 In caves or ventilated mines, or poorly ventilated houses, its concentration climbs to 20–2,000 Bq/m3.Radon concentration can be much higher in mining contexts.Ventilation regulations instruct to maintain radon concentration in uranium mines under the "working level", with 95th percentile levels ranging up to nearly 3 WL (546 pCi 222Rn per liter of air; 20.2 kBq/m3, measured from 1976 to 1985).