The smog calculator above takes a fine particulate concentration in micrograms per cubic metre and reports two things: the value and category that the United States Environmental Protection Agency's Air Quality Index assigns to it, and the cigarette-equivalence figure derived from a published rule of thumb. Both are translations of a single measured number into more familiar terms, and both are presented here as what a named source says rather than as any statement about your health.
Arb Digital builds free calculators that show which authority a number comes from. The AQI on this page is the US scale specifically. Many countries operate their own indices with different breakpoints, different averaging periods and different category names, so a reading that falls in one band under the EPA scale may fall in a different band elsewhere. Always check which index your local air quality authority publishes.
What PM2.5 Is and Why It Is Measured
PM2.5 means particulate matter with an aerodynamic diameter of 2.5 micrometres or less. As the EPA's Particulate Matter (PM) Basics page explains, particles in this size range come from combustion — vehicles, power generation, wood burning, wildfires — and from atmospheric reactions between gases such as sulphur dioxide and nitrogen oxides. They are small enough to remain suspended for long periods and to travel a long way from their source.
Size is the reason PM2.5 is singled out from coarser dust. Larger particles are captured in the upper airway, while fine particles penetrate deep into the lung. That is why air quality reporting worldwide has converged on PM2.5 as the headline pollutant, and why the mass concentration of these particles is what the index is built on.
The concentration is a mass per volume of air: micrograms of particulate per cubic metre. It is measured by reference monitors using gravimetric or beta-attenuation methods, and increasingly by low-cost optical sensors. The two are not equivalent. Optical sensors infer mass from light scattering and commonly overstate concentrations in humid conditions or in smoke unless a correction is applied, which is why public networks apply calibration factors to sensor data.
How to Use It
- Enter the concentration. Use the same averaging period the index expects — the EPA PM2.5 index is defined on a 24-hour average.
- Enter your exposure duration. This scales the cigarette-equivalence figure; it does not change the index value, which describes the air rather than the person.
- Apply a reduction if relevant. Filtration or a well-fitted respirator lowers the concentration you actually breathe, and the effective figure is what the equivalence uses.
- Read the category as the EPA's statement. The category names and their advice are set by the EPA for the United States, and other jurisdictions publish their own.
- Check the equivalence caveats below before quoting the cigarette number anywhere, because it compares particle mass and nothing else.
How the EPA Index Is Calculated
The AQI is a piecewise linear transformation. Each pollutant has a set of concentration breakpoints, each mapped to a fixed range of index values, and the index is interpolated linearly within whichever band the concentration falls into. The formula is the standard one: index equals the low index of the band, plus the concentration above the band's low breakpoint, scaled by the ratio of the index range to the concentration range.
For PM2.5 the EPA revised its breakpoints alongside the annual standard in February 2024. Its fact sheet on the final updates to the Air Quality Index for particulate matter sets the level of the primary annual PM2.5 standard at 9.0 micrograms per cubic metre and revises the index breakpoints to match. This tool implements those revised breakpoints: 0.0 to 9.0 maps to index 0 to 50, 9.1 to 35.4 maps to 51 to 100, 35.5 to 55.4 maps to 101 to 150, 55.5 to 125.4 maps to 151 to 200, 125.5 to 225.4 maps to 201 to 300, and 225.5 to 325.4 maps to 301 to 500.
Work the default through. A concentration of 55.0 sits in the 35.5 to 55.4 band, which maps to index 101 to 150. The interpolation is 101 + (150 − 101) ÷ (55.4 − 35.5) × (55.0 − 35.5), which is 101 + 2.4623 × 19.5, or 149. That falls in the category the EPA labels Unhealthy for Sensitive Groups. The six category names, their colours and the advice attached to each are described on AirNow's AQI Basics page, which is the EPA's public-facing explanation of the scale.
The Cigarette Equivalence, and What It Does Not Mean
In 2015 the research group Berkeley Earth published a widely repeated rule of thumb: a day spent breathing air at roughly 22 micrograms per cubic metre of PM2.5 delivers about the same mass of fine particulate to the lungs as smoking one cigarette. That figure is the default in the rule field above, and the equivalence is why an air quality reading can be made intuitive to people who have never thought about micrograms.
The comparison is about particle mass and nothing else. Cigarette smoke contains carbon monoxide, nicotine, tar, formaldehyde and a long list of other constituents that ambient smog does not deliver in the same proportions, and smoke from wildfires, diesel exhaust and coal combustion each have different compositions again. Equal mass of particulate is not equal exposure to everything else, and the equivalence should never be read as saying two exposures are medically the same.
It is also an average over a day rather than a dose to a person. Actual intake depends on breathing rate, on how much time is spent outdoors, on ventilation and filtration indoors, and on individual factors that this calculation knows nothing about. Treat the cigarette number as an illustration of scale — the thing it is genuinely good at — and not as a quantity.
Why Index Numbers Differ Between Countries
Because an index is a convention. The EPA scale runs 0 to 500 with six categories and specific breakpoints; other national systems use different upper bounds, different numbers of categories and different concentration thresholds, and some are based on different averaging periods. The World Health Organization publishes global air quality guideline levels, which are recommendations on concentration rather than an index, and national standards frequently sit above them.
The practical consequence is that an index number is not portable. Quoting "AQI 149" without naming the scale is ambiguous, and comparing an index value from one country with one from another is meaningless. What is portable is the concentration in micrograms per cubic metre, which is why this tool reports the effective concentration alongside the index and why serious comparisons are always made in concentration terms.
Averaging matters just as much. A short spike during a passing plume can be many times the daily mean, and an index computed from an hourly reading is not the same statistic as the reported daily AQI. Some agencies publish a NowCast value designed to represent current conditions from recent hourly data precisely because of this mismatch.
What Changes the Concentration You Actually Breathe
Indoor concentrations are not outdoor concentrations. In a closed building with mechanical ventilation and effective filtration, indoor PM2.5 can be a small fraction of the outdoor level; in a naturally ventilated building with the windows open it approaches it. Indoor sources — cooking, candles, wood stoves, smoking — push the other way and can produce indoor levels far above anything outside.
The reduction field is where any of that goes. A well-fitted respirator, a portable air cleaner sized for the room, or a building's filtration all reduce the concentration reaching you, and the effective figure is what drives the equivalence output. Poorly fitted face coverings achieve much less than their filter rating suggests, because leakage around the edges bypasses the filter entirely.
Timing is the other lever. Concentrations vary through the day with traffic, temperature inversions, wind and, in wildfire conditions, with the movement of the plume. Where an authority publishes hourly data, moving outdoor activity to the cleaner part of the day changes exposure more than most equipment does.
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Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Comparing index numbers across countries — breakpoints and scales differ, so only the concentration in micrograms per cubic metre is comparable.
- Treating an hourly reading as the reported AQI — the EPA PM2.5 index is defined on a 24-hour average, and agencies publish a NowCast for current conditions.
- Quoting the cigarette figure as a health equivalence — it compares particle mass only, and tobacco smoke contains many constituents that ambient particulate does not.
- Using uncorrected low-cost sensor data — optical sensors commonly overstate concentrations in humid air and in smoke unless a calibration is applied.
- Assuming indoors is safe — without filtration, indoor levels track outdoor ones, and cooking or burning indoors can exceed anything outside.
Related Free Tools From Arb Digital
For the emissions side of combustion, see the Kaya identity calculator, the flight carbon footprint calculator and the food carbon footprint calculator. For building energy and ventilation see the energy use intensity calculator. Since this page mentions cigarettes only as a unit of comparison, the smoking cost calculator and the quit smoking savings calculator deal with actual tobacco spending. The full free online tools hub lists everything.
Frequently Asked Questions
The United States EPA Air Quality Index for PM2.5, using the breakpoints revised in February 2024 alongside the annual standard of 9.0 micrograms per cubic metre. Other countries publish their own indices with different breakpoints and category names, so check the scale your local air quality authority uses.
By linear interpolation within a band. The index equals the band's low index value plus the amount by which the concentration exceeds the band's low breakpoint, scaled by the ratio of the index range to the concentration range for that band.
From a rule of thumb published by Berkeley Earth in 2015, which equates a day at about 22 micrograms per cubic metre of PM2.5 to the particle mass delivered by one cigarette. The field is editable so you can substitute another published equivalence.
No. The comparison is about particle mass alone. Tobacco smoke contains carbon monoxide, nicotine, tar and many other constituents in proportions that ambient particulate does not, and different particulate sources differ from each other as well. It is an illustration of scale, not a medical equivalence.
Low-cost optical sensors infer particle mass from light scattering and commonly read high in humid conditions and in smoke. Public networks apply correction factors to sensor data for that reason, and reference monitors use gravimetric or beta-attenuation methods instead.
You can, but the result is then an indication rather than a reported AQI, because the EPA PM2.5 index is defined on a 24-hour average. Agencies publish a NowCast value for current conditions, computed from recent hourly data with a weighting scheme.
No, and it is not designed to. It reports what the EPA's published scale says about a concentration and what one published rule of thumb equates it to. Decisions about activity, especially for children, older people and anyone with heart or lung conditions, belong with your local air quality authority and a clinician.
This tool is provided for education. It reports the US EPA Air Quality Index value and category for a PM2.5 concentration you supply, and a cigarette equivalence published by Berkeley Earth; neither is a health assessment or advice about you. Air quality scales differ between countries and are revised over time. Follow your local air quality authority, such as the EPA and AirNow in the United States, and consult a clinician about your own health.
