ARE CARPET FLOORS UNHYGIENIC EVEN IF CLEANED REGULARLY?
If you've ever removed padding and carpet that has been in place for a few years, you know that dirt will always be there under the padding, regardless of how frequently you vacuum or wash. Even simply walking barefoot over carpet or padding causes wear and tear, which eventually filters down to the floor. Therefore, the "soil" beneath carpet padding might be the carpet and padding material itself, or dirt from outside the home. One may argue that this particular type of buried dirt is trapped and not airborne, but it's also possible that the same physical forces that shatter carpet fibres also propel it into the air.
Padding and carpet are primarily made of synthetic materials and chemicals, with the exception of very costly carpet. The materials will continue to off gas for many years even if you can't smell it. However, as another poster noted, hospitals have carpet. I've worked in a number of houses where there was a significant plumbing leak. Even though the clean up specialists had arrived right away with the blowers and dryers, carpet that had become wet began to grow mould.
But wouldn't most flooring qualify similarly? If laminate becomes wet, it would be quite difficult to completely dry below. Wood is hardwood, too. Tile? Even grout is porous and retains a staggering quantity of debris in addition to wetness. Unlike carpet cushioning, sheet vinyl doesn't collect as much dirt underneath, although I've observed it there as well after removal.