Radon level crossed threshold

The Radon level crossed threshold trigger fires when the radon reading on one or more air quality sensors crosses into a zone you define: above a level, below a level, into a range, or out of a range. Radon is a naturally occurring radioactive gas that rises from the soil and seeps into buildings through cracks and gaps in floors and walls. It is invisible and odorless. It collects in basements and ground-floor rooms, and long-term exposure to elevated levels increases the risk of lung cancer. Because radon fluctuates with ventilation, weather, and the seasons, a live sensor lets your home respond the moment a level becomes a concern instead of waiting for a periodic test.

Imagine your basement fan starting on its own the moment radon climbs above your limit, and a notification letting you know once it has dropped back to normal. Radon sensors report in becquerels per cubic meter (Bq/m³) or picocuries per liter (pCi/L). You can set the threshold in either unit, and Home Assistant converts between them automatically.

When you target more than one entity, the trigger’s Trigger when option controls when it fires.

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 Radon 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 radon sensor is in (like your basement or bedroom). You can also select a device, a specific entity, or a label.
5. From the triggers shown for that target, select Radon 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 radon concentration, 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 radon unit (Bq/m³ or pCi/L) 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.radon_crossed_threshold. A basic example looks like this:

trigger: air_quality.radon_crossed_threshold
target:
  entity_id: sensor.basement_radon
options:
  threshold:
    type: above
    value:
      number: 100
      unit_of_measurement: "Bq/m³"

This fires at the moment the basement radon sensor crosses from 100 Bq/m³ or below to above 100 Bq/m³. It does not fire again until the reading drops back to 100 Bq/m³ or below and then crosses above it again.

To fire when the reading drops back below a level, with the threshold set in pCi/L:

trigger: air_quality.radon_crossed_threshold
target:
  entity_id: sensor.basement_radon
options:
  threshold:
    type: below
    value:
      number: 2
      unit_of_measurement: "pCi/L"

Options in YAML

threshold:
  type: between
  value_min:
    number: 50
    unit_of_measurement: "Bq/m³"
  value_max:
    entity: input_number.radon_upper_limit

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 radon sensor behind that target.

• Entity: one specific radon sensor, such as sensor.basement_radon.
• Device: every radon sensor that belongs to a device.
• Area: every radon sensor in a room or area.
• Floor: every radon sensor on a floor.
• Label: every radon 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.

To leave out some of these entities, refer to Excluding targets. For example, you can leave out one radon sensor in an area.

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

• Use a sensor with the radon device class as the target.
• If you use an entity as the threshold, it must report its unit of measurement as Bq/m³ or pCi/L. A user-created number helperA helper is a virtual entity you create inside Home Assistant. It is not backed by a physical device. Helpers store values, track state, or do calculations that your automations and dashboards need. [Learn more] without one of those units cannot be used as a threshold, and the trigger does not fire.
• Radon sensors report in Bq/m³ or pCi/L. You can set the threshold in either unit, and Home Assistant converts the sensor reading and the threshold to the same unit before comparing them, so a sensor that reports in pCi/L works with a threshold set in Bq/m³ and the other way around. 1 pCi/L equals 37 Bq/m³.
• Above and Below fire on the crossing moment only. Once the reading is above the threshold, the trigger does not fire again until the reading dips back below it and then crosses above again.
• In range (between) fires when the reading moves from outside the bounds into the bounds. Outside range (outside) fires when the reading moves from inside the bounds past either bound.
• Radon levels fluctuate from hour to hour, so a single reading can briefly cross a threshold without a lasting change. Use For at least to fire only after the reading has stayed past the threshold for a while.
• The trigger only fires when a sensor transitions from a known, valid state. If a sensor comes back from being unavailable (unavailable) or having an unknown state (unknown), the trigger does not fire for that recovery.
• Guidance on indoor radon varies by country. Reference levels between 100 Bq/m³ and 300 Bq/m³ are commonly cited, and 4 pCi/L (about 150 Bq/m³) is a widely used action level. Long-term averages matter more than any single reading, so check the guidance that applies where you live before deciding on remediation.
• Pair this trigger with Radon level changed if you also want to react to 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, 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: start basement ventilation when radon crosses above 100 Bq/m³

Radon builds up in closed, low-lying rooms. This automation turns on the basement ventilation fan the moment the radon reading crosses above 100 Bq/m³, so fresh air starts moving before the level has a chance to climb further.

• Trigger: Radon level crossed threshold
  • Target: Basement radon sensor
  • Threshold type: Above (100 Bq/m³)
• Action: Turn on fan

alias: "Ventilate basement on high radon"
triggers:
  - trigger: air_quality.radon_crossed_threshold
    target:
      entity_id: sensor.basement_radon
    options:
      threshold:
        type: above
        value:
          number: 100
          unit_of_measurement: "Bq/m³"
actions:
  - action: fan.turn_on
    target:
      entity_id: fan.basement_ventilation

Automation: get an alert when any room stays above 4 pCi/L for an hour

Radon readings can spike briefly and settle again. This automation watches every radon sensor in the house and sends a notification only after a reading has stayed above 4 pCi/L for a full hour, so you hear about sustained elevated levels rather than a momentary blip. The threshold is set in pCi/L, and Home Assistant converts readings from sensors that report in Bq/m³ automatically.

• Trigger: Radon level crossed threshold
  • Target: All radon sensors (by label)
  • Threshold type: Above (4 pCi/L)
  • For at least: 01:00:00
• Action: Send a notification message
  • Target: My Device (notify.my_device)

alias: "Alert on sustained radon above 4 pCi/L"
triggers:
  - trigger: air_quality.radon_crossed_threshold
    target:
      label_id: radon_sensors
    options:
      threshold:
        type: above
        value:
          number: 4
          unit_of_measurement: "pCi/L"
      for: "01:00:00"
actions:
  - action: notify.send_message
    target:
      entity_id: notify.my_device
    data:
      title: "Radon level elevated"
      message: >
        {{ trigger.to_state.name }} has been above 4 pCi/L
        for the past hour. Consider improving ventilation
        and checking the long-term average.

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:

• Radon level changed: Triggers when one or more radon levels change.