Sulphur dioxide level crossed threshold

The Sulphur dioxide level crossed threshold trigger fires when the sulphur dioxide (SO2) reading on one or more air quality sensors crosses a specific level. Sulphur dioxide is a sharp-smelling gas produced by burning fossil fuels that contain sulphur, volcanic activity, and some industrial processes. The WHO recommends keeping 24-hour SO2 exposure below 40 micrograms per cubic meter, because elevated levels irritate the respiratory system and worsen conditions like asthma.

If you live near industrial areas or in a region with volcanic activity, this trigger is especially valuable. Have your smart windows close automatically the moment outdoor SO2 crosses your safety limit, or get an alert on your phone so you know to stay indoors until the air clears. Your home reacts to changing conditions in real time, keeping irritating fumes outside where they belong.

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 Sulphur dioxide 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 sulphur dioxide sensor is in (like your garden or balcony). You can also select a device, a specific entity, or a label.
5. From the triggers shown for that target, select Sulphur dioxide 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, 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 concentration unit (ppb or μg/m³) 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.

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.so2_crossed_threshold. A basic example looks like this:

trigger: air_quality.so2_crossed_threshold
target:
  entity_id: sensor.outdoor_so2
options:
  threshold:
    type: above
    value:
      number: 40
      unit_of_measurement: "μg/m³"

This fires when the outdoor SO2 sensor reading rises above 40 μg/m³.

Options in YAML

threshold:
  type: between
  value_min:
    number: 20
    unit_of_measurement: "μg/m³"
  value_max:
    entity: input_number.so2_alert_level

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 sulphur dioxide sensor behind that target.

• Entity: one specific sulphur dioxide sensor, such as sensor.living_room_sulphur_dioxide.
• Device: every sulphur dioxide sensor that belongs to a device.
• Area: every sulphur dioxide sensor in a room or area.
• Floor: every sulphur dioxide sensor on a floor.
• Label: every sulphur dioxide 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 SO2 exposure is 40 μg/m³. If you live near industrial areas or in regions with volcanic activity, a threshold around that value is a good starting point.
• Pair this trigger with Sulphur dioxide level changed if you want to react to every change of the level instead.

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: seal the house when industrial or volcanic SO2 spikes

If you live downwind of a factory or in a volcanic region, SO2 levels change fast. This automation closes your smart windows the moment outdoor SO2 rises above 40 μg/m³, keeping that sharp, irritating gas out of the house so your family breathes easy.

• Trigger: Sulphur dioxide level crossed threshold
  • Target: Outdoor SO2 sensor
  • Threshold type: Above 40 μg/m³
  • Trigger when: Each
• Action: Close cover (windows)

alias: "Close windows on high SO2"
triggers:
  - trigger: air_quality.so2_crossed_threshold
    target:
      entity_id: sensor.outdoor_so2
    options:
      threshold:
        type: above
        value:
          number: 40
          unit_of_measurement: "μg/m³"
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

Explore related triggers:

• Sulphur dioxide level changed: Triggers when one or more sulphur dioxide levels change.