During the first hour after a nuclear explosion, radioactivity levels drop precipitously. Radioactivity levels are further reduced by about 90% after another 7 hours and by about 99% after 2 days. An accurate rule of thumb, applicable in the time-period of days to a few weeks post-detonation which approximates the radioactive dose rate generated by the decay of the myriad of isotopes present in nuclear fallout, is the "7/10 rule". The rule states that for each 7-fold increase in time the dose rate drops by a factor of 10. For example, assuming the fallout process has ended 24 hours post detonation and the dose rate would be lethal if a few hours of exposure occurred, 50 roentgens per hour, then 7 days after detonation the dose rate will be 5 R/hr and 49 days after detonation (7×7 days) the dose rate will be 0.5 R/hr at which point no special precautions would need to be taken and venturing outside into that dose rate for an hour or two would pose a close to negligible health hazard, thus permitting an evacuation to be done with acceptable safety to a known contamination free zone. Following a surface-burst nuclear detonation, approximately 80 percent of the fallout would be deposited on the ground during the first 24 hours.