With the Rad Absorption perk, your radiation level slowly decreases on its own over time.”
With this perk, your radiation level will decrease 1 rad every 20 real-world seconds so long as the player is not currently being exposed to radiation (standing in radioactive water, for instance) - the perk will still be reducing the radiation but the new accumulation of rads will more than overpower the absorption effect. Waiting one in-game hour will remove 6 rads. If you had 999 rads, you would have to wait 167 in-game hours to reduce your rad count to zero. The rate applies to fast travel, sleeping and using the wait function.
Fallout: New VegasEdit
This perk remains functionally identical to the one in Fallout 3. However, Irradiated Beauty exists as a possible alternative.
This perk is useful in preventing the "death from a thousand paper cuts" rad build up, which combined with a high radiation resistance can reduce incidences of serious radiation poisoning to few and far between. It is especially useful in hardcore mode as it effectively eliminates the slow but steady buildup of rads from most food and water.