Radon concentrations are lowest downwind of oceans or large lakes and highest in the mid-continent interior. 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.
That's more than 6 times the action level for homes and above the safety level enforced in uranium mines.