Humans are most comfortable when the air they are breathing is substantially cooler than the radiant temperature of their environment. If both are at 19–24°, humans can be reasonably comfortable, but it’s better to have the radiant temperature at some 40° and the air temperature closer to 20°.
This, of course, implies some kind of continuous energy input — supplied by the sun in our natural environment, whose "solar constant" is 1000 W/m². If the radiant temperature is 40°, without any cooling, things will gradually heat up to 40°, and humans will feel like shit. The air inside a building itself doesn’t absorb a substantial amount of radiant energy at thermal IR wavelengths, but surfaces will — even with low-emissivity coatings, they will absorb at least 10% of the incident radiation, which is the radiation they would be emitting at 40° — that’s 545 W/m² for a black body, 54.5 W/m² for one whose emissivity at the relevant wavelengths is 0.1. And the 1000 W/m² of the solar constant works out to a radiant temperature of some 91°.
54 W/m² is a relatively reasonable amount of heat to remove through airflow and air conditioning, especially compared to the 1000 W/m² supplied by the sun shining through an open window. What does it look like to supply it through radiant heaters?
Note that you have to have the whole 545 W/m² incident on your body for the radiant temperature to be 40°. But only some of it has to come directly from the radiant heaters; other parts can be reflected, or even emitted, by other materials in your environment. So let’s consider what it takes to reach 150 W/m² on the floor directly from ceiling-mounted radiant heaters radiating down.
High-emissivity heating elements that can reach 900° without damage are commonplace. Higher-temperature materials, like the Kanthals, are somewhat expensive. 900° is 107 kW/m², while 1400° (about as high as any Kanthal goes) is 440 kW/m². So each cm² of 900° heater radiator can cover 720 cm² of floor with 150 W/m². To cover a whole 40m² efficiency apartment, you need 560 cm² of 900° heating elements to emit the requisite 6 kW (requiring 25 A at 240 V to produce it through Joule heating).
If you instead use coolant from a phase-change reservoir at, say, 500° (not sure what material melts at 500°, but I’m sure there’s something), it only emits 20 kW/m², so you need 3000 cm² (0.3 m²) of emitting panels.