Short answer
The median is the middle value in a set of test results lined up from lowest to highest: half of the results are above it and half are below.
What median means
The median is not the average. A few very high readings pull the average up, and the median is less affected by them, which is why this site reports the median for a county's reported results.
It is a figure about a set of tests, never about one building.
A median and an average from the same seven numbers
The seven levels on EPA's risk chart make a handy set. Lined up, the fourth of the seven is the median; the average adds them and divides by seven.
| Position | Level, pCi/L | What it is in the set |
|---|---|---|
| 1 | 0.4 | Below the median |
| 2 | 1.3 | Below the median |
| 3 | 2 | Below the median |
| 4 | 4 | The median: three levels below it, three above |
| 5 | 8 | Above the median |
| 6 | 10 | Above the median |
| 7 | 20 | Above the median, and the one that pulls the average up |
What it means for your home
A county's median says what a typical reported test there read. It does not say what your home will read, and homes next to each other can have different indoor radon levels.
Read it beside the number of tests behind it. A median from a handful of tests carries far less weight than one from thousands.
Where to go next
- Radon data explorer. Look up the reported figures for any county or state.
- How each radon figure is read. What is predicted, what is measured and what this site calculates.