 If you live in a house with a basement, the chances are you've had a radon test. But what do the numbers mean, and just how safe or dangerous is it if they're below or above the recommended exposure level? Radon is a naturally occurring radioactive gas. It's one of those substances that it's pretty much impossible to avoid as it's constantly being produced by other materials in the Earth's crust. And because it's radioactive, it has the potential to cause lung cancer if you inhale it at high levels for long enough. For most of us, this isn't worth worrying about too much as we can't avoid breathing small amounts of the gas. But if you're living in an area where there are high levels of radon, it can be an issue. Unfortunately, there doesn't seem to be an exposure level where radon doesn't increase the risk of cancer. And because of this, radiation protection specialists recommend levels in homes are kept as low as you can reasonably get them. But what does this mean if you find yourself living somewhere with high radon levels? In the US, the Environmental Protection Agency recommends you take action to reduce radon levels if they're above 4 pico-curies per litre. The World Health Organization is a little bit more conservative here and suggests you do something about it if the levels are above 2.7 pico-curies per litre. The good news, though, is that for most people, it's not too hard to get indoor concentrations below the EPA action level. But if you are stuck with high radiation levels, how do you know what the additional risk is? To get a handle on this, we need to look at the chances of being killed by this invisible gas. Each year in the US, around 155,000 people a year die from lung cancer, and of these, something like 21,000 deaths are associated with radon. Many of these, though, are also connected with smoking, separating out the non-smokers. Around 2,900 people a year die from radon-related lung cancer. This is a serious number of deaths, but because of the number of people living in the US, the actual risk of dying from radon exposure in any given year is not that high. On average, something like 9,000 people out of every million in the US die each year. 500 people out of every million die from lung cancer, and 70 people out of every million die because they've been exposed to radon for a number of years. But when you only consider people who don't smoke, only 10 people out of every million die from radon exposure. Another way of looking at this is to ask what the chances are that when you die, radon was the cause. And here it turns out that when you pop your clogs, there's around about a 1- to 2-in-1,000 chance that radon will be to blame. It's somewhat comforting to know that if you're inhaling radon, the chances are that something else will kill you first. But this doesn't answer the question of how much safer you'll be if you reduce your radon exposure. Here the science is a little unclear, but research suggests that there's around about a 4% increase in your chances of getting cancer from inhaling radon for every 1 pico-curie per litre increase in your average exposure. This is over a lifetime of exposure though. A few days or weeks inhaling high levels of radon isn't that risky. It's the day-in-day-out exposure over tens of years that's important. Using this estimate of risk versus exposure, it turns out that if the average exposure level for radon was increased by 2 pico-curies per litre across the United States, we'd expect something like 5 additional radon-related deaths per million people each year. Likewise, if we reduce the average exposure level by 2 pico-curies per litre, there'd be 5 fewer radon-related deaths per million people each year on average. In other words, small increases and decreases in exposure do make something of a difference, and this is why it's important to keep your radon exposure as low as you can possibly get it. But if you're just over the recommended levels, it's not something to panic about. And if you're below the recommended levels but can't afford to go lower still, it's probably not worth worrying over. That said, it's always worth taking radon seriously and taking action to keep your exposures low if you can. For more information on radon safety, do check out the links in the blurb below. And as always, stay safe.