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If the air is muddy and dirty it's because there is too much pollution in that area. You should wear a mask and be careful of contamination.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
湖の水質汚濁を防止するために規制が設けられた。
A regulation was established to prevent the pollution of the lake.
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