Radon gas is nine times heavier than air and tends to stay close to ground level.  In fact, studies have shown that radon levels in a home's basement can be two to five times higher than the levels on the upper floors.

Source

Radon enters the home through cracks in foundations or slabs, joints, underground service penetrations, untrapped floor drains, sumps, exposed soil/crawl space areas, and other openings. It can also enter a home through the water supply, particularly if the water is supplied from a well. In the water, radon is released when it is boiled, used in a shower or otherwise agitated. Although rare, radon may also enter a home through the use of certain building materials such as phosphate slag in concrete and gypsum board. Brick, concrete, and tile -- although they have some radiation characteristics -- have proven not to be a problem. 

Health Effects

Exposure to radon gas is believed to be the second leading cause of lung cancer after cigarette smoke.  It is a colorless, odorless and tasteless gas and is relatively harmless until it decays into what are referred to as "radon daughters".  It is these "daughters" ' which pose a threat to human health. If inhaled, they are deposited in the lungs where they continue their decay, emitting alpha and beta particles and gamma radiation.  Radon 'daughters' produce an electrical charge and tend to attach themselves to walls, furniture, and dust particles. If the 'daughters' are attached to large surfaces they generally do not pose a threat however, when they attach to airborne dust particles or are not attached to anything and can be inhaled, then they can cause long term health problems.

Testing 

Air samples from at least two areas within the home should be collected over a 48 to 72 hour minimum period. Resultant measures can be read immediately when samples are collected with using a continuous radon monitor.  If a canister type collector is used, an additional 48 hours may be required and the results must be sent for laboratory analysis. 

Control/Remediation

Two factors control radon levels inside the home...

• the flow of the gas into the home

• the rate at which indoor air is exchanged with outdoor air (ventilation.) 

What can you do to reduce the level of radon gas in the home?

For existing homes...

• Seal all cracks in the walls, basement floor, and joints between the concrete slab and basement walls. 
• Cover crawl space floors and/or areas where the soil is exposed, with a polyethylene ground sheet (moisture barrier).
• Ensure proper crawl space ventilation.
• Provide tightly sealed covers for sumps or use a submersible pump.
• Floor drains should have water traps installed.
• Vent the space under the floor slab where possible.
• Increase your home's ventilation rate by using a balanced, mechanical ventilation system such as a heat recovery ventilator. Unbalanced systems (exhaust only) can create negative pressure in the home and draw more outside air and radon through cracks or holes.

For new home construction...

• Check with your builder to ensure that concrete walls and floors are designed to withstand soil pressures and minimize cracking.
• Place a polyethylene barrier under the concrete slab before it is poured.
• Caulk/seal all joints between the slab and the concrete walls. 
• Avoid open air flow paths between the weeping tile and the floor drain. 

   Exposure Guidelines

Radon is classified as a potential human carcinogen therefore,  indoor levels should be reduced as low as possible. Remedial action should be taken where the average annual concentration in the normal living areas exceeds 800 Becquerels (Bq) per cubic meter.