[Plutonium is] an alpha emitter, so ingested it will kill cells if ingested in high enough concentration.
The 240Pu fraction emits also gammas that have a longer range than alphas. And for cancer, there is no minimum amount nor concentration: the scientific consensus is that the risk decreases linearly with the amount. Inhalation is worse than ingestion.
That is only haft the story. Long time ago i actually majored in nuclear engineering and here is what i remember (note: product usually means integral, since quantities may have dependence on energy):
Damage is the time integral of volume integral of energy integral of the product of the response function and radiation flux.
Here is what it all means:
Radiation has some number density per unit area per second, this is called flux. More scientifically accurate is to say Flux = N*v. Where v is direction vector for velocity and N is number density. Since for our purposes direction does not matter we can integrate over v to get some value for flux F.
F is still a function on energy and varies with energy. A particle (alpha, beta, gamma) has some probability to interact with matter. This probability is called cross section and depends on type of particle, energy of the particle and material that we are considering. Since human body has about fixed composition of hydrogen, oxygen, carbon and other elements we can calculate the average cross section for that composition.
An interaction of a given particle of given energy with human body will cause given amount of damage. So we can define a quantity, which is a function of energy and is the product of damage and cross section. We call this quantity response function. So, now we have flux times response function equal damage. Of course this quantity needs to be integrated over the volume we are considering (lets say volume of the liver) as well as energy spectrum of the radiation flux causing the damage.
Here is a link explaining the same thing in different terms: http://web.utk.edu/~rpevey/NE406/lesson16.htm
The Flux part is what differentiates the damage done by radiation coming from outside of the body vs damage coming from inside the body. Skin can shield from big heavy, high energy types of radiation like alpha particles and it can somewhat shield from other types of radiation. Re result is that damage done to the body by radioactive source located inside the body is much greater than by radiation source located outside the body. The things that do the most damage are also the things that are easiest to shield for, like alpha particles. Meanwhile a weakly interacting type of radiation like neutrons is harder to shield for because it has lower cross section, but it will also do less damage. Not saying that you cant die from neutron radiation, but an alpha particle source located inside the body will do much more damage that the same alpha particle source located outside the body and significantly more damage than neutron or gamma ray source of the same intensity located inside or outside the body.Neutron or gamma ray sources located inside the body will do about the same amount of damage as those same sources (of equal intensity hitting the body) located just outside the body. Equal intensity hitting the body here is necessary to account for the fact that a body will present 4Pi solid body angle to a source located inside of it and about 2PI to a source just outside the body, since half of radiation would fly away from the body.