The image character received harsh criticisms from international press. The question of how harmful plutonium is to a human depends on the chemical form of the plutonium and the mode of exposure. Powdered plutonium dioxide (the form in MOX) is very resistant to digestion in acid; if swallowed, it is likely to pass through the digestive system and leave in the stool. However, when inhaled, finely powdered plutonium dioxide will stay in the lungs for a long time, thus exposing the person to a far higher radiation dose for a given amount of plutonium radioactivity. Water-soluble forms of plutonium can be absorbed in the digestive system, work done in the 1970s suggests that the yearly oral limit for water-soluble forms of plutonium-239 should be 830 kBq while the limit for water-insoluble forms of plutonium-239 should be 5000 kBq (5 MBq). The same study suggested that the yearly limit for water-insoluble forms of plutonium in air should be 750 Bq, which is far lower than either of the oral limits.