According to Weapons of Mass Destruction and Terrorism by James Forest and Russel Howards, what factors influence the biological impacts of a radiological attack, once the material has been dispersed? Why are they important?

According to the book, there are several factors that can influence the biological impacts of a radiological attack, once the material has been dispersed.
First, the radioactive source is important. Officials contend that there are 3 types of mechanisms used to perpetrate radiological attacks. They are radiological incendiary devices (RID), radiological dispersal devices (RDD), and radiation emission devices (RED).
Terrorists use one of the three dispersal methods for various reasons. For example, some terrorists use RIDs because the devices couple fire with radioactive material. Use of such a device can complicate rescue efforts by law enforcement and fire-fighting personnel. In the case of an RID attack, officials have to contend with public panic regarding fire and radioactive material.
The second factor that can influence the biological impacts of a radiological attack is the variety of human exposure pathways for the dispersed material. These pathways are inhalation, ingestion, external exposure, and immersion.
External exposure is the most common pathway and associated with gamma emitters like RED devices. Meanwhile, inhalation requires that the terrorist to convert the radioactive source into an aerosol form. Typically, the terrorist must disperse such a radioactive source through ventilation systems. According to the book, inhalation is the most damaging exposure pathway, especially in the case of alpha emitters. Meanwhile, the immersion method (where people are covered with contaminated water) can lead to grievous external and internal bodily injuries.
So, the factors that influence the biological impact of a radiological attack are the dispersal methods, the kind of radiation emitted by the isotope used (gamma, alpha, or beta particles), and the chemical forms of the isotopes themselves. These factors are important because they determine the scope of the catastrophe and the methods that must be used to contain the ramifications of the attack.