Yellows
Yellow is defined as: "harmful, active process: molecular, (heat) atomic, chemical". Or. Simply. Yellow is bad for humans.
[15.] Exhaust
Yellow streams "flowing" (down and) outwards (more on that later), from one extremity of the "safe" (not really, in this case), (red) enclosure. Knowing that yellow means "adverse conditions for humans" than it seems a really good depiction of "Exhaust". The best part, though, is how directionality (in this case "outwards") is "hinted" implicitly without actually (directly) expressing it (through an arrow or triangle, as seen in other, previous, designs). The way the 4 flow streams are pictured can only make sense if they are "exiting" (going "outwards"), because they widen as soon as they leave the "constriction" of the container they are leaving. If, for one second, one paused and entertained the possibility that the "flow" is actually going in the other direction, it would certainly fall apart immediately in the eye of what one knows about fluid(s) behavior (and common sense). So. That small detail (almost unnoticeable) makes all the difference.
[17.] Radiation Hazard
Simple elements in this one. The enclosure is unbroken (meaning a fully pressurized area) but is fully filled with yellow (which means it is adverse to human life). To confirm that, one can see the same figure used in sign/symbol #4 (meaning a, lying down, human being) but all in black (which means "death" – or “dead”). A clear, unambiguous, "keep out" message.
[18.] High Radioactivity
This seems to have a similar meaning (in relation to the previous sign/symbol, #17), but there is, as I see it one major difference that justifies the existence of both signs/symbols. In this one, triangles appear (denoting directionality and pointing inwards) at alternating colors: yellow (hazardous to human life), and black (explicitly: "vacuum, death, hazard", all seeming to apply, in this particular case). So until now. One have: "something very bad directed inward". If one adds to this the four "holes” in the continuity of the protective enclosure (in red), than it seems to follow, logically, that the origin of the "bad stuff" is "outside" the spacecraft. And that is the difference: the origin of the "bad stuff" ("bad stuff" is an over-simplification, OC), in this case, is exogenous, while in the previous one (#17) the "origin" is either a) unknown or b) endogenous (the one I favor).
Usage of both, as examples: #17 could/would/should be used in the vicinity of fuel storage; while #18 would/could/should be used near a place with diminished (or nonexistent) shielding from space radiation.
