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.

It doesn't at all say that; it notes that you should be using an electric stovetop but that vents can reduce the exposure.

If I already have a gas stove, what steps can I take to reduce the risks?

If you have the means, you can replace your gas stove with an electric one. The Inflation Reduction Act provides rebates of up to $840 for purchasing new electric appliances, including stoves and cooktops. (Eligibility varies by state and income level.)

But if you can’t afford to buy a new stove or if you rent an apartment and can’t change the appliances, experts note that there are still things you can do to reduce your exposure risk.

If your stove has an overhead vent, you should use it every time you cook—and ideally it should vent to the outside. “You should always turn that exhaust fan on anytime that you’re using your stove, no matter if you’re just boiling water,” says Eric Lebel, a senior scientist at PSE Healthy Energy, a nonprofit research and policy institute in Oakland, Calif. “Even if what you’re cooking doesn’t smell, if that flame is on, you should have the exhaust on to help reduce the concentrations of the off-gassing, or those [nitrogen oxide] by-products, in the kitchen.”

Unfortunately, many people don’t use their vents. They work best when they’re running at full blast, which can be pretty loud, and the filter should be changed about every three months, Kephart says. And some overhead vents merely recirculate the air back into a room. If you don’t have a “ducted hood” that vents to the outside, you can open a window and run a fan to increase ventilation, Lebel says. Portable air purifiers may also help, Kephart says, although they don’t completely remove the NO2.