 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 Nitrogen 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 nitrogen dioxide sensor is in (like your kitchen or street-facing 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 Nitrogen 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, 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.no2_changed. A basic example looks like this:

trigger: air_quality.no2_changed
target:
  entity_id: sensor.street_side_no2
options:
  threshold:
    type: above
    value:
      number: 10
      unit_of_measurement: "ppb"

This fires whenever the street-side NO2 sensor reading changes to a value above 10 ppb. To fire on any change, use type: any and leave out value.

Options in YAML

threshold:
  type: outside
  value_min:
    entity: input_number.no2_alert_level
  value_max:
    number: 50
    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 nitrogen dioxide sensor behind that target.

• Entity: one specific nitrogen dioxide sensor, such as sensor.living_room_nitrogen_dioxide.
• Device: every nitrogen dioxide sensor that belongs to a device.
• Area: every nitrogen dioxide sensor in a room or area.
• Floor: every nitrogen dioxide sensor on a floor.
• Label: every nitrogen 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 gas stoves and heaters are common sources of NO2 inside the home. A kitchen NO2 sensor combined with this trigger helps you automate range hood ventilation.
• 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 NO2 crosses a specific concentration in one direction, use Nitrogen 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: turn on the range hood when cooking produces NO2

Every time you fire up the gas stove, NO2 levels in the kitchen rise. This automation turns on the range hood as soon as the kitchen NO2 level changes to a value above 15 ppb, clearing combustion byproducts before they spread through the house.

• Trigger: Nitrogen dioxide level changed
  • Target: Kitchen NO2 sensor
  • Threshold type: Above 15 ppb
• Action: Turn on switch

alias: "Range hood on NO2 change"
triggers:
  - trigger: air_quality.no2_changed
    target:
      entity_id: sensor.kitchen_no2
    options:
      threshold:
        type: above
        value:
          number: 15
          unit_of_measurement: "ppb"
actions:
  - action: switch.turn_on
    target:
      entity_id: switch.range_hood

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:

• Nitrogen dioxide level crossed threshold: Triggers when one or more nitrogen dioxide levels cross a threshold.