Radon gas, which accounts for approximately 21,000 lung cancer deaths each year, is second only to smoking as a source for lung cancer. However, unlike tobacco smoke, it is colorless and odorless, and is undetectable without specialized equipment. Radon is a product of the decay of uranium that is present in most rock, especially rocks containing granite. Radon gas rises out of the soil to be disbursed by surface air movement. If the route of exit from the ground is blocked by a home foundation and concrete slab, the gas enters that structure through cracks and gaps in the building components. When radon escapes the soil in an open space it poses little if any health threat as it is diluted with fresh air and carried away by surface air movement. In contrast, when it finds its way into a home, the absence of that fresh air to dilute it allows the gas to build up to what can sometimes be dangerously high concentrations. Radon is present in some building materials but not in sufficiently high concentrations to pose a health hazard. Radon can also contaminate well water which is then either taken in orally or disbursed into the air through activities such as showering, adding to elevated levels of airborne contamination (although this is less common than exposure to gas in the air).