Outdoor radon concentrations are about 0.4 pCi/L averaged over the US. Radon concentrations are lowest downwind of oceans or large lakes and highest in the mid-continent interior. Radon outdoors in western Iowa-Minnesota-Dakota can exceed 1.5 pCi/L. That is more than the average US house indoor value of 1.2 pCi/L (see map, see research article). Outdoor radon delivers more damage per unit radon concentration than indoor radon since the radon progeny are not "removed" from the air as effectively outdoors.
Indoor radon in public places
The distribution of indoor radon in US buildings other than detached homes is not well known. In many states, schools have been measured but few factories, restaurants, shopping malls, or office buildings. In the upper Midwest, we have found that places workplaces tend to have about one-half of the radon concentration that is found in local houses. Radon tends to be lowest in large, well-ventilated buildings where the indoor concentration is about the same as the outdoor concentration. Radon is highest in small buildings with basements and no balanced mechanical ventilation system. The highest public workplace that we found in a small sample (~150 workplaces) had radon concentrations in excess of 30pCi/L That's more than 6 times the action level for homes and above the safety level enforced in uranium mines.
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