diff --git a/custom_components/localthings/registry/capabilities/air_purifier.py b/custom_components/localthings/registry/capabilities/air_purifier.py
index 5b86e91..b497628 100644
--- a/custom_components/localthings/registry/capabilities/air_purifier.py
+++ b/custom_components/localthings/registry/capabilities/air_purifier.py
@@ -87,6 +87,25 @@ def _has_top_level_modes(rep, resources):
     return isinstance(rep.get("x.com.samsung.da.supportedModes"), (list, tuple))
 
 
+# The fourth column is state_class, which is what makes Home Assistant keep
+# long-term statistics for a sensor -- without one, a reading is only in the
+# short-term recorder history and disappears with the next purge (10 days by
+# default), so it can't back a long-range air-quality graph. The values are
+# already numeric (sensor_item_value returns int), so nothing else was in the
+# way; three sensors in this same module (filter_progress, fan_speed_level,
+# hepa_filter_usage) already declare one.
+#
+# Only the three particulate readings get it. They fall monotonically with
+# particle size on three independent board families -- 11/9/5 on ARTIK051_TVTL
+# (issue #56), 10/9/6 on AVT-WW-TP1 (issue #190), 18/14/9 on the range hood --
+# which is concentration behaviour, and an average over time is meaningful for
+# it. Odor and CleanLevel read 0-2 on every fixture and look like graded
+# indices instead, where the mean of a grade isn't obviously meaningful; left
+# without a state_class rather than guessing.
+#
+# Deliberately no device_class/unit here: pm1/pm25/pm10 would assert the
+# reading is a µg/m³ concentration, and the dumps never say so. That's a
+# separate call from making the series recordable at all.
 _AIR_QUALITY_SENSORS = (
     ("dust", "mdi:blur", "Dust"),
     ("fine_dust", "mdi:blur", "FineDust"),
@@ -95,6 +114,30 @@ _AIR_QUALITY_SENSORS = (
     ("clean_level", "mdi:air-filter", "CleanLevel"),
 )
 
+# Sensors that get a state_class, which is what makes Home Assistant keep
+# long-term statistics -- without one a reading only lives in the short-term
+# recorder history and is dropped at the next purge (10 days by default), so it
+# can't back a long-range air-quality graph. Kept as a key set rather than a
+# fourth column because air_monitor.py imports the tuple above and unpacks it
+# as a triple.
+#
+# Only the three particulate readings are listed. They fall monotonically with
+# particle size on three independent board families -- 11/9/5 on ARTIK051_TVTL
+# (issue #56), 10/9/6 on AVT-WW-TP1 (issue #190), 18/14/9 on the range hood --
+# which is concentration behaviour, and an average over time is meaningful for
+# it. Odor and CleanLevel read 0-2 on every fixture and look like graded
+# indices instead, where the mean of a grade isn't obviously meaningful.
+#
+# Note air_monitor.SENSORS does stamp all five, and its docstring says it is
+# "matching air_purifier.AIR_QUALITY's existing precedent" -- a precedent this
+# module did not actually set. Extending to all five here is a one-line change
+# if consistency is preferred over the grade/concentration split.
+#
+# Deliberately no device_class/unit: pm1/pm25/pm10 would assert the reading is
+# a µg/m³ concentration, and no dump says so. That is a separate call from
+# making the series recordable at all.
+_RECORDED_AIR_QUALITY = frozenset({"dust", "fine_dust", "super_fine_dust"})
+
 AIR_QUALITY = Capability(
     href="/sensors/vs/0",
     poll_tier="warm",
@@ -103,6 +146,7 @@ AIR_QUALITY = Capability(
             key=key,
             field="x.com.samsung.da.items",
             icon=icon,
+            state_class="measurement" if key in _RECORDED_AIR_QUALITY else None,
             value_fn=lambda items, t=sensor_type: sensor_item_value(items, t),
         )
         for key, icon, sensor_type in _AIR_QUALITY_SENSORS
diff --git a/tests/test_air_purifier_air_quality_statistics.py b/tests/test_air_purifier_air_quality_statistics.py
new file mode 100644
index 0000000..74376fa
--- /dev/null
+++ b/tests/test_air_purifier_air_quality_statistics.py
@@ -0,0 +1,65 @@
+"""The particulate sensors must declare a state_class so Home Assistant keeps
+long-term statistics for them; the graded readings must not.
+
+Without a state_class a sensor only lives in the short-term recorder history
+and is dropped at the next purge, so a long-range air-quality graph is not
+possible -- that is the bug this guards against reappearing.
+"""
+
+from custom_components.localthings.registry.capabilities import air_purifier
+
+PARTICULATE = ("dust", "fine_dust", "super_fine_dust")
+GRADED = ("odor", "clean_level")
+
+
+def _desc(key):
+    return next(d for d in air_purifier.AIR_QUALITY.entities if d.key == key)
+
+
+def test_particulate_sensors_record_long_term_statistics():
+    for key in PARTICULATE:
+        assert _desc(key).state_class == "measurement", key
+
+
+def test_graded_sensors_are_left_without_a_state_class():
+    """Odor and CleanLevel read 0-2 on every fixture -- graded indices, not
+    concentrations. Whether averaging a grade is meaningful is a separate
+    call, so they stay unstamped rather than being guessed into statistics."""
+    for key in GRADED:
+        assert _desc(key).state_class is None, key
+
+
+def test_no_unit_or_device_class_is_asserted():
+    """state_class alone makes the series recordable. pm1/pm25/pm10 with
+    µg/m³ would additionally assert the reading is a mass concentration,
+    which no dump states."""
+    for key in PARTICULATE + GRADED:
+        desc = _desc(key)
+        assert desc.unit is None, key
+        assert desc.device_class is None, key
+
+
+def test_shared_sensor_tuple_keeps_its_three_column_shape():
+    """air_monitor.py imports _AIR_QUALITY_SENSORS and unpacks it as a triple,
+    so widening the tuple here breaks that module's import outright."""
+    for row in air_purifier._AIR_QUALITY_SENSORS:
+        assert len(row) == 3, row
+
+
+def test_air_monitor_still_imports():
+    """Guard the coupling above end to end, not just by row width."""
+    from custom_components.localthings.registry.capabilities import air_monitor
+
+    assert air_monitor.SENSORS.href == "/sensors/vs/0"
+
+
+def test_every_air_quality_sensor_still_reads_a_plain_int():
+    """A state_class is only honoured for a numeric state, so the value
+    contract this depends on is asserted here too."""
+    from tests.conftest import _load_device
+
+    resources = _load_device("air_purifier")
+    rep = resources["/sensors/vs/0"]
+    for key in PARTICULATE + GRADED:
+        value = _desc(key).value_fn(rep["x.com.samsung.da.items"])
+        assert isinstance(value, int), (key, value)