Three things reduce a dose, and they are the whole of the practical advice: less time near the source, more distance from it, and something solid in between.
Go inside. A building, and the middle of it: an interior room, a basement, away from windows and exterior walls. Brick and concrete are better than wood and glass. Close windows and doors and turn off ventilation and air conditioning.
Stay inside. Sheltering for the first hours is usually far safer than driving through a plume. Do not evacuate on your own initiative unless you are told to, and do not go to collect somebody unless you are told it is safe.
Stay informed. Radio, official channels, and the civil-protection instruction for your area. This is a situation where the official instruction genuinely matters more than anything on this page, because it depends on what was released, how much, and where the wind is.
If you were outside during a release.
- Remove outer clothing before going into the shelter area, and bag it away from people. This removes a large proportion of contamination.
- Wash exposed skin and hair with soap and plenty of water. Do not use conditioner — it binds particles to the hair. Do not scrub hard enough to break the skin.
- Blow your nose, and wipe eyelids and ears.
Iodine tablets. Stable iodine protects only the thyroid and only against radioactive iodine. It is not a shield against radiation, it does not protect any other organ, and it is harmful to some people — those with particular thyroid conditions, and those allergic to it. Take it only when the authorities say to, at the dose they say, at the time they say. Taking it early, late, or repeatedly on your own initiative gives no protection and carries real risk.
Food and water. Sealed and indoor supplies are fine. Follow the instructions about local produce, milk and water afterwards.
Source. World Health Organization, radiation emergencies, https://www.who.int/health-topics/radiation-emergencies . The International Atomic Energy Agency publishes the international standards for emergency preparedness and response; its site refuses automated requests from this server, so it is named here rather than linked as though we had read it today.
What this is not. Reading, not training. Follow the emergency services and the civil-protection authority where you are; where they differ from this page, they govern.