What is nitrogen dioxide (NO₂)?
In one line: nitrogen dioxide (NO₂) is a gas from combustion — traffic, gas hobs and boilers — that irritates the airways and worsens asthma. Because it's a gas, a HEPA filter can't remove it; ventilation and, to a degree, activated carbon help, but cutting it at source matters most.
What it is and where it comes from
NO₂ is one of the nitrogen oxides (NOₓ) produced whenever fuel burns at high temperature. Outdoors, the main source is road traffic — especially diesel — so homes near busy roads see higher levels. Indoors, the big source is gas cooking: a gas hob or oven raises kitchen NO₂ sharply while in use. Gas boilers and open flames add to it. Short spikes irritate the eyes, nose and throat, and ongoing exposure is linked to reduced lung function and worse asthma, particularly in children.
Why HEPA can't remove it
NO₂ is an individual gas molecule, far smaller than any particle a HEPA or MERV-13 filter can catch — it passes straight through, the same as other gases and VOCs. So a particle-only purifier does nothing for NO₂ (though it still handles the PM2.5 that combustion also produces).
How to reduce indoor NO₂
- Extract at source when cooking — use a cooker hood vented outside, or open a window, every time you use a gas hob. This is the single biggest lever.
- Ventilate — fresh air dilutes NO₂ quickly.
- Activated carbon — a genuine carbon stage adsorbs some NO₂, a useful supplement but not a substitute for extraction.
- Consider electric/induction cooking — it removes the indoor NO₂ source entirely.
Next step
Near a busy road or cooking on gas? A carbon-plus-HEPA unit covers both the gas and the particles — see options at Best Air Purifier UK and how carbon works in VOCs & activated carbon.
FAQ
Does an air purifier remove NO₂? A HEPA filter doesn't — NO₂ is a gas. Activated carbon adsorbs some of it, but extraction at source (cooker hood, ventilation) does far more.
Is gas-hob NO₂ dangerous? Gas cooking causes sharp indoor NO₂ spikes that irritate airways and can worsen asthma. Always extract or ventilate while cooking on gas.