Key facts
- Most people spend roughly 20 hours a day indoors, where PM2.5 is typically well below outdoor levels.
- Breathing rate matters as much as concentration — cycling inhales far more air per hour than sitting in a car.
- The WHO 24-hour PM2.5 guideline is 15 µg/m³; the annual guideline is 5 µg/m³.
- A well-fitted N95 removes roughly 90% of fine particles during the time it is worn.
Personal Exposure Calculator
Live PM2.5 for your city, adjusted for how you spend your day -- your commute, time outdoors, and the hours indoors -- to estimate what you actually breathe.
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Why the city reading is not your exposure
Air-quality indexes report ambient outdoor concentration at a fixed location. Actual exposure depends on where you spend your hours and how hard you are breathing while you are there. Two people in the same city on the same day can differ several-fold.
A motorcyclist commuting two hours through traffic can breathe considerably more particulate matter than a colleague working from a filtered apartment, even though both look up the same AQI number.
The microenvironment model
Each part of the day is assigned an exposure factor — the concentration in that setting divided by the ambient outdoor concentration — and an inhalation rate in cubic metres per hour, which rises with physical activity.
| Setting | Exposure factor | Inhalation rate (m³/h) |
|---|---|---|
| Car | 0.7 | 0.5 |
| Bus | 1.3 | 0.6 |
| Metro / subway | 0.9 | 0.6 |
| Motorcycle | 2.0 | 0.6 |
| Bicycle | 1.8 | 2.0 |
| Walking | 1.5 | 1.1 |
| Outdoors, general | 1.0 | 0.8 |
| Indoors, windows open | 0.65 | 0.55 |
| Indoors, HEPA purifier running | 0.15 | 0.55 |
Reading the output
The dose figure is often the more useful of the two. Cycling through moderately polluted air can produce a higher inhaled dose than sitting in a car through severely polluted air, because the breathing rate is roughly four times greater.
- Time-weighted average — the concentration you were effectively exposed to across 24 hours.
- Inhaled dose — total micrograms drawn into the lungs, which accounts for breathing rate.
- Percentage of the WHO daily guideline of 15 µg/m³.
- Cigarette equivalent, derived from the time-weighted average rather than the raw ambient reading.
What actually moves the number
- Indoor filtration has the largest effect for most people, simply because indoor hours dominate the day.
- Mask use during commuting and outdoor time cuts those segments by roughly 90%.
- Switching commute mode changes both the concentration and the breathing rate simultaneously.
- Reducing outdoor exercise during peak pollution hours lowers the highest-dose portion of the day.
Frequently asked questions
Why is my exposure lower than the city AQI suggests?
Because most of your day is spent indoors, where PM2.5 is typically 15–65% of the outdoor concentration depending on filtration. The city reading is an outdoor ambient value, not a personal one.
Is cycling in polluted air worse than driving?
For inhaled dose, often yes. A cyclist breathes roughly four times as much air per hour as a car passenger, which can outweigh the lower in-cabin concentration a car provides. The calculator models both effects.
How much does an air purifier actually help?
In this model a running HEPA purifier reduces the indoor exposure factor from 0.65 to 0.15. Because indoor hours usually dominate a 24-hour day, that is typically the single largest available reduction.
Does this replace a personal air quality monitor?
No. This is a modeled estimate built from typical exposure factors. A wearable monitor measures your actual environment and will be more accurate for your specific circumstances.
Sources
This reference was prepared from the following primary public-health and regulatory sources:
- WHO Global Air Quality GuidelinesWorld Health Organization
- Particulate Matter (PM) BasicsUnited States Environmental Protection Agency
- Health and Environmental Effects of Particulate MatterUnited States Environmental Protection Agency