Short answers, pulled from the story.
Ionizing radiation consists of subatomic particles or electromagnetic waves with enough energy per particle or photon to strip electrons from atoms and molecules. This breaks chemical bonds, creates reactive free radicals, and damages DNA in living cells, which can lead to radiation burns, radiation sickness at high doses, and cancer after lower cumulative exposure.
The main types are alpha particles (helium-4 nuclei), beta particles (high-energy electrons or positrons), neutrons, and photon radiation (gamma rays and X-rays). Alpha particles have the lowest penetrating power and are stopped by a few centimeters of air or the top layer of skin; gamma rays and neutrons are the most penetrating.
The global average exposure to ionizing radiation is about 3 millisieverts per year, with 80% coming from natural sources such as cosmic radiation, terrestrial radiation, and radon. The remaining 20% comes mainly from medical imaging procedures.
The highest level of purely natural background radiation recorded on Earth's surface is 90 microgray per hour on a Brazilian black beach made of monazite. The most exposed inhabited area is Ramsar, Iran, where naturally radioactive limestone used as a building material exposes roughly 2,000 residents to an average of 10 milligray per year.
Radon-222 is a gas produced by the alpha decay of radium-226, part of the natural uranium decay chain. Because of its high density, radon seeps from bedrock and accumulates in poorly ventilated buildings. It is the leading cause of lung cancer among non-smokers and the second-leading cause overall.
Airline flight crew consistently receive more average radiation dose than any other worker, including those in nuclear power plants, according to the United Nations UNSCEAR 2000 Report. Crew flying polar routes at high altitudes receive the highest doses; one measured polar flight, London Heathrow to Tokyo Narita, recorded 6 microsieverts per hour.