PM2.5 level crossed threshold

The PM2.5 level crossed threshold trigger fires when the PM2.5 (particulate matter 2.5 micrometers or smaller) reading on one or more air quality sensors crosses a specific level. PM2.5 is one of the most widely tracked air quality metrics because these fine particles travel deep into the lungs and affect your health. When PM2.5 rises above 25 μg/m³, air quality is already poor enough to bother sensitive groups, including children and anyone with asthma or allergies.

Have your air purifier start the second PM2.5 crosses the safe limit, or close your windows automatically when wildfire smoke pushes outdoor readings into unhealthy territory. You also get a notification on your phone so you always know what is happening, whether you are at home or away. This trigger helps your home react to air quality changes faster than you ever could on your own.

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 PM2.5 level crossed threshold.
6. Under Threshold type, set the PM2.5 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.pm25_crossed_threshold. A basic example looks like this:

trigger: air_quality.pm25_crossed_threshold
target:
  entity_id: sensor.outdoor_pm25
options:
  threshold: 35
  behavior: any

This fires whenever the outdoor PM2.5 sensor crosses 35 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 want to monitor only one direction, add a condition that checks whether the current PM2.5 level is above or below your threshold.
• The WHO guideline for 24-hour average PM2.5 exposure is 15. Many people use a threshold between 25 and 50 for automations depending on their sensitivity and local conditions.
• Pair this trigger with PM2.5 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: close windows when outdoor pollution rises

Wildfire smoke, traffic exhaust, and smog all push PM2.5 to unhealthy levels. This automation closes your smart windows the moment outdoor PM2.5 crosses 35, sealing your home off from pollution before it drifts inside.

• Trigger: PM2.5 level crossed threshold
• Target: Outdoor PM2.5 sensor
• Threshold type: 35
• Trigger when: Any
• Condition: PM2.5 is above 35
• Action: Close cover (windows)

alias: "Close windows on high PM2.5"
triggers:
  - trigger: air_quality.pm25_crossed_threshold
    target:
      entity_id: sensor.outdoor_pm25
    options:
      threshold: 35
      behavior: any
conditions:
  - condition: numeric_state
    entity_id: sensor.outdoor_pm25
    above: 35
actions:
  - action: cover.close_cover
    target:
      entity_id: cover.living_room_windows

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:

• PM2.5 level changed - Triggers after one or more PM2.5 levels change.