When radiation of high enough energy strikes another atom, it strips away an electron. The resulting positively charged atom is called an ion, which explains why high energy radiation is called ionizing radiation. The release of the electron produces 33 electron volts (eV) of energy, which heats the surrounding tissues and disrupts certain chemical bonds. Extremely high-energy radiation can even destroy the nuclei of atoms, releasing even more energy and causing more damage. Radiation sickness is the cumulative effect of all this damage on a human body that's been bombarded with radiation.
Ionizing radiation comes in three flavors: alpha particles, beta particles and gamma rays. Alpha particles are the least dangerous in terms of external exposure. Each particle contains a pair of neutrons and a pair of protons. They don't penetrate very deeply into the skin, if at all -- in fact, clothing can stop alpha particles. Unfortunately, alpha particles can be inhaled or ingested, usually in the form of radon gas. Once ingested, alpha particles can be very dangerous. However, even then they don't typically cause radiation sickness -- instead, they lead to lung cancer [source: EPA].
Beta particles are electrons that move very quickly -- that is, with a lot of energy. Beta particles travel several feet when emitted from a radioactive source, but they're blocked by most solid objects. A beta particle is about 8,000 times smaller than an alpha particle -- and that's what makes them more dangerous. Their small size allows them to penetrate clothing and skin. External exposure can cause burns and tissue damage, along with other symptoms of radiation sickness. If radioactive material enters food or water supplies or is dispersed into the air, people can inhale or ingest beta particle emitters unknowingly. Internal exposure to beta particles causes much more severe symptoms than external exposure.
Gamma rays are the most dangerous form of ionizing radiation. These extremely high energy photons can travel through most forms of matter because they have no mass. It takes several inches of lead -- or several feet of concrete -- to effectively block gamma rays. If you're exposed to gamma rays, they pass through your entire body, affecting all of your tissues from your skin to the marrow of your bones. This causes widespread, systemic damage.
How much radiation does it take to cause radiation sickness, and what effect does this damage have on a human body? That's next. For more detailed information on different types of radiation and where they come from, take a look at How Radiation Works.