Smog Calculator
Enter an 8-hour ozone concentration in ppm to get the EPA Air Quality Index, category, and health guidance for smog conditions.
🌫️ What is the Smog Calculator?
The Smog Calculator converts a ground-level ozone reading, the primary chemical marker of photochemical smog, into the EPA's official Air Quality Index (AQI), a 0-300+ scale that translates raw pollutant concentration into an everyday health category from Good to Very Unhealthy. Enter an 8-hour average ozone concentration in parts per million and get the matching AQI number, category, and health guidance instantly.
This calculator is useful for anyone checking whether outdoor exercise is safe on a hazy day, students studying atmospheric chemistry and air pollution, people with asthma or other respiratory conditions planning their day around air quality, and anyone who wants to understand a raw pollutant sensor reading (in ppm) in the same AQI terms used by weather apps and news reports.
A common misconception is that "smog" and stratospheric ozone depletion are related. They are not, the beneficial ozone layer sits 10 to 50 km overhead and blocks harmful UV radiation, while the ground-level ozone this calculator measures is a pollutant formed at breathing height by sunlight reacting with vehicle and industrial emissions, and it damages lungs, crops, and materials rather than protecting them.
Instead of memorizing the EPA's breakpoint table, this calculator applies the official piecewise linear AQI formula directly to your reading, so you get an accurate AQI and category in one step, along with a chart showing exactly where your reading sits relative to the health category thresholds.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Clean, low-ozone day
Ozone concentration: 0.045 ppm
Example 2 — Typical summer afternoon
Ozone concentration: 0.062 ppm
Example 3 — Severe smog alert day
Ozone concentration: 0.120 ppm
❓ Frequently Asked Questions
🔗 Related Calculators
What is smog and what causes it?
Smog is a form of air pollution, historically a mix of smoke and fog, but today most commonly meaning photochemical smog, a hazy layer of ground-level ozone and particulates formed when sunlight triggers chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs), usually from vehicle exhaust and industrial emissions.
How is the smog Air Quality Index (AQI) calculated?
This calculator converts an 8-hour average ozone concentration in parts per million into the EPA Air Quality Index using the official piecewise linear breakpoint formula, AQI = ((AQIhigh - AQIlow) / (Chigh - Clow)) x (C - Clow) + AQIlow, where the breakpoint values are looked up from the concentration range the reading falls into.
Why is ground-level ozone used to measure smog instead of measuring smog directly?
Ground-level ozone is the key secondary pollutant responsible for photochemical smog's health effects and haze, and it is continuously monitored by regulatory air-quality networks, making it a practical, standardized proxy that regulatory agencies like the EPA use to define official air quality categories.
What AQI range counts as unhealthy?
AQI 0-50 is Good, 51-100 is Moderate, 101-150 is Unhealthy for Sensitive Groups, 151-200 is Unhealthy for everyone, and 201-300 is Very Unhealthy. Sensitive groups, children, older adults, and people with asthma or lung disease, should start limiting outdoor exertion once AQI passes 100.
Why does ozone smog get worse on hot, sunny days?
Photochemical smog formation is driven by sunlight, ultraviolet radiation supplies the energy that splits nitrogen dioxide and drives the reaction chain that produces ozone. Higher temperatures also speed up these reactions, which is why cities issue more smog and ozone action day alerts during summer heat waves.
Is ground-level ozone the same as the ozone layer that protects Earth?
No, and this is a common point of confusion. Stratospheric ozone, roughly 10 to 50 km up, is beneficial and blocks harmful UV radiation. Ground-level (tropospheric) ozone, the ozone measured by this calculator, is a pollutant that damages lungs, crops, and materials at breathing height.
What ozone concentration corresponds to AQI 100?
An 8-hour ozone concentration of 0.070 ppm corresponds to exactly AQI 100, the top of the Moderate category and the threshold the EPA uses to trigger air quality advisories for sensitive groups.
Can ppb ozone readings be used with this calculator?
Yes, but convert first, divide a parts-per-billion (ppb) reading by 1000 to get parts per million (ppm) before entering it. For example, a monitoring station reading of 75 ppb ozone equals 0.075 ppm.
Does this calculator account for particulate matter (PM2.5) smog too?
No, this calculator computes AQI specifically from ground-level ozone, the classic photochemical smog pollutant. Particulate matter uses a separate EPA breakpoint table with its own concentration ranges and is common in wildfire smoke and industrial haze rather than sunlight-driven ozone smog.
What should I do if the AQI from this calculator is above 150?
At Unhealthy (151-200) and Very Unhealthy (201-300) AQI, health authorities recommend everyone reduce prolonged or heavy outdoor exertion, sensitive groups should stay indoors when possible, and anyone experiencing symptoms like coughing or chest tightness during outdoor activity should seek shade and rest.
Why does the AQI-versus-ozone chart look like connected straight-line segments instead of one smooth curve?
The EPA AQI formula is deliberately piecewise linear, each health category (Good, Moderate, Unhealthy, and so on) is its own straight-line segment mapped between fixed concentration and AQI endpoints, which is why the chart shows connected line segments with visible breakpoints rather than a single smooth curve.