These effects of radiation exposure are known as ‘early effects’. They only occur if an individual has received a relatively large dose, in a short period of time, which exceeds a certain threshold level. If a dose is received below this threshold then no early effects are observed. The thresholds for these early effects start at around
1 Sv. A dose of 5 Sv is generally considered lethal. Therefore, to put this into context, you would have to receive approximately 400 years worth of natural UK background radiation in a matter of hours to reach these thresholds.