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 rises above 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 PM2.5 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 PM2.5 sensor is in (like your living room or bedroom). You can also select 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, 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 in μg/m³, 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 Trigger when (see Behavior), pick Each, First, or All to control how the trigger behaves when multiple entities are targeted.
8. 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.
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. 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.pm25_crossed_threshold. A basic example looks like this:

trigger: air_quality.pm25_crossed_threshold
target:
  entity_id: sensor.outdoor_pm25
options:
  threshold:
    type: above
    value:
      number: 35

This fires when the outdoor PM2.5 sensor reading rises above 35 μg/m³.

Options in YAML

threshold:
  type: above
  value:
    number: 35

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 PM2.5 sensor behind that target.

• Entity: one specific PM2.5 sensor, such as sensor.living_room_pm25.
• Device: every PM2.5 sensor that belongs to a device.
• Area: every PM2.5 sensor in a room or area.
• Floor: every PM2.5 sensor on a floor.
• Label: every PM2.5 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 24-hour average PM2.5 exposure is 15 μg/m³. Many people use a threshold between 25 and 50 μg/m³ for automations depending on their sensitivity and local conditions.
• Pair this trigger with PM2.5 level changed if you want to react to every change of the level instead.
• The threshold is in μg/m³. A sensor that reports PM2.5 in another unit never matches. If you use an entity as the threshold, it must also report μg/m³.

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: 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 rises above 35 μg/m³, sealing your home off from pollution before it drifts inside.

• Trigger: PM2.5 level crossed threshold
• Target: Outdoor PM2.5 sensor
• Threshold type: Above 35 μg/m³
• Trigger when: Each
• 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:
        type: above
        value:
          number: 35
actions:
  - action: cover.close_cover
    target:
      entity_id: cover.living_room_windows

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:

• PM2.5 level changed: Triggers when one or more PM2.5 levels change.