Nitrogen dioxide level crossed threshold

The Nitrogen dioxide level crossed threshold trigger fires when the nitrogen dioxide (NO2) reading on one or more air quality sensors crosses a specific level. Nitrogen dioxide is a reddish-brown gas with a sharp odor, produced primarily by vehicle traffic and gas appliances. Elevated NO2 irritates the airways and worsens respiratory conditions like asthma, so the WHO recommends keeping short-term exposure below 25 micrograms per cubic meter.

If you live near a busy road, this trigger is a game-changer. Have your ventilation system close its fresh-air intake automatically when street-side NO2 rises past your limit, keeping traffic pollution out of the house. Or get an alert on your phone during rush hour so you know to keep the kids inside until levels drop. Your home watches the air for you and reacts the instant conditions change.

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 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 nitrogen dioxide sensor is in (like your kitchen or street-facing room). You can also select a device, a specific entity, or a label.
5. From the triggers shown for that target, select Nitrogen dioxide 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.no2_crossed_threshold. A basic example looks like this:

trigger: air_quality.no2_crossed_threshold
target:
  entity_id: sensor.street_side_no2
options:
  threshold:
    type: above
    value:
      number: 40
      unit_of_measurement: "μg/m³"

This fires when the street-side NO2 sensor reading rises above 40 μg/m³.

Options in YAML

threshold:
  type: between
  value_min:
    number: 25
    unit_of_measurement: "μg/m³"
  value_max:
    entity: input_number.no2_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 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.

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.
• The WHO guideline for short-term NO2 exposure is 25 μg/m³. If you live near a busy road, a threshold in that range helps protect indoor air quality.
• Pair this trigger with Nitrogen dioxide 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: block traffic pollution during rush hour

Living near a busy road means NO2 spikes every rush hour. This automation closes the fresh-air intake on your ventilation system the moment your street-side sensor rises above 40 μg/m³, keeping traffic fumes out of the house while you go about your evening.

• Trigger: Nitrogen dioxide level crossed threshold
  • Target: Street-side NO2 sensor
  • Threshold type: Above 40 μg/m³
  • Trigger when: Each
• Action: Close cover (fresh-air intake)

alias: "Close intake on high NO2"
triggers:
  - trigger: air_quality.no2_crossed_threshold
    target:
      entity_id: sensor.street_side_no2
    options:
      threshold:
        type: above
        value:
          number: 40
          unit_of_measurement: "μg/m³"
actions:
  - action: cover.close_cover
    target:
      entity_id: cover.fresh_air_intake

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:

• Nitrogen dioxide level changed: Triggers when one or more nitrogen dioxide levels change.