Irradiated Wasteland adds a new mechanic to Minecraft: Radiation.
In some new and old biomes, as well as in some blocks and items, you can be affected by radiation, which damages you based on its level. In order to survive the irradiated biomes (as well as the basalt deltas), you will need to either craft a hazmat mask, or another way of cleansing the radiation.
New features include: The Irradiated Desert and Irradiated Forest biomes, the new Charred wood type, the Radiation effect, new radiation themed blocks (and creative tab), as well as new radiation themed mobs