SOUTH LAKE TAHOE - Giant warning labels aside, most people know smoking is bad for them.
But what they may not know is that smoking combined with radon exposure makes it even worse.
"Smoking combined with radon is a health risk," said Christy Kessler, supervising health education coordinator for the El Dorado County Health Services Department. "Stop smoking and lower your radon levels to reduce lung cancer risk."
Radon is an odorless, colorless, radioactive gas that arises from the decay of naturally occurring uranium and thorium in soil.
The gas is linked to 21,000 lung cancer deaths a year, second only to cigarette smoking, according to the Environmental Protection Agency.
It is the leading cause of death among nonsmokers.
"Radon increases lung cancer among everyone," said Lisa Sheretz, lung health program manager for the American Lung Association in Reno.
An estimated 23,400 people in the Lake Tahoe area live in homes likely to equal or exceed EPA's recommended action level of 4 picocuries per liter. The highest concentrations are in South Lake Tahoe, Truckee and north of Emerald Bay.
While much attention is paid to eliminating tobacco, health officials say radon should not be ignored.
"Pretty amazing, isn't it?" said Pam Graber, spokesperson for Carson City Health and Human Services. "(Radon is) the No. 1 cause of lung cancer in nonsmokers."
While Thursday marked the 35th annual Great American Smokeout, an American Cancer Society event encouraging smokers to either quit or make a plan to quit, Graber said it's a good time to address exposure to radon.
Free test kits are available from public agencies in California and Nevada. The kits also sell for $5-$20 at hardware stores.
If a problem is detected, the gas can be vented from the home's crawl space through the roof, or the crawl space can be fortified.
Quitting smoking, however, may take even more planning. That's where the Great American Smokeout comes in.
"What they want folks to do is use that day to decide, 'I'm going for it,'" Graber said. "Effective quitting requires pre-contemplation and contemplation and really getting your head straight to quit."
"Getting ready and bam, that's day one," she added.