diff --git a/src/layers/airQuality/edgeCases.test.mjs b/src/layers/airQuality/edgeCases.test.mjs
new file mode 100644
index 000000000..e1a22d6a8
--- /dev/null
+++ b/src/layers/airQuality/edgeCases.test.mjs
@@ -0,0 +1,95 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  AIR_QUALITY_BANDS,
+  AIR_QUALITY_UNKNOWN_COLOR,
+  airQualityColor,
+  normalizeAqhiObservations,
+  normalizeAqhiStations,
+  readingsInView,
+} from './model.js';
+
+const NOW = Date.parse('2026-01-02T12:00:00Z');
+const OBSERVED = '2026-01-02T11:00:00Z';
+
+const stationFeature = (id, lon) => ({
+  geometry: { coordinates: [lon, 50] },
+  properties: { location_id: id, location_name_en: `Station ${id}` },
+});
+const observation = (id, aqhi) => ({
+  properties: { location_id: id, aqhi, observation_datetime: OBSERVED },
+});
+
+const IDS = ['zero', 'missing', 'blank', 'b3', 'b4', 'b6', 'b7', 'b10', 'b11'];
+const stations = normalizeAqhiStations({
+  features: IDS.map((id, index) => stationFeature(id, -100 + index)),
+});
+
+test('edge:air-quality: a zero reading and a missing one give different cards', () => {
+  const readings = normalizeAqhiObservations(
+    {
+      features: [
+        observation('zero', 0),
+        observation('missing', null),
+        observation('blank', ''),
+      ],
+    },
+    stations,
+    NOW,
+  );
+  // Zero is a real low reading, published on ECCC's floor of 1.
+  assert.deepEqual(
+    readings.map(({ id, value, band, label }) => ({ id, value, band, label })),
+    [{ id: 'eccc:zero', value: 1, band: 'low', label: '1' }],
+  );
+  assert.equal(readings[0].color, AIR_QUALITY_BANDS[0].color);
+  // A station that reported nothing never becomes a confident "AQHI 1" row.
+  const card = readingsInView(readings, null).map((reading) => reading.id);
+  assert.deepEqual(card, ['eccc:zero']);
+});
+
+test('edge:air-quality: IDs and colour-band boundaries are fixed by the model', () => {
+  const values = { b3: 3, b4: 4, b6: 6, b7: 7, b10: 10, b11: 11 };
+  const readings = normalizeAqhiObservations(
+    {
+      features: Object.entries(values).map(([id, value]) =>
+        observation(id, value),
+      ),
+    },
+    stations,
+    NOW,
+  );
+  const byId = Object.fromEntries(readings.map((row) => [row.id, row]));
+  const expected = {
+    'eccc:b3': ['low', '3'],
+    'eccc:b4': ['moderate', '4'],
+    'eccc:b6': ['moderate', '6'],
+    'eccc:b7': ['high', '7'],
+    'eccc:b10': ['high', '10'],
+    'eccc:b11': ['very-high', '10+'],
+  };
+  assert.deepEqual(
+    Object.keys(byId).toSorted(),
+    Object.keys(expected).toSorted(),
+  );
+  for (const [id, [band, label]] of Object.entries(expected)) {
+    assert.equal(byId[id].band, band, id);
+    assert.equal(byId[id].label, label, id);
+    // Both renderers take the colour from the reading itself.
+    assert.equal(byId[id].color, airQualityColor(band), id);
+    assert.notEqual(byId[id].color, AIR_QUALITY_UNKNOWN_COLOR, id);
+  }
+  // A second normalization of the same response is identical.
+  assert.deepEqual(
+    normalizeAqhiObservations(
+      {
+        features: Object.entries(values).map(([id, value]) =>
+          observation(id, value),
+        ),
+      },
+      stations,
+      NOW,
+    ),
+    readings,
+  );
+});