Smog Calculator
EcologyCalculate Air Quality Index (AQI) and health risk from PM2.5, PM10, NO₂, and ozone concentrations. Instantly know your AQI category and recommended precautions for air quality.
Air Quality Index (AQI)
What is a Smog?
The Smog Calculator computes the Air Quality Index (AQI) from four key pollutant concentrations — PM2.5, PM10, nitrogen dioxide (NO₂), and ground-level ozone (O₃). Using the US EPA's standard linear interpolation formula, it converts raw concentration measurements into a single AQI score, an AQI category, and a plain-language health risk level. The AQI score is the primary output: a number between 0 and 500 that translates complex atmospheric chemistry into an actionable health signal.
India's air quality crisis makes this tool particularly relevant. India accounts for roughly 40 of the world's 50 most polluted cities by annual average PM2.5. Delhi's AQI regularly exceeds 400 — the "Hazardous" threshold — during winter months, when cold air traps vehicle exhaust and crop-residue smoke over the Indo-Gangetic Plain. India's Central Pollution Control Board (CPCB) operates a National AQI with six categories closely mirroring the EPA system, and their Sameer app reports the same pollutants this calculator accepts. Whether you are a researcher, a concerned resident, or a public health professional, the Smog Calculator converts measured concentrations into a risk category in seconds.
How to use this Smog calculator
Obtain your pollutant concentration data. Source PM2.5 and PM10 readings (in µg/m³) from India's CPCB Sameer app, IQAir, or a local continuous ambient air quality monitoring station (CAAQMS). Obtain NO₂ and O₃ values in ppb — divide µg/m³ values by 1.88 (NO₂) or 1.96 (O₃) to convert if needed.
Enter PM2.5 concentration. Use the PM2.5 (µg/m³) slider or number field. The range is 0–500 µg/m³; the default is 25 µg/m³ (WHO 24-hour interim target 3). The WHO annual guideline is just 5 µg/m³ — most Indian cities far exceed this value on most days.
Enter PM10 concentration. Use the PM10 (µg/m³) slider (0–600 µg/m³, default 50). PM10 includes dust, pollen, and construction debris in addition to combustion particles. On dust-heavy days — common in Rajasthan, Delhi, and other arid regions — PM10 can independently push AQI into poor categories even when PM2.5 is moderate.
Enter NO₂ concentration. Use the NO₂ (ppb) slider (0–2000 ppb, default 40). NO₂ is primarily emitted by vehicles and power plants. It is a precursor to both ozone and secondary particulate matter, so elevated NO₂ often signals that future ozone formation is likely.
Enter O₃ concentration. Use the O₃ (ppb) slider (0–500 ppb, default 30). Ground-level ozone forms through photochemical reactions between NO₂ and volatile organic compounds in sunlight. It is typically highest in afternoon hours on hot, sunny days — relevant for cities at lower latitudes like Hyderabad, Chennai, and Pune.
Read the AQI Score, Category, and Health Risk Level. The results update instantly. The highlighted AQI Score is your primary reference. Cross-reference the AQI Category against CPCB guidelines if you are working with official Indian reporting. Use the Health Risk Level to communicate recommendations to non-technical audiences or to decide on personal protective measures.
Formula & Methodology
### AQI Linear Interpolation (US EPA Method) The AQI for each pollutant is computed using a piecewise linear function that maps a measured concentration (Cₚ) onto the AQI scale using breakpoint pairs: AQI = ((I_Hi − I_Lo) / (C_Hi − C_Lo)) × (Cₚ − C_Lo) + I_Lo Where: - Cₚ = measured concentration of the pollutant - C_Lo = concentration breakpoint ≤ Cₚ - C_Hi = concentration breakpoint ≥ Cₚ - I_Lo = AQI value corresponding to C_Lo - I_Hi = AQI value corresponding to C_Hi ### PM2.5 Breakpoints (24-hour average, µg/m³) | Concentration range | AQI range | Category | |---|---|---| | 0.0 – 12.0 | 0 – 50 | Good | | 12.1 – 35.4 | 51 – 100 | Moderate | | 35.5 – 55.4 | 101 – 150 | Unhealthy for Sensitive Groups | | 55.5 – 150.4 | 151 – 200 | Unhealthy | | 150.5 – 250.4 | 201 – 300 | Very Unhealthy | | 250.5 – 350.4 | 301 – 400 | Hazardous | | 350.5 – 500.4 | 401 – 500 | Hazardous | ### Final AQI Final AQI = max(AQI_PM2.5, AQI_PM10, AQI_NO₂, AQI_O₃) The reported AQI score is the highest sub-index across all entered pollutants. ### Health Risk Categories | AQI | Category | Health Risk | |---|---|---| | 0–50 | Good | Minimal risk; outdoor activity recommended | | 51–100 | Moderate | Acceptable; unusually sensitive individuals may be affected | | 101–150 | Unhealthy for Sensitive Groups | Sensitive groups at risk; general public unaffected | | 151–200 | Unhealthy | General public may experience health effects | | 201–300 | Very Unhealthy | Health alert; everyone should limit outdoor exertion | | 301–500 | Hazardous | Emergency conditions; avoid all outdoor exposure | ### Worked Example Inputs: PM2.5 = 60 µg/m³, PM10 = 90 µg/m³, NO₂ = 50 ppb, O₃ = 45 ppb PM2.5 sub-index: Cₚ = 60 falls in the 55.5–150.4 range (AQI 151–200) AQI_PM2.5 = ((200 − 151) / (150.4 − 55.5)) × (60 − 55.5) + 151 = (49 / 94.9) × 4.5 + 151 = 2.32 + 151 = 153 PM10 sub-index: Cₚ = 90 falls in the 55–154 µg/m³ range (AQI 51–100) AQI_PM10 = ((100 − 51) / (154 − 55)) × (90 − 55) + 51 = (49 / 99) × 35 + 51 = 17.3 + 51 = 68 Final AQI = max(153, 68, …) = 153 → Category: Unhealthy for Sensitive Groups This example shows PM2.5 as the binding constraint — a common scenario in Indian urban environments where fine particulate dominates air quality. For a broader perspective on how national energy and economic decisions drive the emission levels that produce these concentrations, see the Kaya Identity Calculator and the CO₂ Breathing Emission Calculator. For ecological impact at a community level — such as how persistent high AQI affects species composition in urban green spaces — the Carrying Capacity Calculator provides useful framing.
Frequently Asked Questions