PM2.5 Concentration to AQI Calculator

This calculator uses the standard linear interpolation between breakpoint ranges:

I = (I_high − I_low) / (C_high − C_low) × (C − C_low) + I_low

• C is the PM2.5 concentration used in the calculation (µg/m³).
• C_low, C_high are the concentration breakpoints that bracket C.
• I_low, I_high are the AQI breakpoints that correspond to the concentration range.
• PM2.5 can be truncated to one decimal place for consistent reporting.

1. Enter the measured PM2.5 concentration in µg/m³.
2. Keep truncation enabled to match common AQI practice.
3. Click Calculate AQI to see results above the form.
4. Review the category, color label, and health guidance.
5. Use Download CSV or Download PDF for records.

• Breakpoints are designed for 24-hour PM2.5 averages in outdoor air.
• Short-term spikes may feel worse than a daily AQI suggests.
• If you have respiratory or cardiac conditions, consider stricter personal limits.
• Always follow local air quality advisories during smoke or dust events.

1) What PM2.5 measures

PM2.5 is particulate matter with aerodynamic diameter at or below 2.5 micrometers. These particles penetrate deep into the lungs and can enter the bloodstream. Concentrations are reported as micrograms per cubic meter (µg/m³) and are commonly averaged over 24 hours for regulatory comparisons.

2) Why AQI uses breakpoints

The Air Quality Index converts a concentration into a standardized 0–500 scale so different pollutants can be communicated consistently. AQI is not a direct dose metric; it is a category-driven scale built from concentration “breakpoints” that align with health-based guidance and public messaging. It also enables consistent comparisons across days and regions.

3) Breakpoint ranges used in this calculator

This calculator follows the widely used PM2.5 (24-hour) AQI breakpoint bands: 0.0–12.0, 12.1–35.4, 35.5–55.4, 55.5–150.4, 150.5–250.4, 250.5–350.4, and 350.5–500.4 µg/m³. Each band maps to an AQI interval from “Good” through “Hazardous.” The corresponding AQI bands are 0–50, 51–100, 101–150, 151–200, 201–300, 301–400, and 401–500.

4) Linear interpolation converts concentration to AQI

Within a single breakpoint band, AQI is computed by linear interpolation between the band’s lower and upper endpoints. This preserves smooth changes inside a category while keeping category transitions aligned with published thresholds. The output is then rounded to the nearest whole number for reporting.

5) Truncation and rounding practice

Operational AQI workflows often truncate the input concentration to one decimal place before calculation, then round AQI to a whole number. Truncation avoids overstating air quality impacts when measurements are near a boundary. If you disable truncation here, results may shift slightly near breakpoint edges.

6) Typical sensor and sampling considerations

Low-cost sensors can drift with humidity, temperature, and aerosol type. For best comparability, use calibrated or reference-grade instruments such as FEM/FRM monitors, apply recommended correction factors, and prefer stable averaging windows. Short bursts from cooking, dust, or nearby traffic can inflate momentary readings relative to 24-hour guidance.

7) Health interpretation by AQI category

Categories provide actionable risk communication. “Moderate” may affect unusually sensitive individuals, while “Unhealthy for Sensitive Groups” can trigger symptoms in asthma or heart disease. As AQI rises above 150, broader populations can be affected, and protective actions become more urgent.

8) Practical actions aligned to your result

For elevated AQI, reduce outdoor exertion, close windows during smoke or dust events, and use high-efficiency filtration indoors. Well-fitted respirators can reduce exposure outdoors when needed. Track trends across hours and days, because sustained exposure matters more than a single point reading.

1) Is this AQI based on 24-hour PM2.5?

Yes. The breakpoint mapping is intended for 24-hour average PM2.5 concentrations. If you only have short-term data, treat the AQI as an approximate indicator and check local advisories.

2) Why does the calculator truncate to one decimal place?

Many AQI workflows truncate PM2.5 to one decimal before computing AQI. This reduces boundary effects and aligns results with common reporting practice. You can disable truncation to see the difference.

3) What happens if my value is above 500.4 µg/m³?

The AQI scale is defined up to 500. This tool caps results at 500 for extremely high concentrations and labels the condition as hazardous. Use official emergency guidance for severe events.

4) Why can two apps show different AQI for the same place?

Differences come from averaging period, sensor type, corrections, and whether readings are modeled or measured. Some systems also report “NowCast” style estimates rather than strict 24-hour values.

5) Does indoor PM2.5 use the same AQI breakpoints?

AQI breakpoints were designed for outdoor public reporting. Indoors, sources and exposure patterns differ. You can still compute an equivalent AQI, but interpret it alongside ventilation, filtration, and time spent indoors.

6) Is AQI proportional to health risk?

Not exactly. AQI is a communication scale based on categories and thresholds. Health risk depends on duration, individual susceptibility, pollutant mix, and activity level. Use AQI to guide protective decisions.

7) What is the most reliable way to improve indoor air during high PM2.5?

Run a properly sized HEPA purifier, upgrade HVAC filters where possible, and limit infiltration by closing doors and windows. Combine filtration with source control, especially during cooking or nearby smoke.