context: Ask anybody to name the most common cause of lung cancer, and they'll probably know the answer: smoking. But you're more likely to get a blank stare if you ask them about the second most common cause. The answer, if you didn't know, is radon. It's an invisible, odorless, radioactive gas that can build up in homes. Experts say that nearly 1 out of 15 houses in the U. S. has elevated levels of radon. That sounds pretty alarming, but there are steps you can take to protect yourself from radon and lung cancer. You just need to test your home for radon, and then -- if you have high levels -- make some changes to get them lower. If you don't know much about radon, you're not alone. For such a serious health issue, it seems to have a low profile. Many only find out about it when they're buying or selling a house, since testing for radon sometimes happens along with home inspections. So what is it? It's a gas that comes from the ground naturally. When uranium deep in the earth breaks down, it creates radon. Most of this gas finds its way up through the ground and into the air. Outside air always has some radon in it, but the level is usually low enough that it doesn't cause trouble. Radon can become a problem when it seeps up from the ground into your house, through cracks in your foundation, gaps around pipes or wires, or other openings. When radon gets trapped inside, like in a basement, the levels can build up and become dangerous. Since you can't smell or see it, you won't know it's there. While most radon comes from the ground, there are a few other sources. Sometimes it can get into well water. Small amounts of it can also come from building materials, like concrete, brick, and granite, but experts say that the levels are usually low enough they don't cause problems on their own. When you breathe in radon, tiny radioactive particles get trapped in your lungs and cause damage there. If you're in contact with these levels of radon day after day for years, that damage can lead to lung cancer. Experts most often measure radon by the amount that's in the air. The higher the level, and the longer you're in contact with it, the greater your risk of lung cancer. To put the risks of radon in perspective, about 21,000 people die each year from lung cancer related to radon. It's a serious home hazard, since it kills more people than falls in the home, drownings, and house fires. And keep in mind that smoking and radon are a bad combination. If you smoke and live in a home with high radon levels, your risk of getting lung cancer is especially high. While radon can be a problem in any building, including schools and workplaces, the biggest danger for most people is at home, where they spend so much time. So find out about the radon levels where you live. These levels vary across the country, but even states with relatively low radon levels can still have areas with high radon. Sometimes, houses next door to each other can have different levels. The type of home doesn't matter either. Old homes and new homes can both have radon. That's why experts say that everyone needs to test their home. There's no way to know otherwise. Radon tests are easy to get online or in hardware stores. Usually, you just set up the testing kit in the lowest level of your home that you use regularly, such as your basement or first floor. It should not be placed in a kitchen or bathroom. Then after a few days, you send it off to a lab for results. If you have well water, you may want to test your water for radon, too. When you get the results, you may see some numbers in front of units called "p Ci/ L." That's how the amount of radon is measured. According to the Environmental Protection Agency: 1.3 p Ci/ L is an average indoor level of radon. 2.0 to 3.9 p Ci/ L is higher than average, and you should think about fixing your radon problem. 4.0 p Ci/ L is high enough that you need to take action. If you find out that radon is high in your home, you don't need to panic or flee your house. Remember, lung cancer from radon is a long-term risk. It takes years of contact with it, not days. But you do need to deal with the problem. Get in touch with a contractor who's an expert in solving radon issues. Fixes can include things like sealing up cracks in your basement or installing a pipe under your home to suck out the radon and blow it outside. If you've never tested your home for radon, or think you might have but can't remember, now is the time to do it. Testing is so easy to do, and the benefits, for you and your family, could be huge.