The peculiarity of the way radon gas affects humans is based on the difference between its chemistry and its physics, according to a 1995 United States Geological Survey report.

Chemically, radon is what chemists call a "noble gas." It is chemically inert, entering into no known biochemical process in the human body.

Radioactivity, however, is not a chemical process, but a process controlled by physics. A radon atom's nucleus is unstable. It disintegrates, emitting alpha radiation and the radioactive metal polonium, which is chemically reactive in the body.

In 2003, the U.S. Environmental Protection Agency (EPA) estimated 21,000 cancer deaths resulted from radon inhalation every year. It is colorless and tasteless and undetectable to human senses.

The Stealth Effect

According the U.S. Geological Survey, radioactive decay of tiny amounts of uranium and radium in soils and household stonework produce minute amounts of radon. Radon diffuses through your foundation and accumulates inside your home. Radon also enters in water from deep wells.

The U.S. Department of Health and Human Services says there are no short-term effects of radon, but you may be breathing it unaware for many years. Radiation damage is cumulative.

Lung Cancer

Breathing radon for many years eventually raises your risk of developing lung cancer. According to the Surgeon General of the United States, only cigarette smoking causes more lung cancer than radon in the United States. Physicians at the Mayo Clinic also recognize radon as a known lung cancer agent in their patient population. They advise home testing to detect accumulations of radon.

In January 2009, the EPA published guidelines for indoor-air radon testing and hazards. A radiation level of 1.3 picocuries per liter of air is predicted to cause lung cancer in two people out of every 1,000. Radiation over 2 picocuries per liter of air triggers EPA recommendations for remediation, usually by enhanced ventilation, water treatment and removal of low-level radiation sources. If radiation reaches 4 picocuries, EPA estimates as many as seven people out of 1,000 may develop lung cancer, comparable to the rate of auto fatalities.

Stomach Cancer

According to the EPA's 2009 report, deep wells often penetrate deep mineral deposits where radon is continually produced and its product polonium may accumulate. This water contamination increases a user's lifetime risk for stomach cancer. About 168 cancer deaths occur every year from this cause, based on EPA estimates. Stomach cancer accounts for 11% of these cases. Lung cancer accounts for the other 89%. Household activities such as showering and washing liberate radon into the air, explaining water's effect on the lung cancer statistics.

The Safe Drinking Water Act of 1974 prompted the EPA to devise new regulations for technology to eliminate radon from household water. Point-of-entry water treatment extracts radon outdoors, yielding safe water inside the home. Point-of-use water treatment is placed at every tap. Dishwashers and washing machines may be left unprotected if additional installations and costs are not planned.