Radiation levels in the reactor building were estimated to be 5.6 roentgens per second (R/s), or 20,000 R/hr. In modern SI units, this is equivalent to 200 Gray per hour (Gy/hr). Unprotected workers would have received the threshold dose for gastrointestinal radiation sickness of 3 Gy in one minute, and the LD50/30 acute lethal dose of 5 Gy in less than two minutes. (The LD50/30 is the lethal dose for 50% of the population in 30 days). Dosimeters capable of reading such levels were destroyed or unavailable. Conventional biological dosimeters "peg out" at >0.001 R/s, the detector "locks" and ceases to report discrete decay events ("clicks"). That is, actual radiation levels inside the reactor building were under-reported by a factor of > 5,000 x.
The low dosimeter readings led the reactor crew chief Aleksandr Akimov to assume that the reactor was intact. Akimov stayed with his crew in the reactor building until morning, sending members of his crew to try to pump water into the reactor. None of them wore any protective gear. Most, including Akimov, died from radiation exposure within three weeks.