Nuclear Radiation or Fallout

Time after detonation	Fraction of 1-hour dose rate	Example (1,000 R/hr at 1 hour)
1 hour	100%	1,000 R/hr
2 hours	44%	435 R/hr
4 hours	19%	189 R/hr
7 hours	10%	97 R/hr
12 hours	5%	51 R/hr
24 hours	2.2%	22 R/hr
48 hours	1%	9.6 R/hr
3 days	0.6%	5.9 R/hr
1 week	0.2%	2.1 R/hr
2 weeks	0.1%	0.9 R/hr
1 month	0.04%	0.37 R/hr
3 months	0.01%	0.1 R/hr

• The underlying formula is the Way-Wigner approximation: R(t) = R1 × t^-1.2, where R1 is the dose rate at 1 hour and t is hours since detonation.
• Time is measured from the detonation, not from when fallout arrives. While fallout is still coming down, readings at your location will rise before they start to fall.
• Accuracy: it holds to within about 25% for the first two weeks and within a factor of 2 out to roughly six months. After that, real decay is faster than the formula predicts.
• Nuclear weapon fallout only: it does not apply to reactor accidents or dirty bombs, where long-lived isotopes like cesium-137 dominate and decay is far slower.

Short-Term Whole-Body Radiation Dose (Rad)	Acute Symptoms (Nausea and Vomiting Within 4 Hours) (%)	Acute Death from Radiation Without Medical Treatment (%)	Acute Death from Radiation With Medical Treatment (%)
<50	0	0	0
100	5-30	<5	0
150	40	<5	<5
200	60	5	<5
300	75	30-50	15-30
600	100	95-100	50
1,000	100	100	>90

• For external gamma radiation from fallout: 1 R (roentgen) ≈ 1 rad ≈ 1 rem ≈ 1 cGy = 0.01 Gy.
• Acute effects listed are likely to be reduced by about one-half if radiation exposure occurs over weeks.
• Acute deaths are likely to occur from 30 to 180 days after exposure.