The radon concentration in the basement needs to be measured.
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The character 氡 stacks 气 (gas) over 冬 (winter) - a noble gas whose name borrows the sound of 冬. Most Chinese element names for gases use the 气 radical (氢 hydrogen, 氧 oxygen, 氮 nitrogen).
Gas/vapour radical wrapping the top-right - the indexing radical. Modern chemistry chars for noble gases all live in this family: 氢 (hydrogen), 氧 (oxygen), 氦 (helium), 氮 (nitrogen), 氪 (krypton). The radical labels 氡 as a gaseous element on sight.
Inside 冬 supplies the sound dōng directly - a clean phonetic borrowing. Radon was named in Chinese using this transparent sound-loan strategy: pick a gas-radical, glue on a phonetic that matches the syllable, done. 冬 carries no semantic weight here.
