Air quality·sulfur dioxide 40
sulfur dioxide 40ppb
AQI 57
moderate · moderate

sulfur dioxide 40 ppb — AQI 57

sulfur dioxide at 40 ppb scores 57 on the US index, and the Korean grade calls it moderate.

Both systems agree here

Concentration
40 ppb
US index (AQI)
57
US category
moderate
Korean grade
moderate
Concentration that makes 100
75 ppb

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.

Nearby cells

35 ppb
AQI 50
50 ppb
AQI 69

Same pollutant, other concentrations

5 ppb
AQI 7
10 ppb
AQI 14
15 ppb
AQI 21
20 ppb
AQI 29
25 ppb
AQI 36
30 ppb
AQI 43
35 ppb
AQI 50
40 ppb
AQI 57
50 ppb
AQI 69
60 ppb
AQI 82
75 ppb
AQI 100
100 ppb
AQI 112
120 ppb
AQI 121
150 ppb
AQI 134
200 ppb
AQI 157
300 ppb
AQI 198
500 ppb
AQI 266
800 ppb
AQI 399

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. How bad is sulfur dioxide at 40 ppb?

57 on the US index, and moderate on the Korean scale.

Q. Do the two systems agree?

Yes, both read it the same way here.

Q. What concentration would make the index 100?

75 ppb. This page is at 40 ppb.

Q. What is this pollutant?

sulfur dioxide. The cigarette comparison is only applied to PM2.5.