I suspect the allergy/immune system link was hypothesized well after Caves was written. It is not obvious, even now.
The "clean" thing is a theme of the entire work -- the city dwellers, however limited in their bathing and sewage facilities, consider themselves "cleaner" than those who must work "outside" in the yeast swamps and other wild places. That Spacers consider even the city dirty, and that the Spacers' worlds are SO clean that even their "outsides" are cleaner than the city insides, is simply another step on the continuum.
And this isn't unreasonable in an artistic view of our own world. We urban dwellers put up with far more ticks, spiders and skeeters in camping -- outdoors -- than we permit in our houses. One roach and we break out the boron dust ... The more urban, the fewer pests; and the pests that DO slip in are considered even more disgusting.
With regard to population, again, the timing is an issue. The "Green Revolution" of Norman Borlaug had yet to be demonstrated. Simple irrigation, fertilization, and hybridization weren't (a few decades ago) considered sufficient improvement to feed billions. But in practice and in fact bringing developing nations up to a mere 1950's technology in agriculture has produced an awesome improvement, and the notion of "yeast vats" to convert all available solar input to edible caloric output has not proved needful.
That said, the population figure, the yeast vat tech, and the cities themselves suggest a collapse of the climate or ecosystem that is not directly addressed in the books. I suspect a ret-con could be easily worked up if needful.
About two years back I actually used a two-track two-speed "slidewalk" (on rubber belts) at an airport -- exactly as described in Caves, if more limited.
immune system
The "clean" thing is a theme of the entire work -- the city dwellers, however limited in their bathing and sewage facilities, consider themselves "cleaner" than those who must work "outside" in the yeast swamps and other wild places. That Spacers consider even the city dirty, and that the Spacers' worlds are SO clean that even their "outsides" are cleaner than the city insides, is simply another step on the continuum.
And this isn't unreasonable in an artistic view of our own world. We urban dwellers put up with far more ticks, spiders and skeeters in camping -- outdoors -- than we permit in our houses. One roach and we break out the boron dust ... The more urban, the fewer pests; and the pests that DO slip in are considered even more disgusting.
With regard to population, again, the timing is an issue. The "Green Revolution" of Norman Borlaug had yet to be demonstrated. Simple irrigation, fertilization, and hybridization weren't (a few decades ago) considered sufficient improvement to feed billions. But in practice and in fact bringing developing nations up to a mere 1950's technology in agriculture has produced an awesome improvement, and the notion of "yeast vats" to convert all available solar input to edible caloric output has not proved needful.
That said, the population figure, the yeast vat tech, and the cities themselves suggest a collapse of the climate or ecosystem that is not directly addressed in the books. I suspect a ret-con could be easily worked up if needful.
About two years back I actually used a two-track two-speed "slidewalk" (on rubber belts) at an airport -- exactly as described in Caves, if more limited.