Pollutant
NO₂ is the pollutant most strongly tied to UK roadside air. Indoors, it arrives through infiltration and is generated directly by gas hobs and unflued heaters — and at levels that routinely exceed WHO guidance.
WHO 2021 annual
10 µg/m³
WHO 24-hour
25 µg/m³
Gas-hob peaks
>200 µg/m³ at the cooker
UK limit
40 µg/m³ annual
Outdoor infiltration. Diesel traffic dominates urban NO₂. Buildings on busy corridors typically run 50–70% of outdoor concentrations indoors when windows are closed and higher when opened.
Gas cooking. Unvented gas hobs are the largest single indoor source. Peak concentrations during cooking commonly exceed WHO 24-hour guidance in poorly extracted kitchens.
Unflued heaters and faulty appliances. Rare but acute risk; flagged by CO₂/CO monitoring.
NO₂ exposure is linked to asthma onset and exacerbation, reduced lung function in children, and cardiovascular outcomes. Children with bedrooms above gas kitchens, and adults in roadside flats, sit at the top of the UK exposure distribution.
Use ducted extract over gas hobs at >60 l/s, prefer induction where replacing. Add ePM1 50% (MERV 13) filtration to mechanical systems on roadside elevations. Monitor NO₂ where exposure is plausibly high — sensors are now affordable enough to deploy in homes. IAQ monitoring →