Cigarette smoking.

[CajunBlazer]

214. Probably refering to the presence of Radon 222 in the water

Radon 222 is a colorless, odorless gas that often enters a home from ground vapors and through the water supply, especially if that water supply is obtained from wells. It is radio active and is the second leading cause of lung cancer after smoking.

From what I have read, random readily separates from water during aggregation, such as during showers. If your water has a fairly high radon content, radon concentrations can reach high levels of in the air of a shower stall and the gas then escapes into the air of your home. I would imagine that the hotter the water, the more residual radon would be released and of course the longer the shower, the more radon will escape into the air of the shower stall.

There is no doubt that radon causes lung cancer; uranium miners who were exposed to high levels of radon often had lung cancer in much higher percentages than the general population. How many deaths Radon in homes cause due to lung cancer is an open question - but evidently evidently smoking and radon exposure is a double whammy.

You an find a lot of information on this subject on the internet. Though I remember better sources, I found this one quickly: http://www.maine.gov/dhhs/mecdc/environmental-health/rad/radon/radonfaq.htm

Although some scientists dispute the precise number of deaths due to radon, all major health organizations (like the Centers for Disease Control and Prevention, the American Lung Association and the World Health Organization) agree with estimates that radon causes thousands of preventable lung cancer deaths every year. This is especially true among smokers, since the risk to smokers is much greater than to non-smokers.