diff --git a/Localizable.xcstrings b/Localizable.xcstrings
index 503704c2c..c96646b51 100644
--- a/Localizable.xcstrings
+++ b/Localizable.xcstrings
@@ -1,6 +1,15 @@
 {
   "sourceLanguage" : "en",
   "strings" : {
+    "AQI" : {
+      "comment" : "Air Quality Index label (acronym) shown on the air quality gauge."
+    },
+    "AQI " : {
+      "comment" : "Air Quality Index label prefix (acronym) preceding the numeric AQI value."
+    },
+    "AQI %lld" : {
+      "comment" : "Air Quality Index label with the numeric AQI value."
+    },
     "Air Quality" : {
       "comment" : "Node detail section header for particulate-matter air quality readings."
     },
diff --git a/Meshtastic.xcodeproj/project.pbxproj b/Meshtastic.xcodeproj/project.pbxproj
index 95d6bd736..4263f7f19 100644
--- a/Meshtastic.xcodeproj/project.pbxproj
+++ b/Meshtastic.xcodeproj/project.pbxproj
@@ -447,6 +447,7 @@
 		DDA1C48E28DB49D3009933EC /* ChannelRoles.swift in Sources */ = {isa = PBXBuildFile; fileRef = DDA1C48D28DB49D3009933EC /* ChannelRoles.swift */; };
 		DDA3DFDA2F10B39600D8F103 /* UIKeyboardType.swift in Sources */ = {isa = PBXBuildFile; fileRef = DDA3DFD92F10B39600D8F103 /* UIKeyboardType.swift */; };
 		DDA6B2E928419CF2003E8C16 /* MeshPackets.swift in Sources */ = {isa = PBXBuildFile; fileRef = DDA6B2E828419CF2003E8C16 /* MeshPackets.swift */; };
+		E65184282B90D941CB5626CE /* EPAAirQuality.swift in Sources */ = {isa = PBXBuildFile; fileRef = 36D8DBD29BF3C32998C5A0A3 /* EPAAirQuality.swift */; };
 		AABBCCDD0000000000000001 /* MeshPackets+Geofence.swift in Sources */ = {isa = PBXBuildFile; fileRef = AABBCCDD0000000000000002 /* MeshPackets+Geofence.swift */; };
 		DDA9515A2BC6624100CEA535 /* TelemetryWeather.swift in Sources */ = {isa = PBXBuildFile; fileRef = DDA951592BC6624100CEA535 /* TelemetryWeather.swift */; };
 		DDA9515C2BC6631200CEA535 /* TelemetryEnums.swift in Sources */ = {isa = PBXBuildFile; fileRef = DDA9515B2BC6631200CEA535 /* TelemetryEnums.swift */; };
@@ -1105,6 +1106,7 @@
 		DDA28B1B2D32C89200EF726F /* MeshtasticDataModelV 48.xcdatamodel */ = {isa = PBXFileReference; lastKnownFileType = wrapper.xcdatamodel; path = "MeshtasticDataModelV 48.xcdatamodel"; sourceTree = "<group>"; };
 		DDA3DFD92F10B39600D8F103 /* UIKeyboardType.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = UIKeyboardType.swift; sourceTree = "<group>"; };
 		DDA6B2E828419CF2003E8C16 /* MeshPackets.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MeshPackets.swift; sourceTree = "<group>"; };
+		36D8DBD29BF3C32998C5A0A3 /* EPAAirQuality.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = EPAAirQuality.swift; sourceTree = "<group>"; };
 		AABBCCDD0000000000000002 /* MeshPackets+Geofence.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "MeshPackets+Geofence.swift"; sourceTree = "<group>"; };
 		DDA951592BC6624100CEA535 /* TelemetryWeather.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TelemetryWeather.swift; sourceTree = "<group>"; };
 		DDA9515B2BC6631200CEA535 /* TelemetryEnums.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TelemetryEnums.swift; sourceTree = "<group>"; };
@@ -2383,6 +2385,7 @@
 				DD7E5740000000000000007D /* LockdownPassphraseStore.swift */,
 				DD913638270DFF4C00D7ACF3 /* LocalNotificationManager.swift */,
 				DDA6B2E828419CF2003E8C16 /* MeshPackets.swift */,
+				36D8DBD29BF3C32998C5A0A3 /* EPAAirQuality.swift */,
 				AABBCCDD0000000000000002 /* MeshPackets+Geofence.swift */,
 				84CCF00CDD3D48C9B9DCA554 /* WatchSessionManager.swift */,
 				DD3619142B1EF9F900C41C8C /* LocationsHandler.swift */,
@@ -3209,6 +3212,7 @@
 				23A1AFB72E42BD2500E46C96 /* RXTXIndicatorView.swift in Sources */,
 				DDB75A1E2A0B0CD0006ED576 /* LoRaSignalStrengthIndicator.swift in Sources */,
 				DDA6B2E928419CF2003E8C16 /* MeshPackets.swift in Sources */,
+				E65184282B90D941CB5626CE /* EPAAirQuality.swift in Sources */,
 				AABBCCDD0000000000000001 /* MeshPackets+Geofence.swift in Sources */,
 				DDCE4E2C2869F92900BE9F8F /* UserConfig.swift in Sources */,
 				BCB613852C68703800485544 /* NodePositionIntent.swift in Sources */,
diff --git a/Meshtastic/Extensions/SwiftData/NodeInfoEntityExtension.swift b/Meshtastic/Extensions/SwiftData/NodeInfoEntityExtension.swift
index 89364b5cd..79d2d9fc8 100644
--- a/Meshtastic/Extensions/SwiftData/NodeInfoEntityExtension.swift
+++ b/Meshtastic/Extensions/SwiftData/NodeInfoEntityExtension.swift
@@ -119,6 +119,18 @@ extension NodeInfoEntity {
 		return try? ctx.fetch(descriptor).first
 	}
 
+	/// NowCast-derived AQI (0–500) computed from the last 12 hours of PM2.5 telemetry, or `nil`
+	/// when there isn't enough recent history to compute a NowCast. Per design#54, callers should
+	/// fall back to showing the raw PM2.5 reading rather than a misleading instantaneous AQI.
+	var airQualityNowCastAQI: Int? {
+		let twelveHoursAgo = Calendar.current.date(byAdding: .hour, value: -12, to: Date()) ?? .distantPast
+		let readings = safeTelemetries(ofType: 3).compactMap { telemetry -> (date: Date, pm25: Double)? in
+			guard let time = telemetry.time, time >= twelveHoursAgo, let pm25 = telemetry.pm25Standard else { return nil }
+			return (date: time, pm25: Double(pm25))
+		}
+		return EPAAirQuality.nowCastAQI(from: readings)
+	}
+
 	var hasAirQualityMetrics: Bool {
 		guard let ctx = modelContext else { return false }
 		let nodeNum = self.num
diff --git a/Meshtastic/Helpers/EPAAirQuality.swift b/Meshtastic/Helpers/EPAAirQuality.swift
new file mode 100644
index 000000000..5d6756978
--- /dev/null
+++ b/Meshtastic/Helpers/EPAAirQuality.swift
@@ -0,0 +1,125 @@
+//
+//  EPAAirQuality.swift
+//  Meshtastic
+//
+//  US EPA air quality math for PM2.5 (issue #2040 / design#54):
+//   • NowCast — a 12-hour weighted rolling average that favors recent hours.
+//   • AQI — the 0–500 index derived from a PM2.5 concentration via EPA Equation 1.
+//
+//  References:
+//   • EPA/AirNow "Technical Assistance Document for the Reporting of Daily Air Quality"
+//     (Equation 1, truncation + rounding rules, NowCast data-sufficiency rule).
+//   • EPA AQS breakpoint code table (parameter 88101, PM2.5 24-hour), current
+//     post-2024 concentration breakpoints.
+//
+
+import Foundation
+
+enum EPAAirQuality {
+
+	/// One row of the EPA AQI breakpoint table: a concentration range (µg/m³) mapped to an AQI range.
+	struct PM25Breakpoint {
+		let concentrationLow: Double
+		let concentrationHigh: Double
+		let aqiLow: Int
+		let aqiHigh: Int
+	}
+
+	/// Current (2024) EPA PM2.5 24-hour AQI breakpoints (µg/m³ → AQI index).
+	/// Contiguous at 0.1 µg/m³ resolution; concentrations are truncated to one decimal
+	/// place before lookup so the 0.1 gaps between rows (e.g. 9.0 / 9.1) never fall through.
+	/// The app's `Aqi` scale tops out at 500, so concentrations above the final breakpoint
+	/// clamp to 500 rather than extending into the 501–999 "beyond the AQI" range.
+	static let pm25Breakpoints: [PM25Breakpoint] = [
+		PM25Breakpoint(concentrationLow: 0.0, concentrationHigh: 9.0, aqiLow: 0, aqiHigh: 50),
+		PM25Breakpoint(concentrationLow: 9.1, concentrationHigh: 35.4, aqiLow: 51, aqiHigh: 100),
+		PM25Breakpoint(concentrationLow: 35.5, concentrationHigh: 55.4, aqiLow: 101, aqiHigh: 150),
+		PM25Breakpoint(concentrationLow: 55.5, concentrationHigh: 125.4, aqiLow: 151, aqiHigh: 200),
+		PM25Breakpoint(concentrationLow: 125.5, concentrationHigh: 225.4, aqiLow: 201, aqiHigh: 300),
+		PM25Breakpoint(concentrationLow: 225.5, concentrationHigh: 325.4, aqiLow: 301, aqiHigh: 500)
+	]
+
+	/// Minimum weight factor for the PM NowCast; approximates a 3-hour average when
+	/// pollutant levels are changing rapidly.
+	static let nowCastMinimumWeight = 0.5
+
+	/// AQI (0–500) from a PM2.5 concentration (µg/m³) using EPA Equation 1:
+	///   I = (Ihi − Ilo) / (BPhi − BPlo) · (C − BPlo) + Ilo
+	/// The concentration is truncated to one decimal place; the result is rounded to the
+	/// nearest integer. Returns `nil` for negative input.
+	static func aqi(fromPM25 concentration: Double) -> Int? {
+		guard concentration >= 0 else { return nil }
+		// Truncate to one decimal place (EPA rule for PM2.5). Direct sensor readings are integer
+		// µg/m³ (Stage 1 stores pm25Standard as UInt32) so they truncate exactly; NowCast averages
+		// are fractional, where an exact-boundary hit is measure-zero and at most ±1 AQI. `.towardZero`
+		// == floor here because the value is already guarded non-negative.
+		let c = (concentration * 10).rounded(.towardZero) / 10.0
+		for bp in pm25Breakpoints where c <= bp.concentrationHigh {
+			let slope = Double(bp.aqiHigh - bp.aqiLow) / (bp.concentrationHigh - bp.concentrationLow)
+			let index = slope * (c - bp.concentrationLow) + Double(bp.aqiLow)
+			return min(500, max(0, Int(index.rounded())))
+		}
+		// Above the top breakpoint: clamp to the ceiling of the app's AQI scale.
+		return 500
+	}
+
+	/// EPA NowCast for PM2.5 from up to 12 hourly averages.
+	/// `hourly[0]` is the most recent clock hour, `hourly[11]` is 11 hours earlier; `nil`
+	/// marks a missing hour. Returns `nil` when there isn't enough recent data — EPA requires
+	/// at least two of the three most recent hours to be valid.
+	static func nowCastPM25(hourly: [Double?]) -> Double? {
+		let hours = Array(hourly.prefix(12))
+		// Data sufficiency: 2 of the 3 most recent hours must be present.
+		guard hours.prefix(3).compactMap({ $0 }).count >= 2 else { return nil }
+
+		let valid = hours.compactMap { $0 }
+		guard let cMax = valid.max(), let cMin = valid.min() else { return nil }
+		// If the max is 0 every reading is 0 → NowCast is 0 (avoids divide-by-zero on the weight).
+		guard cMax > 0 else { return 0 }
+
+		// Weight factor = 1 − (scaled rate of change), floored at the minimum weight.
+		let weightFactor = max(cMin / cMax, nowCastMinimumWeight)
+
+		var numerator = 0.0
+		var denominator = 0.0
+		for (hoursAgo, value) in hours.enumerated() {
+			guard let concentration = value else { continue }
+			let weight = pow(weightFactor, Double(hoursAgo))
+			numerator += weight * concentration
+			denominator += weight
+		}
+		guard denominator > 0 else { return nil }
+		return numerator / denominator
+	}
+
+	/// Buckets timestamped PM2.5 readings into the most recent 12 clock hours, computes the
+	/// NowCast, and maps it to an AQI (0–500). Returns `nil` when NowCast can't be computed
+	/// (insufficient recent history) — callers should fall back to showing the raw reading.
+	static func nowCastAQI(
+		from readings: [(date: Date, pm25: Double)],
+		now: Date = Date(),
+		calendar: Calendar = .current
+	) -> Int? {
+		guard !readings.isEmpty else { return nil }
+		let currentHourStart = calendar.dateInterval(of: .hour, for: now)?.start ?? now
+
+		var buckets: [Int: (sum: Double, count: Int)] = [:]
+		for reading in readings {
+			guard let hourStart = calendar.dateInterval(of: .hour, for: reading.date)?.start else { continue }
+			guard let hoursAgo = calendar.dateComponents([.hour], from: hourStart, to: currentHourStart).hour else { continue }
+			guard hoursAgo >= 0, hoursAgo < 12 else { continue }
+			var bucket = buckets[hoursAgo] ?? (sum: 0, count: 0)
+			bucket.sum += reading.pm25
+			bucket.count += 1
+			buckets[hoursAgo] = bucket
+		}
+
+		let hourly: [Double?] = (0..<12).map { hoursAgo in
+			guard let bucket = buckets[hoursAgo], bucket.count > 0 else { return nil }
+			return bucket.sum / Double(bucket.count)
+		}
+
+		guard let nowCast = nowCastPM25(hourly: hourly) else { return nil }
+		return aqi(fromPM25: nowCast)
+	}
+}
diff --git a/Meshtastic/Views/Helpers/Weather/AirQualityIndex.swift b/Meshtastic/Views/Helpers/Weather/AirQualityIndex.swift
index 66307a913..5d5b4a181 100644
--- a/Meshtastic/Views/Helpers/Weather/AirQualityIndex.swift
+++ b/Meshtastic/Views/Helpers/Weather/AirQualityIndex.swift
@@ -19,7 +19,7 @@ enum AqiDisplayMode: Int, CaseIterable, Identifiable {
 
 struct AirQualityIndex: View {
 	var aqi: Int
-	var displayMode: IaqDisplayMode = .pill
+	var displayMode: AqiDisplayMode = .pill
 	let gradient = Gradient(colors: [.green, .yellow, .orange, .red, .purple, .magenta])
 
 	var body: some View {
@@ -31,7 +31,7 @@ struct AirQualityIndex: View {
 				RoundedRectangle(cornerRadius: 10)
 					.fill(aqiEnum.color)
 					.frame(width: 125, height: 30)
-				Label("IAQ \(aqi)", systemImage: aqi < 100 ? "aqi.low" : ((aqi > 100 && aqi < 201) ? "aqi.medium" : "aqi.high"))
+				Label("AQI \(aqi)", systemImage: aqi < 100 ? "aqi.low" : ((aqi > 100 && aqi < 201) ? "aqi.medium" : "aqi.high"))
 					.padding(.leading, 4)
 			}
 		case .dot:
@@ -49,7 +49,7 @@ struct AirQualityIndex: View {
 		case .gauge:
 			Gauge(value: Double(aqi), in: 0...500) {
 
-						Text("IAQ")
+						Text("AQI")
 							.foregroundColor(aqiEnum.color)
 					} currentValueLabel: {
 						Text("\(Int(aqi))")
@@ -59,10 +59,10 @@ struct AirQualityIndex: View {
 		case .gradient:
 			HStack {
 				Gauge(value: Double(aqi), in: 0...500) {
-							Text("IAQ")
+							Text("AQI")
 							.foregroundColor(aqiEnum.color)
 						} currentValueLabel: {
-							Text("IAQ ")+Text("\(Int(aqi))")
+							Text("AQI ")+Text("\(Int(aqi))")
 								.foregroundColor(.gray)
 						}
 						.tint(gradient)
diff --git a/Meshtastic/Views/Nodes/Helpers/NodeDetail.swift b/Meshtastic/Views/Nodes/Helpers/NodeDetail.swift
index e4dbdb902..9ba1702a6 100644
--- a/Meshtastic/Views/Nodes/Helpers/NodeDetail.swift
+++ b/Meshtastic/Views/Nodes/Helpers/NodeDetail.swift
@@ -46,6 +46,7 @@ struct NodeDetail: View {
 	@State private var latestDeviceMetrics: TelemetryEntity?
 	@State private var latestEnvironmentMetrics: TelemetryEntity?
 	@State private var latestAirQualityMetrics: TelemetryEntity?
+	@State private var airQualityNowCastAQI: Int?
 	@State private var latestPowerMetrics: TelemetryEntity?
 	@State private var logAvailability = NodeDetailLogAvailability()
 
@@ -496,20 +497,27 @@ struct NodeDetail: View {
 		if let metrics = latestAirQualityMetrics,
 		   metrics.pm25Standard != nil || metrics.pm10Standard != nil || metrics.pm100Standard != nil {
 			Section("Air Quality") {
-				// design#54: with insufficient history to compute a NowCast AQI, show the raw
-				// particulate-matter readings (µg/m³) rather than a misleading instantaneous AQI.
-				LazyVGrid(columns: gridItemLayout) {
-					if let pm25 = metrics.pm25Standard {
-						ParticulateMatterCompactWidget(label: "PM2.5", value: pm25)
-					}
-					if let pm10 = metrics.pm10Standard {
-						ParticulateMatterCompactWidget(label: "PM1.0", value: pm10)
+				VStack {
+					// design#54: once ~12h of PM2.5 history exists, headline the NowCast-derived EPA AQI
+					// gauge above the raw particulate-matter readings (µg/m³). Without enough history the
+					// gauge is omitted and only the raw readings show — never a misleading instantaneous AQI.
+					if let aqi = airQualityNowCastAQI {
+						AirQualityIndex(aqi: aqi, displayMode: .gradient)
+							.padding(.vertical)
 					}
-					if let pm100 = metrics.pm100Standard {
-						ParticulateMatterCompactWidget(label: "PM10", value: pm100)
+					LazyVGrid(columns: gridItemLayout) {
+						if let pm25 = metrics.pm25Standard {
+							ParticulateMatterCompactWidget(label: "PM2.5", value: pm25)
+						}
+						if let pm10 = metrics.pm10Standard {
+							ParticulateMatterCompactWidget(label: "PM1.0", value: pm10)
+						}
+						if let pm100 = metrics.pm100Standard {
+							ParticulateMatterCompactWidget(label: "PM10", value: pm100)
+						}
 					}
+					.padding(.bottom)
 				}
-				.padding(.vertical)
 			}
 			.accessibilityElement(children: .combine)
 		}
@@ -836,11 +844,13 @@ struct NodeDetail: View {
 		let deviceMetrics = node.latestDeviceMetrics
 		let environmentMetrics = node.latestEnvironmentMetrics
 		let airQualityMetrics = node.latestAirQualityMetrics
+		let airQualityAQI = node.airQualityNowCastAQI
 		let powerMetrics = node.latestPowerMetrics
 		let position = node.latestPosition
 		latestDeviceMetrics = deviceMetrics
 		latestEnvironmentMetrics = environmentMetrics
 		latestAirQualityMetrics = airQualityMetrics
+		airQualityNowCastAQI = airQualityAQI
 		latestPowerMetrics = powerMetrics
 		latestPosition = position
 		logAvailability = NodeDetailLogAvailability(
diff --git a/MeshtasticTests/MeshPacketsAndTelemetryTests.swift b/MeshtasticTests/MeshPacketsAndTelemetryTests.swift
index e95681d64..2ab3b79b4 100644
--- a/MeshtasticTests/MeshPacketsAndTelemetryTests.swift
+++ b/MeshtasticTests/MeshPacketsAndTelemetryTests.swift
@@ -85,6 +85,96 @@ struct LocalStatsTelemetryExportTests {
 	}
 }
 
+// EPA PM2.5 → AQI breakpoints and NowCast — issue #2040 / design#54 (Stage 2)
+@Suite("EPA air quality math")
+struct EPAAirQualityTests {
+
+	// MARK: AQI from PM2.5 concentration (current 2024 EPA breakpoints)
+
+	@Test func aqiBreakpointBoundaries() {
+		#expect(EPAAirQuality.aqi(fromPM25: 0.0) == 0)
+		#expect(EPAAirQuality.aqi(fromPM25: 9.0) == 50)     // top of Good
+		#expect(EPAAirQuality.aqi(fromPM25: 9.1) == 51)     // bottom of Moderate
+		#expect(EPAAirQuality.aqi(fromPM25: 35.4) == 100)   // top of Moderate
+		#expect(EPAAirQuality.aqi(fromPM25: 35.5) == 101)   // bottom of USG
+		#expect(EPAAirQuality.aqi(fromPM25: 55.4) == 150)
+		#expect(EPAAirQuality.aqi(fromPM25: 125.4) == 200)
+		#expect(EPAAirQuality.aqi(fromPM25: 225.4) == 300)
+		#expect(EPAAirQuality.aqi(fromPM25: 225.5) == 301)  // bottom of Hazardous
+		#expect(EPAAirQuality.aqi(fromPM25: 325.4) == 500)  // top of the app's AQI scale
+	}
+
+	@Test func aqiClampsAndTruncates() {
+		#expect(EPAAirQuality.aqi(fromPM25: 400.0) == 500)  // above the table → clamp to 500
+		#expect(EPAAirQuality.aqi(fromPM25: 9.05) == 50)    // truncates to 9.0, not rounded up
+		#expect(EPAAirQuality.aqi(fromPM25: -1.0) == nil)   // invalid
+	}
+
+	// MARK: NowCast
+
+	@Test func nowCastConstantSeriesEqualsValue() {
+		let hourly: [Double?] = Array(repeating: 10.0, count: 12)
+		#expect(EPAAirQuality.nowCastPM25(hourly: hourly) == 10.0)
+	}
+
+	@Test func nowCastRequiresTwoOfThreeRecentHours() {
+		// Only the most recent hour present → insufficient.
+		let insufficient: [Double?] = [10.0, nil, nil, 10.0, 10.0, 10.0]
+		#expect(EPAAirQuality.nowCastPM25(hourly: insufficient) == nil)
+		// Two of the three most recent hours present → computes.
+		let sufficient: [Double?] = [10.0, nil, 10.0]
+		#expect(EPAAirQuality.nowCastPM25(hourly: sufficient) == 10.0)
+	}
+
+	@Test func nowCastWeightsRecentHoursMoreHeavily() {
+		// cMin/cMax = 1/100 < 0.5, so the weight factor floors at 0.5.
+		// NowCast = (0.5^0·1 + 0.5^1·100) / (1 + 0.5) = 51 / 1.5 = 34.
+		let hourly: [Double?] = [1.0, 100.0]
+		let nowCast = EPAAirQuality.nowCastPM25(hourly: hourly)
+		#expect(nowCast != nil)
+		#expect(abs((nowCast ?? 0) - 34.0) < 0.0001)
+	}
+
+	@Test func nowCastAQIFromTimestampedReadings() {
+		let calendar = Calendar.current
+		let now = calendar.date(from: DateComponents(year: 2025, month: 6, day: 1, hour: 12, minute: 30)) ?? Date()
+		let readings: [(date: Date, pm25: Double)] = [
+			(now, 10.0),
+			(calendar.date(byAdding: .hour, value: -1, to: now) ?? now, 10.0),
+			(calendar.date(byAdding: .hour, value: -2, to: now) ?? now, 10.0)
+		]
+		// NowCast of a constant 10 µg/m³ → 10.0 → AQI 53 (Moderate).
+		#expect(EPAAirQuality.nowCastAQI(from: readings, now: now, calendar: calendar) == 53)
+	}
+
+	@Test func nowCastAQIReturnsNilWithoutEnoughHistory() {
+		let calendar = Calendar.current
+		let now = calendar.date(from: DateComponents(year: 2025, month: 6, day: 1, hour: 12, minute: 30)) ?? Date()
+		// A single reading is not enough (needs 2 of the 3 most recent hours).
+		#expect(EPAAirQuality.nowCastAQI(from: [(now, 42.0)], now: now, calendar: calendar) == nil)
+	}
+
+	@Test func nowCastAllZeroReadingsIsZero() {
+		// All-zero series: max is 0, so the weight can't be derived — NowCast short-circuits to 0.
+		let hourly: [Double?] = [0.0, 0.0, 0.0]
+		#expect(EPAAirQuality.nowCastPM25(hourly: hourly) == 0.0)
+		#expect(EPAAirQuality.aqi(fromPM25: 0.0) == 0)
+	}
+
+	@Test func nowCastAQIExcludesReadingsOutsideTwelveHourWindow() {
+		let calendar = Calendar.current
+		let now = calendar.date(from: DateComponents(year: 2025, month: 6, day: 1, hour: 12, minute: 30)) ?? Date()
+		// Two recent hours plus one stale reading 20h ago that must be ignored.
+		let readings: [(date: Date, pm25: Double)] = [
+			(now, 10.0),
+			(calendar.date(byAdding: .hour, value: -1, to: now) ?? now, 10.0),
+			(calendar.date(byAdding: .hour, value: -20, to: now) ?? now, 999.0)
+		]
+		// The 999 reading is outside the 12h window; NowCast of the constant 10 → AQI 53.
+		#expect(EPAAirQuality.nowCastAQI(from: readings, now: now, calendar: calendar) == 53)
+	}
+}
+
 // Air Quality (particulate matter) telemetry — issue #2040
 @Suite("Air quality telemetry export")
 struct AirQualityTelemetryExportTests {