Ozone (O3) AQI Calculator

Example data table

Illustrative results. Actual values depend on averaging, truncation, and conditions.

Case	Mode	Input	Unit	Approx. AQI	Category
1	8-hour	0.052	ppm	48	Good
2	8-hour	0.067	ppm	86	Moderate
3	8-hour	0.078	ppm	126	Unhealthy for Sensitive Groups
4	1-hour	0.162	ppm	148	Unhealthy for Sensitive Groups
5	1-hour	0.420	ppm	~320	Hazardous

Formula used

The AQI is computed with a linear mapping between concentration breakpoints:

I = (I_Hi − I_Lo) / (C_Hi − C_Lo) × (C − C_Lo) + I_Lo

C is the truncated ozone concentration in ppm (to 0.001 ppm).
C_Lo, C_Hi are the ozone breakpoints that bound C.
I_Lo, I_Hi are the AQI values at those breakpoints.

If you enter µg/m³, the calculator converts to ppm using the ideal gas relation:

µg/m³ = ppm × (P × MW) / (R × T)

Where P is pressure (Pa), T is temperature (K), MW for O3 is 48.0 g/mol, and R is 8.314462618.

How to use this calculator

Select 8-hour, 1-hour, or Auto mode.
Choose your input unit: ppm or µg/m³.
If using µg/m³, confirm temperature and pressure for accurate conversion.
Enter the ozone concentration value(s) and press Calculate AQI.
Review the AQI value, category, and guidance shown above the form.
Use the CSV or PDF buttons to export your result.

Ozone AQI Guide

1) Why ozone is tracked

Ground-level ozone is a reactive gas that irritates airways and reduces lung function. It forms when nitrogen oxides and volatile organic compounds react in sunlight, so it often rises on warm, bright days in urban and downwind areas. Unlike stratospheric ozone, this “bad ozone” is a near-surface pollutant tied to smog.

2) What the AQI number means

The Air Quality Index converts ozone concentration into a 0–500 scale based on breakpoint bands. The tool uses linear interpolation between the bounding breakpoints, so the AQI increases smoothly as ozone increases within a band.

3) 8-hour versus 1-hour averaging

Health impact depends on exposure duration. The 8-hour average is the primary daily indicator and covers AQI up to 300. The 1-hour metric targets short, high peaks; values below 0.125 ppm are typically not used for 1-hour AQI reporting.

4) Breakpoints used by this calculator

For 8-hour ozone, breakpoints span 0.000–0.200 ppm, including 0.000–0.054 (Good) and 0.055–0.070 (Moderate). For 1-hour ozone, breakpoints span 0.125–0.604 ppm and can produce AQI values up to 500. These bands define the category thresholds. The calculator truncates ozone to 0.001 ppm (1 ppb) before indexing to match common reporting practice.

5) Unit conversion approach

If you enter µg/m³, the calculator converts to ppm using the ideal gas relation with pressure, temperature, and ozone molar mass (48.0 g/mol). Accurate local station pressure and realistic ambient temperature improve mass-to-volume conversions. At sea level and 25 °C, 0.070 ppm corresponds to roughly 137 µg/m³ for reference.

6) Who is most at risk

Children, older adults, and people with asthma or COPD are more likely to feel effects at the same AQI. Sensitive groups can be affected in the 101–150 range, while broader impacts become more common above 150 during outdoor exertion.

7) Common daily pattern

Ozone often peaks in mid‑afternoon when sunlight and temperature are highest. Morning values can be lower, then rise as photochemistry accelerates. Transport by wind can shift peaks to suburbs or other downwind locations.

8) Turning results into actions

Use the category to plan outdoor work and exercise. Above 100, reduce intensity, shorten duration, and take more breaks. Above 150, reschedule strenuous activity when possible and prioritize indoor, filtered environments. If outdoor activity is unavoidable, choose early morning or evening and monitor symptoms.

Note: Agency breakpoints and reporting rules may change. Use this tool for education and planning.

FAQs

1) Why does the calculator truncate ozone to 0.001 ppm?

Truncation to 0.001 ppm (1 ppb) aligns with common AQI reporting steps and avoids borderline breakpoint flips caused by tiny measurement noise. It helps produce stable, repeatable AQI values.

2) When should I use 8-hour versus 1-hour ozone?

Use 8-hour ozone for routine daily air-quality reporting and general planning. Use 1-hour ozone for high, short-term peaks during ozone episodes, typically when concentrations are at least 0.125 ppm.

3) Can I enter µg/m³ instead of ppm?

Yes. Choose µg/m³ and provide temperature and pressure. The calculator converts to ppm using the ideal gas relation and ozone molar mass, then computes AQI using the breakpoint interpolation.

4) Why is 1-hour ozone not accepted below 0.125 ppm?

Below 0.125 ppm, 1-hour ozone is generally not used for AQI calculations. The 8-hour metric is the preferred indicator at lower concentrations, so switch to 8-hour mode for those cases.

5) What does Auto mode do?

Auto mode calculates AQI for both 8-hour and 1-hour inputs (when provided) and reports the higher index. This mirrors the idea of highlighting the worse short-term ozone condition.

6) Does conversion affect the AQI category?

It can if you start from µg/m³. Temperature and pressure change the ppm equivalent, which can move the value across a breakpoint. Using realistic local conditions reduces conversion-driven category shifts.

7) Is this output suitable for regulatory compliance?

No. Compliance decisions require validated monitoring data, official averaging rules, and agency QA procedures. This tool is intended for education, screening, and communication of likely AQI categories.