The time factor means that the more time an individual remains in a radiation field, the more exposure that individual will receive.

The figure shows the effect of time spent in a field of 100 mR/hr. If you remain in a 100 mR/hr field of radiation for one hour, you will be exposed to 100 mR. If you remain in the same 100 mR/hr field for three hours, you will be exposed to 300 mR (3 x 100), and with nine hours of exposure, you will receive a 900 mR dose.