This study compares the external hazard posed by radioactive material trapped in the C7 filter canister of the Canadian C4 full-face mask to the internal hazard from the portion of the material that bypasses the mask and is inhaled. Published measured protection factors (PFs) are used to define the ratio of radioisotope concentration outside of the mask to that inside the mask. The hazards for a variety of radioisotopes are quantified using a Monte Carlo model for the external hazard from the contaminated canister and International Commission on Radiological Protection Publication 68 internal dose coefficients for 1 micron internalized particulate material. In general, the external hazard from a contaminated canister exceeds the internal hazard from material that bypasses the filters for only the most highly protective negative-pressure masks and then only for gamma emitting materials. Our model shows that it is highly unlikely that a canister can become contaminated with enough radioactive material to pose an immediate threat to the wearer, even for pessimistic radiological dispersal device scenarios, when the mask is being worn properly. The "as low as reasonably achievable" (ALARA) principle, however, suggests that filters should be changed as frequently as practical, and the dose measured in the filter may be useful for determining dose of record and for forensic investigations.