 How much radiation is too much? As it turns out, just measuring the amount of radiation you're exposed to isn't that helpful. Instead, we need to know how much harm that radiation can do if it gets into your body, especially if it's ionizing radiation, the type of radiation that can rep through cells and damage DNA. This takes more than just listening to the clicks on a radiation counter though, and in fact, just knowing there's radiation around you isn't that helpful in working out how risky it is. Instead, it means understanding how different sources and types of radiation cause biological damage, and this is where seaverts come in. Seaverts are a measure of the potential biological impact of exposure to ionizing radiation, and in particular, how exposure might lead to health problems by damaging your DNA. They help us understand, for instance, how much exposure to radiation might increase the chances of developing cancer, or how it might affect any children you have after being exposed. To do this, seaverts combine three different aspects of ionizing radiation. First, there's the amount of energy that the radiation is capable of dumping into your body, and this changes with different sources and different situations. If you're having a dental x-ray, for instance, this would be the amount of energy the x-rays deposit in the tissues around your mouth and your head. Then there's the type of radiation. Boulder-like alpha particles, for instance, can do far more damage as they rip through your cells than x-rays or gamma rays. And finally, there's which parts of your body are being exposed. So radiation that gets into your lungs or your guts, for instance, does more harm than if it's just on your skin. By combining these together, seaverts give us a good idea of how much damage exposure might cause. For example, if you're exposed to four or five seaverts over a short period of time, it might well kill you. And one seavert, if delivered all at once, will make you pretty sick. But you need to be in some pretty extreme situations to get exposures like these, like being inside a nuclear reactor when there's a major leak. On the other hand, because we live on a radioactive planet, we know that our bodies are pretty good at repairing the damage from low-level exposures. For instance, you're probably exposed to somewhere between one and five thousandths of a seavert every year from the Earth's natural background radiation. A dental x-ray will expose you to something like ten millionths of a seavert. And if you live within 50 miles of a safely functioning nuclear power plant, you can expect to be exposed to a miniscule of 90 billionths of a seavert, which coincidentally is less than if you're living the same distance from a coal-fired power plant. This doesn't mean we can afford to become complacent about radiation, though. And the International Commission on Radiological Protection recommends not exceeding background radiation levels by more than one thousandth of a seavert per year. But at least seaverts help us weigh up the pros and cons of radiation exposure, and they allow us to differentiate between exposures that are probably okay and those that are not.