108 air-quality readings — PM10 of 150 is "bad" in Korea but AQI 98
For every meeting of 6 pollutants and 18 concentrations: the US index and the Korean grade, side by side. You can see exactly where the same number gets two different verdicts.
The index is one straight line
The US index pins one concentration range to one index range and draws a straight line between them. PM2.5 at 35.4 µg/m³ is exactly 100; 9 µg/m³ is exactly 50. With the table you can turn any concentration into an index — and any index back into a concentration.
Same number, different verdict
A Korean forecast calls PM10 of 150 "bad", but the same value in the US index is 98 — still moderate. The other way round, PM2.5 of 40 has only just crossed into "bad" in Korea while the US index already reads 112, unhealthy for sensitive groups. Neither is wrong; they hold different rulers. Move country and the same air reads differently.
Put in cigarettes
Breathing air at 22 µg/m³ of PM2.5 for a day is reckoned to be about one cigarette. The figure comes from Berkeley Earth, and it turns an abstract number into something the body understands: a day outdoors at 75 µg/m³ works out to three cigarettes.
A mask is about fit, not the number
KF94 and N95 filter to a similar standard; what actually differs is how well the mask seals to your face. If it gapes at the sides, air takes the shortcut and the rating stops mattering. Wear one once fine dust is in the bad range, but for a long walk pick the one you can actually breathe through.
These figures are a starting point
These indexes are usually built on daily or eight-hour averages, so they differ from whatever the sensor says this minute — and a main road differs from the alley behind it. Countries revise their tables from time to time; these figures follow the tables as they stand.
Find it by pollutant and concentration
US index (AQI) · Korean grade
fine dust PM2.5Concentration that makes 100 35.4µg/m³
coarse dust PM10Concentration that makes 100 154µg/m³
ozoneConcentration that makes 100 70ppb
nitrogen dioxideConcentration that makes 100 100ppb
carbon monoxideConcentration that makes 100 9.4ppm
sulfur dioxideConcentration that makes 100 75ppb
How to read this
- On the left is the US index, on the right the Korean grade — the same concentration measured with two rulers.
- Index values of 50, 100 and 150 are the boundaries; that is where the category name changes.
- Forecast grades are usually daily averages, not the reading at this moment.
- The cigarette figure is only for PM2.5 and assumes a full day of exposure.
Frequently asked questions
Q. Is PM2.5 of 35 bad?
In Korea it sits at the top of "moderate"; on the US index it is 99, also the top of moderate. From 36 the Korean grade turns bad.
Q. Why does every app show a different number?
Because they use different scales. The US index maps concentration onto 0–500; the Korean grade sorts it into four bands. Same air, different display.
Q. How bad is an AQI of 150?
It is the top of the band that is unhealthy for sensitive groups, just short of unhealthy for everyone — around 55 µg/m³ of PM2.5.
Q. What is the difference between PM10 and PM2.5?
PM10 is anything up to 10 micrometres across; PM2.5 up to 2.5. The smaller particles go deeper into the lungs, which is why their limits are stricter.
Q. At what level should I wear a mask?
Once PM2.5 passes 36 µg/m³ and the Korean grade turns bad, it is worth it. But how well the mask seals matters more than its rating.