Using the 7-10 Rule to Plan Nuclear Fallout Shelter Time

For every 7-fold increase in time after detonation, radiation intensity decreases 10-fold. At 7 hours = 1/10 the radiation. At 49 hours (7×7) = 1/100 the radiation. At 343 hours (~14 days, 7×7×7) = 1/1000 the radiation. This decay continues predictably regardless of initial fallout density. The critical insight: sheltering through the first 7 hours prevents the highest accumulated lifetime dose. Each additional 7-hour period multiplies protection by 10.

Within 10 miles of ground zero: shelter minimum 24 hours before brief exits. Within 10-50 miles with visible heavy fallout: shelter 12-18 hours. Beyond 50 miles with light/no fallout: shelter 6-12 hours. After 7 hours, radiation drops enough for 5-15 minute necessary exits (retrieving water, assessing shelter, signaling). After 49 hours, 30-minute outdoor tasks become survivable. The first 7 hours present maximum dose risk—plan shelter provisions accordingly.

Sheltering reduces effective dose to 1/10 or less (basement better than first floor). Evacuation during or immediately after fallout exposes you to active radioactive contamination. Evacuate only if: shelter fails structurally, medical emergency requires outside aid, or confirmed safe zone exists within walking distance with verified fallout reports. A basement or interior room for 24-48 hours delivers far less lifetime dose than 2-3 hours of evacuation travel.

Before 7 hours: exit only for critical emergencies (trapped person, structural collapse). After 7 hours: brief necessary tasks (15 minutes) are survivable. After 49 hours: 30-minute outdoor tasks acceptable. Minimize exposed skin, cover nose/mouth with cloth, decontaminate upon re-entry (shower, change clothes). Track cumulative outside time—each exit adds dose. Prioritize tasks: locate water/food, assess shelter integrity, signal for help. Conserve shelter supplies for 7-14 days of potential occupation.

Without instruments or broadcasts: visible heavy ash/dust near you in first 2 hours = close proximity = shelter 48-72 hours minimum. Light or no visible fallout = probable distance = 24-48 hours sufficient. If battery radio accessible: follow emergency broadcast guidance (broadcasters use 7-10 rule). If isolated: shelter conservatively for 5-7 days before outdoor survival tasks become necessary. Children, elderly, and injured require extended shelter—their dose limit is lower. After 7-14 days, radiation reaches levels survivable for extended outdoor exposure (foraging, water retrieval).