 Radiation with enough energy to cause changes at the atomic level is called ionizing radiation. It can damage cells, so it's important to measure its potential health effects. But how? Units measuring ionizing radiation are commonly named after leading physicists in the field. The unit for radioactivity, for example, was initially named after Marie and Pierre Curie, pioneers of radiology. Scientists now use becquerel, gray, and sievert as standard units to quantify radiation and estimate its impact on people. Let's take an example. Suppose Alex is standing close to a radiation source. The source emits radiation through decay or loss of energy of its unstable atoms. This radioactivity is measured in a unit called becquerel. Becquerel describes the number of atom decays over time. When Alex stands too close to a source of ionizing radiation, his body absorbs a dose of energy. This dose is measured in grays. Grays describe the energy Alex's body has absorbed, divided by his weight. But grays don't give a full picture of the effects of radiation. Some types do more damage than others. Certain body tissues are more or less sensitive to ionizing radiation. To calculate potential health effects, we use a unit called sievert. Sieverts take into account the type of radiation the tissue is exposed and how much energy they could have absorbed. The bigger the sievert, the higher the potential health risk for Alex. Since one sievert is a large quantity, radiation doses are usually expressed in fractions of sieverts. A person's exposure due to all natural sources of radiation, for example, is on average around 2.4 millisieverts a year.