New research by Which? suggests cooking on a gas hob can create more air pollution indoors than levels found on one of the UK’s busiest roads.
In order to see how everyday cooking activities affect levels of pollution in the average home, Which? conducted a snapshot investigation which looked specifically at Nitrogen Dioxide (NO2) and fine particulate matter (PM2.5).
In November 2024, Which? gave air quality monitors to five volunteers – four with gas hobs and one with an induction hob– and asked them to carry out a variety of cooking scenarios in addition to their normal usage over the course of a week. All of them used extractor hoods. They were asked to keep windows and doors shut, aside from one test scenario where they fully ventilated the kitchen.
Which? found that NO2 rose with the use of a gas hob and increased in line with the amount of time gas was used for or the number of gas rings used. In Which?’s slow cooking scenario (using one ring), all participants’ NO2 levels more than doubled.
Once levels of NO2 had spiked, Which? found that they remained elevated for significant periods of time – often several hours – suggesting that people spending their evening in the same room where they have cooked (as with open-plan homes) could be exposed to sustained elevated levels well in excess of the WHO guidelines. Previous studies have highlighted the high levels of NO2 put out by gas hobs – a byproduct of burning natural gas.
Using an induction hob removed the risk of NO2 from this source. The NO2 levels experienced by the volunteer in Which?’s study that used the induction hob were background levels (mainly from outdoor pollution).
Over the period that volunteers were testing, air pollution levels near the Which? office on London’s notoriously busy Marylebone Road, were as follows: average NO2 over an hourly mean for November 2024 was 33µg/ m³; average PM2.5 over a 24-hour mean for November 2024 was 14µg/m³
In Which?’s experiment, nearly all Which?’s volunteers using a hob experienced PM2.5 peaks of over 100µg/m³ on several occasions, and one volunteer experienced a peak PM2.5 of nearly 650µg/m³, (the WHO 24-hour mean guidance limit is 15), when frying Padron peppers; while another experienced nearly 600 when frying peppers and tomatoes. Two volunteers also had peaks of nearly 500 when cooking a fry-up.
Overall, base levels before cooking varied from less than 1µg/m³ to no more than around 20µg/m³. Once they spiked, levels remained elevated for a long time. Which? experts averaged out the PM2.5 over a 24-hour period for the five days each person used the air-quality monitors. One household averaged over the WHO guidance limit for four days. Two other homes exceeded the limit for two days.
For one of the cooking scenarios, Which? also asked people to move their monitors into an adjoining room, or into the living area if their house was open plan. Levels of both NO2 and PM2.5 still reached high peaks and elevated levels for some hours afterwards, showing that air pollution spreads through the home rapidly.
Which? also asked all volunteers to fry food in oil for 10 minutes, but with all windows (and any external doors) open during frying and for 10 minutes afterwards. PM levels rose by far less than in all the other frying scenarios. The average peak reached by the five participants in this case was 27µg/m³, whereas the next lowest average for any of the frying scenarios Which? carried out was 100µg/m³. Crucially, PM levels returned much more quickly to those recorded before cooking with doors or windows open. In three out of five homes, PM levels fell back to pre-cooking levels within 45 minutes.
Which? also found awareness of indoor air pollution is low. In a survey of 2,002 UK adults about air pollution, only a quarter (25%) of those with gas hobs and one in five (18% ) with electric hobs said they were concerned about the possible impact on their health. But around a third (36%) of people were concerned about outdoor air pollution coming in through open windows.
Which?’s findings back up scientific studies on the subject, but there are still significant knowledge gaps around the impact of air pollution on the human body. It is difficult to attribute ill health to specific air pollutants as it is near impossible to isolate them (and their effects) from each other. Humans move constantly between different cocktails of chemicals from different sources – diesel engines, woodsmoke, cooking and cleaning products, to name a few.
At the moment, scientists simply do not know if the effects of indoor and outdoor PM are the same but current WHO guidance limits are set on the basis that there are no safe levels of air pollution.
