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 this trigger 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 air quality sensor is in (like your living room or bedroom). You can also select a floor, 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, set the nitrogen dioxide level the reading must cross for the trigger to fire.
7. Under Trigger when (see Behavior), pick Any, First, or Last to control how multiple targets interact.
8. Under For at least, set how long the level must stay past the threshold before the trigger fires. Leave at the default to fire immediately.
9. 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, 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: 40
  behavior: any

This fires whenever the street-side NO2 sensor crosses 40 in either direction.

Options in YAML

YAML sometimes provides additional options for more complex use cases that are not available through the UI.

Targets

This trigger supports targets. A target tells Home Assistant what the trigger should watch. You can point it at 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, and Home Assistant watches every matching air_quality entity behind that target.

• Entity: one specific air_quality entity, such as air_quality.living_room.
• Device: every air_quality entity that belongs to a device.
• Area: every air_quality entity in a room or area.
• Floor: every air_quality entity on a floor.
• Label: every air_quality entity that shares a label.

You can also mix target types in one trigger. For example, combine a specific entity with an area to watch both at once.

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:

• Any (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 all targeted entities have reset 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.
• Last: 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 trigger fires on any crossing, up or down. If you only want one direction, add a condition that checks whether the current NO2 level is above or below your threshold.
• The WHO guideline for short-term NO2 exposure is 25. 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 also want to track smaller fluctuations between crossings.

Try it yourself

Ready to test this? Go to Settings > Automations & scenes, create a new automation, and add this trigger. Save the automation, then change the state of the targeted entity to watch the trigger fire on your actual entitiesAn 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].

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup.

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 crosses 40, 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: 40
• Trigger when: Any
• Condition: NO2 is above 40
• 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: 40
      behavior: any
conditions:
  - condition: numeric_state
    entity_id: sensor.street_side_no2
    above: 40
actions:
  - action: cover.close_cover
    target:
      entity_id: cover.fresh_air_intake

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

These triggers work well alongside this one:

• Nitrogen dioxide level changed - Triggers after one or more nitrogen dioxide levels change.