Acidic Air

Harkat had spent a lot of time testing the air, and didn't think it was poisonous to him — it was slightly different to the air on Earth, more acidic — but he kept his mask on anyway, to be safe. I'd coughed a lot for the first few days, but I was OK now — my hardened lungs had adapted to the bitter air.

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Cirque Du Freak: The Lake of Souls, Chapter 5