PM4 level crossed threshold

The PM4 level crossed threshold trigger fires when the PM4 (particulate matter 4 micrometers or smaller) reading on one or more air quality sensors crosses a specific level. PM4 sits between the finest particles (PM2.5) and the coarser dust and pollen (PM10), capturing a range of irritants that affect breathing and comfort. Sources include household dust, pollen, mold spores, and cooking emissions.

Think of a nursery where clean air really matters. This trigger lets you boost the air filter to high speed the moment PM4 levels rise, or send a notification to your phone when spring pollen pushes particle counts past your comfort level. You stay one step ahead, keeping the air cleaner for young children and anyone with allergies.

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 PM4 level crossed threshold.
6. Under Threshold type, set the PM4 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.pm4_crossed_threshold. A basic example looks like this:

trigger: air_quality.pm4_crossed_threshold
target:
  entity_id: sensor.nursery_pm4
options:
  threshold: 30
  behavior: any

This fires whenever the nursery PM4 sensor crosses 30 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 PM4 level is above or below your threshold.
• PM4 sits between PM2.5 and PM10 in particle size. If your sensor reports multiple particulate sizes, combining thresholds for each gives you a more complete picture of air quality.
• Pair this trigger with PM4 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: protect the nursery by boosting the air filter

Young children are more sensitive to airborne particles. This automation switches the nursery air filter to high speed the moment PM4 crosses 30, keeping the room cleaner so your children breathe easier.

• Trigger: PM4 level crossed threshold
• Target: Nursery PM4 sensor
• Threshold type: 30
• Trigger when: Any
• Action: Turn on fan (air filter)

alias: "Nursery air filter on high PM4"
triggers:
  - trigger: air_quality.pm4_crossed_threshold
    target:
      entity_id: sensor.nursery_pm4
    options:
      threshold: 30
      behavior: any
actions:
  - action: fan.turn_on
    target:
      entity_id: fan.nursery_air_filter

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:

• PM4 level changed - Triggers after one or more PM4 levels change.