"The greatest challenge in environmental communication is translating complex scientific data into simple, actionable information that everyone can understand and use."
— WIA Philosophy on Public Health Communication
The Air Quality Index (AQI) is a standardized numerical scale that transforms complex pollutant concentration data into a simple, color-coded system that communicates health risks to the public. Think of it as a "weather forecast for air quality"—just as temperature tells you how to dress, AQI tells you how to protect your health.
Definition: An Air Quality Index is a dimensionless number (typically 0-500+) that represents the health impact of air pollution. Higher values indicate greater health risk. The WIA-ENE-017 standard harmonizes global AQI methodologies while allowing regional customization.
The United States Environmental Protection Agency developed the most internationally adopted AQI system in 1999 (revised 2024):
| AQI Range | Category | Color | Health Implications | Cautionary Statement |
|---|---|---|---|---|
| 0-50 | Good | Green | Air quality is satisfactory; pollution poses little or no risk | None. Enjoy outdoor activities! |
| 51-100 | Moderate | Yellow | Acceptable quality; some pollutants may be a concern for unusually sensitive people | Unusually sensitive individuals consider reducing prolonged outdoor exertion |
| 101-150 | Unhealthy for Sensitive Groups | Orange | Sensitive groups (children, elderly, respiratory/heart disease) may experience health effects | Sensitive groups: Reduce prolonged or heavy outdoor exertion. General public: No restrictions |
| 151-200 | Unhealthy | Red | Everyone may begin to experience health effects; sensitive groups at serious risk | Sensitive groups: Avoid prolonged outdoor exertion. General public: Reduce prolonged exertion |
| 201-300 | Very Unhealthy | Purple | Health alert: Risk of health effects increased for everyone | Sensitive groups: Avoid all outdoor exertion. General public: Avoid prolonged exertion |
| 301-500 | Hazardous | Maroon | Health warning of emergency conditions; entire population more likely to be affected | Everyone: Avoid all outdoor exertion. Stay indoors with air filtration if possible |
The EPA uses a piecewise linear function to convert pollutant concentrations to AQI values:
AQI Calculation Formula (Sub-Index Method):
Given:
C_p = Pollutant concentration (measured)
BP_Hi = Breakpoint concentration ≥ C_p
BP_Lo = Breakpoint concentration ≤ C_p
I_Hi = AQI value corresponding to BP_Hi
I_Lo = AQI value corresponding to BP_Lo
AQI = [(I_Hi - I_Lo) / (BP_Hi - BP_Lo)] × (C_p - BP_Lo) + I_Lo
Example: PM2.5 = 55.5 μg/m³ (24-hour average)
From breakpoint table:
BP_Lo = 35.5 μg/m³ → I_Lo = 101
BP_Hi = 55.4 μg/m³ → I_Hi = 150
AQI = [(150 - 101) / (55.4 - 35.5)] × (55.5 - 35.5) + 101
= [49 / 19.9] × 20.0 + 101
= 2.462 × 20.0 + 101
= 49.24 + 101
= 150.24
≈ 150 (round to nearest integer)
Result: AQI 150 = "Unhealthy for Sensitive Groups" (Orange category)
PM2.5 Breakpoints (24-hour average):
| AQI Category | AQI Range | PM2.5 Range (μg/m³) |
|---|---|---|
| Good | 0-50 | 0.0 - 9.0 |
| Moderate | 51-100 | 9.1 - 35.4 |
| Unhealthy for Sensitive Groups | 101-150 | 35.5 - 55.4 |
| Unhealthy | 151-200 | 55.5 - 125.4 |
| Very Unhealthy | 201-300 | 125.5 - 225.4 |
| Hazardous | 301-400 | 225.5 - 325.4 |
| Hazardous | 401-500 | 325.5 - 500.4 |
Ozone Breakpoints (8-hour average):
| AQI Category | AQI Range | O₃ Range (ppm) | O₃ Range (μg/m³) |
|---|---|---|---|
| Good | 0-50 | 0.000 - 0.054 | 0 - 108 |
| Moderate | 51-100 | 0.055 - 0.070 | 109 - 140 |
| Unhealthy for Sensitive Groups | 101-150 | 0.071 - 0.085 | 141 - 170 |
| Unhealthy | 151-200 | 0.086 - 0.105 | 171 - 210 |
| Very Unhealthy | 201-300 | 0.106 - 0.200 | 211 - 400 |
Sub-Index Approach: Calculate AQI for each pollutant separately. The overall AQI is the maximum (worst) sub-index value. The pollutant responsible is called the "dominant pollutant."
Example Multi-Pollutant AQI Calculation:
Location: Downtown Los Angeles
Date: July 15, 2025 (14:00 local time)
Measured Concentrations:
- PM2.5 (24-hr avg): 35.5 μg/m³ → AQI = 101
- PM10 (24-hr avg): 68 μg/m³ → AQI = 64
- O₃ (8-hr avg): 0.078 ppm → AQI = 125
- NO₂ (1-hr): 65 ppb → AQI = 55
- SO₂ (1-hr): 8 ppb → AQI = 11
- CO (8-hr avg): 2.4 ppm → AQI = 25
Overall AQI = max(101, 64, 125, 55, 11, 25) = 125
Dominant Pollutant: Ozone (O₃)
Category: Unhealthy for Sensitive Groups (Orange)
Public Message: "Air quality is acceptable for most but sensitive groups
(children, elderly, people with asthma) may experience respiratory symptoms.
Consider reducing prolonged outdoor exertion."
Different countries have developed their own AQI methodologies. The WIA-ENE-017 standard provides conversion algorithms between systems:
| System | Scale | Categories | Key Differences from US EPA |
|---|---|---|---|
| China MEP AQI | 0-500+ | 6 categories (same colors as EPA) | Stricter PM2.5 breakpoints; includes PM10 in overall AQI |
| India CPCB AQI | 0-500 | 6 categories with different colors | Includes PM2.5, PM10, NO₂, SO₂, O₃, CO, NH₃, Pb |
| Europe CAQI | 0-100+ | 5 categories | Hourly index for roadside, background, and forecast |
| UK DAQI | 1-10 | 4 health advice bands | Simplified 10-point scale; different breakpoints |
| Canada AQHI | 1-10+ | 4 risk categories | Health risk based on mortality/morbidity studies |
| Australia AQI | 0-200+ | 6 categories | State-specific variations; visibility considerations |
China's AQI system, implemented in 2012, addresses severe pollution events more comprehensively:
| PM2.5 (μg/m³) | US EPA AQI | China MEP AQI | Category Difference |
|---|---|---|---|
| 35.5 | 101 (Unhealthy for Sensitive) | 100 (Moderate) | China: less alarming at this level |
| 75 | 158 (Unhealthy) | 150 (Unhealthy for Sensitive) | China: more lenient categorization |
| 150 | 200 (Very Unhealthy) | 200 (Unhealthy) | Similar numeric, different category name |
| 250 | 331 (Hazardous) | 300 (Very Unhealthy) | China: reserves "Hazardous" for >300 |
Public Health Impact of AQI Differences: A PM2.5 concentration of 55 μg/m³ would trigger "Unhealthy for Sensitive Groups" alerts in the US but only "Moderate" in China. This can lead to inadequate public health protection if thresholds are set too leniently for political or economic reasons.
The WIA standard provides API endpoints for real-time AQI conversion:
// Universal AQI Conversion API
POST /api/v1/aqi/convert
Request Body:
{
"pollutant": "pm25",
"concentration": 55.5,
"unit": "ug/m3",
"averaging_period": "24h",
"source_standard": "measured",
"target_standards": ["epa", "china", "india", "europe", "who"]
}
Response:
{
"source_concentration": {
"value": 55.5,
"unit": "ug/m3",
"pollutant": "pm25"
},
"aqi_values": {
"epa": {
"aqi": 150,
"category": "Unhealthy for Sensitive Groups",
"color": "#FF7E00",
"health_advisory": "Sensitive groups reduce prolonged outdoor exertion"
},
"china": {
"aqi": 146,
"category": "Lightly Polluted",
"color": "#FF7E00"
},
"who_guideline_exceedance": {
"ratio": 3.7,
"status": "Exceeds WHO guideline (15 μg/m³) by 270%"
}
},
"timestamp": "2025-07-15T14:30:00Z"
}
WIA-ENE-017 Communication Guidelines:
| AQI Level | Poor Messaging | Effective Messaging (WIA Standard) |
|---|---|---|
| AQI 45 (Good) | "Air quality is good." | "🟢 AQI 45 (Good) - Perfect conditions for all outdoor activities! Enjoy your day outside." |
| AQI 85 (Moderate) | "Unusually sensitive people should consider reducing prolonged outdoor exertion." | "🟡 AQI 85 (Moderate) - Air quality is acceptable for most. If you have severe asthma, consider shorter outdoor workouts. General public: No restrictions." |
| AQI 135 (Unhealthy for Sensitive) | "Sensitive groups may experience health effects." | "🟠 AQI 135 - High ozone levels. 👶 Children, 👴 elderly, and people with asthma: Limit outdoor sports to < 1 hour. Plan activities before noon when ozone is lower." |
| AQI 180 (Unhealthy) | "Everyone may experience health effects." | "🔴 AQI 180 - Unhealthy PM2.5 from wildfire smoke. Everyone: Avoid prolonged outdoor exertion. Stay indoors if possible. Use HEPA air purifiers. Check again at 6 PM for updates." |
| AQI 250 (Very Unhealthy) | "Health alert." | "🟣 AQI 250 - VERY UNHEALTHY. Hazardous smoke conditions. 🚨 ALL RESIDENTS: Stay indoors. Close windows. Run air purifiers. Cancel outdoor events. Check on elderly neighbors. Air quality emergency in effect." |
The EPA's NowCast algorithm provides more responsive AQI values for PM2.5 and PM10 during rapidly changing conditions (e.g., wildfire smoke):
NowCast Formula:
Given: 12 hours of hourly PM2.5 measurements (H1, H2, ..., H12)
H1 = most recent hour
H12 = 12 hours ago
Step 1: Calculate Weight Factor (w)
w = min(H_min / H_max, 0.5)
Where H_min and H_max are minimum and maximum concentrations
in the past 12 hours.
Step 2: Calculate NowCast Concentration
∑(w^i × H_i)
NowCast = ―――――――――――――――――
∑(w^i)
where i ranges from 0 to 11 (hours)
Step 3: Convert to AQI using standard breakpoint table
Example:
Hours: [85, 82, 79, 75, 68, 62, 58, 55, 53, 51, 50, 48] μg/m³
H_max = 85, H_min = 48
w = 48/85 = 0.565 → use 0.5 (capped)
NowCast = (1.0×85 + 0.5×82 + 0.25×79 + ...) / (1.0 + 0.5 + 0.25 + ...)
= 72.3 μg/m³
→ AQI = 156 (Unhealthy)
vs. Simple 12-hour average = 63.8 μg/m³ → AQI = 136
NowCast is more responsive to recent spike!
When to Use NowCast:
When to Use Standard AQI:
The WIA-ENE-017 standard extends AQI concepts to indoor environments where people spend 90% of their time:
| Pollutant | Indoor Source | Good (Green) | Moderate (Yellow) | Unhealthy (Red) |
|---|---|---|---|---|
| PM2.5 | Cooking, smoking, candles | < 12 μg/m³ | 12-35 μg/m³ | > 35 μg/m³ |
| CO₂ | Breathing, combustion | < 600 ppm | 600-1000 ppm | > 1000 ppm |
| VOCs | Cleaning products, furniture | < 220 μg/m³ | 220-660 μg/m³ | > 660 μg/m³ |
| Humidity | Cooking, showers, humidifiers | 30-50% | 20-30% or 50-60% | < 20% or > 60% |
The standard supports individualized health thresholds:
Personalized AQI Example:
User Profile:
- 8-year-old child
- Diagnosed asthma (moderate severity)
- Lives in Phoenix, AZ (high ozone region)
- Enjoys outdoor soccer
Standard EPA AQI Threshold: 101 (Orange - caution for sensitive groups)
Personalized Threshold: 75 (adjusted based on individual factors)
Alert Trigger:
When AQI > 75, send notification:
"⚠️ Air Quality Alert for Emma
Current AQI: 82 (approaching your personal threshold)
Dominant pollutant: Ozone
Recommendations for your child:
✅ OK for light outdoor play (< 30 min)
⚠️ Postpone intense sports practice
✅ Have rescue inhaler readily available
📅 Forecast: AQI improving to 65 by evening
Perfect for outdoor activities after 6 PM!"
This chapter covered the transformation of air quality data into actionable public health information:
Next Chapter Preview: Chapter 4 explores the infrastructure behind air quality monitoring—designing sensor networks, IoT architecture, and deployment strategies for comprehensive spatial coverage.