What it is
Fallout is radioactive material lifted into the air by a nuclear detonation that falls back to earth downwind, sometimes many miles away. It emits gamma radiation that passes through ordinary walls. Most of the danger comes in the first hours and days after the event.
How radiation is stopped
Two things protect people from fallout: mass and time. Dense material between you and the fallout absorbs gamma radiation. Each 2.2 inches of concrete or 3.3 inches of packed earth cuts it roughly in half. Ten of those halving layers reduce exposure about a thousandfold, a protection factor of 1,000.
Time does the rest. Fallout decays quickly: by the 7:10 rule of thumb, every sevenfold increase in time after detonation reduces the exposure rate about tenfold. Radiation that is extreme two hours after a detonation is a small fraction of that two weeks later.
What the guidance says
Federal guidance is to get inside, stay inside and stay tuned: shelter in a basement or the interior of a large brick or concrete building, away from the roof and outer walls, for at least 24 hours unless officials say otherwise. Cars and mobile homes do not protect. Potassium iodide protects only the thyroid, and only from radioactive iodine, and should be taken only when public health officials direct it.
How we design for it
A Styrkholm estate puts its quarters below grade under engineered cover: a reinforced concrete roof beneath compacted earth, designed together with the entry so radiation cannot shine straight in. Air comes through a filtered intake that keeps fallout particles out and holds the quarters at positive pressure. The quarters are stocked to stay below grade for weeks if needed, so the family can wait for the radiation to decay instead of guessing when it is safe.
What you can do now
Use the Shelter Configurator to see how overhead cover changes the protection factor, and the Medicine and Nutrition Planner for age-specific potassium iodide doses.
This page is general information to help you plan. It is not engineering, medical or legal advice. Every Styrkholm project is designed by licensed professionals for the specific site and household.