Do cheap USB fume fans do anything?
Our position, and the 'at least it's moving air' counterpoint.
Our position
Amazon is full of $20 products labeled 'solder fume absorber' that are just a 120mm computer fan in a plastic shell, sometimes with a blue foam square that is not activated carbon. These are pure theater. They move smoke horizontally across the bench (toward the next kid's face) without filtering anything. If the product page does not mention 'activated carbon' with a replaceable filter, it is not an extractor.
Other voices
Reputable sources worth reading before you decide. Labels reflect our honest read of each source's general stance, not direct quotes.
EEVblog teardowns
Broadly agreesDave Jones has done teardowns of cheap fume extractors showing the 'filter' is usually just foam. The results are reproducible.
Hakko / Weller real extractor specs
Broadly agreesReal fume extractors from professional brands list activated-carbon filter specs explicitly. The feature is table-stakes for a real extractor.
Some air movement is better than none (conditional)
Nuanced / mixedEven air movement without filtration is better than stagnant air - if the moved air is going out a window or into an extractor hose. A $20 fan blowing air toward a window might be useful; blowing air at the next kid is not.