Carl Brahe
It might seem unlikely that elevated radon levels would be found in high rise apartments but it is common. The colorless, odorless gas enters homes as the result of the natural rising of warm air that lowers the air pressure behind it that sucks gases out of the ground through cracks, joints and leaks in floor or foundation. The same action can happen in high rise buildings as the rising warm air, called the "stack effect", draws radon and other ground gases up the elevator shaft, utility raceways, wall cavities and other leaks. Most newer buildings have underground parking that is highly ventilated and creates a break between the earth and the living areas. In newer buildings it's more common for radon levels to be elevated by off-gassing from concrete ceilings and floors and from stone counter tops. Exposed concrete ceilings and floors release radon in the natural process of decaying stone.
The off-gassing can't be stopped so the only solution is to add fresh air to keep levels diluted. Green buildings tend to have adequate fresh air intake that keeps radon levels low, but many buildings do not. Whole buildings can be remediated by adding a fresh air heat exchanger at the top level that provides a constant supply of fresh air to all levels. These systems heat, or cool, incoming air by passing it through a radiator that is heated, or cooled, by the exhausted air and loses only a small amount of energy in the exchange. Individual units can be remediated using smaller heat exchangers that typically use the dryer vent, or intake through an exterior wall, to provide necessary fresh air. Countering the stack effect is accomplished by increasing air pressure at lower levels or vacuuming ground gasses from beneath the foundation.Off-gassing is remediated by adding sufficient fresh air to dilute the radon to relatively safe levels.
You can order low cost radon tests from Kansas State University's National Radon Services. Short-term tests are left out for 3-4 days then returned to the lab for results. The EPA recommends long-term testing of 90 -120 days. Both short-term and long-term charcoal canister test kits are available. Short-term kits are $15 and long-term are $25. All costs including postage and lab fees are included.