100 ppb is not the relevant standard, holy shit. 💀 The World Health Organization – the relevant authority on public health here – sets air quality guidelines around pollutants like NO2, wherein "[e]xceedance of the air quality guideline (AQG) levels is associated with important risks to public health." These guidelines set the standard at 10 μg/m^3^, which is essentially 5 ppb. Likewise, they set the 99th percentile amount at about 20 μg/m^3^, or about 10 ppb.

And then we get to the following facts: 1) if they do completely mitigate the hazard as you describe, plenty of people are not adequately using the fan. It's a simple fact that gas and propane stoves are attributable to around 50,000 current cases of pediatric asthma (up to 200,000 by some counts) and thousands of annual deaths. Of course, however, they don't completely mitigate it. 2) As noted in the article, using the built-in fan literally perfectly as intended resulted in a concentration of 15 ppb, which is 1.5x the WHO's 99th percentile limit. 3) The article notes: "our results suggest that manufacturer specified flow rates alone may not provide sufficient information" about their efficacy, meaning that even if you were willing to pay hundreds of dollars to upgrade the fan which 99.99% of people aren't, good luck without a laboratory setup figuring out which ones aren't still well past safe NO2 levels even when run as directed. 4) No amount of NO2 is good for you. 5) NO2 isn't the only pollutant let off by gas stoves; NO2 is thought to be responsible for only a quarter of pediatric asthma cases caused by gas stoves. 6) The article suggests that "in homes with gas ranges levels were generally between 5 and 12 ppb [of NO2]" (2–5 without) which as defined by the WHO is definitely unhealthy.