Air Quality Index (AQI) calculator that converts pollutant concentrations into EPA categories and matching health recommendations.
Four tabs: compute AQI from PM2.5 / PM10 / O3 / NO2 / SO2 / CO, category and health guidance (Good → Hazardous), mask guidance (surgical / KN95 / N95), and air-purifier CADR sizing.
Disclaimer: Real-time AQI data is available from IQAir or the EPA AirNow app. Consult a doctor if you have respiratory symptoms.
Calculator information
📋 How to use this calculator
- Select the pollutant to calculate: PM2.5, PM10, ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), or carbon monoxide (CO).
- Enter the pollutant concentration using the correct units: PM2.5/PM10 in ug/m3, gases (O3/NO2/SO2) in ppb, CO in ppm.
- Pick the averaging period: PM2.5 24-hour, O3 8-hour, CO 8-hour, NO2/SO2 1-hour (per EPA standards).
- The calculator converts the value to an AQI (0-500) and shows the color category from Good (green) to Hazardous (maroon).
- For the health recommendations tab, enter your status: healthy, child, senior, asthma, heart disease, or pregnant.
- Mask guidance: AQI under 100 a surgical mask is adequate, 100-200 KN95, 200-300 N95, above 300 avoid outdoor activity.
- For air purifier sizing, enter room area (sq ft or m2) and ceiling height to compute the required CADR.
🧮 EPA AQI Linear Interpolation
AQI = ((I_hi - I_lo) / (BP_hi - BP_lo)) x (C - BP_lo) + I_lo
- C = measured pollutant concentration
- BP_hi = upper breakpoint concentration containing C
- BP_lo = lower breakpoint concentration containing C
- I_hi = AQI value at the upper breakpoint
- I_lo = AQI value at the lower breakpoint
- Final AQI = the highest value across all pollutants calculated
EPA categories: 0-50 Good (green), 51-100 Moderate (yellow), 101-150 USG (orange), 151-200 Unhealthy (red), 201-300 Very Unhealthy (purple), 301-500 Hazardous (maroon). The WHO 2021 guideline is stricter: annual PM2.5 5 ug/m3 (vs the EPA NAAQS of 9 ug/m3 as of 2024).
💡 Worked example: Calculate AQI for a high-pollution day at PM2.5 75 ug/m3
Given:- Pollutant: PM2.5
- 24-hour concentration: 75 ug/m3
- EPA breakpoint: 55.5-150.4 ug/m3 = AQI 151-200 (Unhealthy)
Steps:- BP_lo = 55.5, BP_hi = 150.4.
- I_lo = 151, I_hi = 200.
- AQI = ((200-151)/(150.4-55.5)) x (75-55.5) + 151.
- AQI = (49/94.9) x 19.5 + 151 = 0.5163 x 19.5 + 151.
- AQI = 10.07 + 151 = 161.
- Category: Unhealthy (red).
Result: AQI 161 - Unhealthy. The general population may experience health effects; sensitive groups (asthma, seniors, children) will feel serious effects. Recommendations: avoid strenuous outdoor activity, wear an N95 mask if you must go outside, and run an indoor air purifier with CADR 200+ for a 200 sq ft room.
❓ Frequently asked questions
What's the difference between the EPA AQI and WHO guidelines?
The US EPA AQI uses a 0-500 scale; the 24-hour PM2.5 breakpoint for Moderate begins at 9 ug/m3 (revised in 2024 from 12). WHO's 2021 guideline is much stricter: 24-hour PM2.5 should stay under 15 ug/m3 and annual under 5 ug/m3. Many other countries have their own indexes with different breakpoints. Use the EPA AQI for US comparisons; reference WHO for stricter health-based targets.
Are cloth masks enough for air pollution?
Not for PM2.5. Cloth masks filter only 30-50% of particles larger than 3 microns, while PM2.5 particles are under 2.5 microns and pass through. Effective options: surgical mask 60-80% for PM2.5, KN95/KF94 90-95%, N95 95%+. Make sure the mask fits snugly at the nose and chin - side leaks tank filtration. Replace masks after about 8 hours of active wear.
What AQI is safe for children?
The WHO and the American Academy of Pediatrics recommend keeping long-term exposure at AQI under 50 (Good) for children. At AQI 51-100 (Moderate), outdoor activity is generally fine but limit it for kids with asthma. AQI above 100 (USG) means cut back on outdoor play; above 150 (Unhealthy) move indoors with an air purifier. Long-term PM2.5 exposure above 10 ug/m3 in children is linked to asthma, reduced lung function, and cognitive impairment (Schultz et al. 2017).
How do I size an air purifier's CADR correctly?
AHAM formula: CADR (cfm) = room_volume x ACH_target / 60. Target 4-5 ACH for heavy pollution. Example: a 13 x 16 x 9 ft room = 1,872 ft3, at 5 ACH = 156 cfm minimum. A simpler rule of thumb (AHAM's 2/3 rule): CADR (cfm) >= 2/3 x room area (sq ft). Pick a unit with True HEPA H13 (99.97% of 0.3-micron particles) plus an activated carbon stage for VOCs and smoke.
Does AQI account for all pollutants at once?
The EPA AQI computes the index for the 5 criteria pollutants (PM2.5, PM10, O3, NO2, SO2, plus CO) separately and reports the highest value as the overall AQI. For example, if PM2.5 yields 161 and O3 yields 80, the overall AQI is 161 (Unhealthy). Monitoring stations measure each pollutant simultaneously using optical sensors (PM), electrochemical cells (gases), and UV photometry (O3). Apps like AirNow, IQAir, and PurpleAir aggregate official and crowdsourced data.
📚 Sources & references
Last updated: May 11, 2026