Short answers, pulled from the story.
The average human receives 3 millisieverts of exposure annually, with 80 percent of that dose coming from nature itself. This natural phenomenon includes cosmic rays originating from distant supernova explosions and the heart of the Sun. These particles bombard the atmosphere constantly, creating a cascade of secondary particles that reach the ground.
Alpha particles consist of two protons and two neutrons bound together into a helium-4 nucleus and can be stopped by a single sheet of paper. Beta particles are high-speed electrons or positrons that travel further than alpha particles but are still halted by an aluminum plate. When beta particles pass through matter, they can produce X-rays known as bremsstrahlung, which complicates the process of shielding them effectively.
The linear no-threshold model holds that the risk increases linearly with effective radiation dose at a rate of 5.5 percent per sievert. This model suggests that natural background radiation is the most hazardous source of radiation to general public health, followed closely by medical imaging. The mechanism involves the mutation of somatic cells, leading to stochastic effects that may not appear for years or decades after exposure.
The highest background radiation in an inhabited area has been recorded in the city of Ramsar, Iran, where some residents receive an average radiation dose of 10 millisieverts per year. This effective radiation dose due to external radiation was 135 millisieverts per year and the committed dose from radon was 640 millisieverts per year. The residents of Ramsar show no compelling evidence of experiencing a greater health risk despite these levels being over 200 times higher than the world average background radiation.
Airline flight crew workers receive more cosmic rays on average than any other worker because the Earth's magnetic field offers less protection at high altitudes and near the poles. A measured dose of 6 microsieverts per hour has been recorded on a polar route from London Heathrow to Tokyo Narita. The United States Federal Aviation Administration requires airlines to provide flight crew with information about cosmic radiation and has a recommended limit of 1 millisievert total for a pregnancy.