What type of radiation is only a hazard when inhaled or ingested?

Although there are many forms of "radiation" when assessing radiation hazards we usually think in terms of just three forms of radiation: alpha, beta, and gamma. Of these, alpha radiation is only a hazard if a source is inhaled or ingested - the other two are a hazard even without inhalation or ingestion.

In general, external alpha radiation is not harmful since alpha particles are effectively shielded by a few centimeters of air, a piece of paper, or the thin layer of dead skin cells. On the other hand, if you inhale or ingest an alpha emitter, it can cause significant damage if the radiation is intense enough.

Beta particles (electrons) are more penetrating, but still can be absorbed by a few millimeters of aluminum. Whatever harm beta radiation can cause, it doesn't have to be inhaled or ingested to do it.

Gamma radiation is the most penetrating - that's the kind of radiation we usually think of using lead to shield us from. Again, gamma radiation is a hazard whether or not you inhale or ingest it.