More pollution from cooking on a gas hob than from one of Britain’s busiest roads, Which? finds
In order to see how everyday cooking activities affect levels of pollution in the average home, the consumer champion conducted a snapshot investigation which looked specifically at Nitrogen Dioxide (NO2) and fine particulate matter (PM2.5) - two of the most damaging air pollutants found both indoors and outdoors.
NO2 is a significant contributor to children developing asthma, as well as exacerbating other respiratory conditions in both children and adults. Meanwhile, PM is everything in the air that is not a gas. Some of these small particles are known to be toxic. PM2.5 can be particularly damaging as the tiny particles travel deep into lungs and enter the bloodstream to reach all organs of the body. Here, they can induce changes that promote many different diseases, such as Parkinson’s disease, diabetes, lung cancer, COPD and heart disease, as well as impacts on reproductive and foetal health.
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.
Emily Seymour, Which? Energy and Sustainability Editor, said:
“It’s shocking to think that something as routine as cooking dinner could be releasing harmful pollutants into our own homes, but our snapshot research shows that once it spikes, air pollution can spread rapidly through the house and linger for long periods of time.
“We still don’t know the full impact of indoor pollutants on our health but if it is something you are concerned about, ventilating by using your extractor hood and opening windows can make a big difference. If you are using a gas hob you could consider switching to induction when you next need to replace your hob.”
-ENDS-
Notes to editors:
Research:
In order to see how everyday cooking activities affect levels of NO2 and PM2.5 in the home Which? gave air quality monitors to five volunteers – four with gas hobs and one with induction – 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. The study was conducted in November 2024.
Marylebone Road levels on NO2 and PM5.2 were gathered from Air Quality in England
While Which? used air quality monitors that are normally used in scientific observational studies, these were not scientifically calibrated or recalibrated each time they entered a new home. There was some variation in readings between the two monitors used, particularly with NO2 readings. One of the monitors consistently recorded higher starting base levels.
The NO2 levels in the graph when slow-cooking shows how they changed compared to the base level in each property.
Which? found that one volunteer who used a gas hob had less variation than the others during specific scenarios in Which?’s study suggesting that personal cooking methods may also be a factor to how much air pollution is created.
Which? surveyed 2,002 UK adults between 29th November and 2nd of December 2024. Fieldwork was carried out online by Deltapoll and data has been weighted to be representative of the UK population (aged 18+)
Useful links
Advice to consumers:
Extractor hood
Use your extractor hood every time you cook and cook on back burners where the hood will be more effective. Extractors are much more effective if they’re vented outside, instead of recirculating the air. Higher fan settings will work best. Clean your extractor hood often (some mesh panels can be put in the dishwasher – check the instruction manual) and replace the filters every six months.
Windows
Monica Mateo-Garcia of Birmingham City University, co-founder of the Centre for Future Homes, says the best practice is Stoßlüften, ‘shock ventilation’. This involves fully opening all windows and internal doors in a home for a few minutes on a dry day, for a quick exchange of air and to remove indoor pollutants. This is a more energy-efficient way of ventilating than keeping some windows a little bit open all the time, as less heat is lost.
More ventilation options
Shock ventilation might not be such a good option if, for example, your house is next to a busy road with high traffic pollution. HEPA air purifiers can do a good job of removing PM from the air, as they can trap fine particles. They won’t help for NO2 as it’s a gas. Mechanical air ventilation systems are another possible and effective option. UV air purifiers aren’t recommended, as they can produce dangerous levels of ozone (O3) if misused.
Upgrade to induction
Swapping from gas to an induction hob will remove much of the NO2 from your home environment and could reduce your exposure to PM. It also reduces your greenhouse gas emissions. Your existing gas supply will need to be capped off by a Gas Safe engineer and you’ll need to ensure your current electrical connection matches your new hob’s requirements.
Background
Professor Sir Chris Whitty, England’s Chief Medical Officer made air pollution the subject of his 2022 annual report. This linked long term exposure to outdoor air pollution to between 26,000 and 38,000 deaths a year, and attributed many people’s avoidable ill health to it. The public health burden if you include indoor air pollution is likely to be significantly higher.
Gas hobs have had a lot of negative attention in the press and recent studies have highlighted the high levels of Nitrogen Dioxide (NO2) that they put out – a byproduct of burning natural gas. A 2023 study by CLASP, a global non-profit campaigning for efficient appliances, carried out over seven countries found that over half of UK households that used gas exceeded WHO daily guideline value and 25% exceeded the UK/EU hourly limits.
All cooking, whether using a gas or electric hob or, indeed other appliances such as ovens and toasters, can give off pollutants from volatile organic compounds (VOCs) to particulate matter. The act of heating food creates some of these pollutants - frying generally causes the highest levels and boiling or steaming the least. The INGENIOUS project - led by Professor Nicola Carslaw at the University of York, measured air pollution, including PM2.5, in 310 Bradford homes and found peak concentrations of PM2.5 in homes exceeding 500µg/m³ when cooking food.
Some studies have found that levels of VOCs and Particulate Matter are increased by using gas hobs as these are also increased by the combustion process. Gas stoves can also be a source of benzene exposure which is a known carcinogen.
Professor Frank Kelly of Imperial College London and a former Chair of the government Committee on the Medical Effects of Air pollution (COMEAP) will shortly be publishing the results of a major air quality study called WellHome looking at the impact of indoor air pollution on over one hundred homes half of which had an asthmatic child. He says, ‘Gas hobs and ovens are a major source of indoor air pollution, including NO2, which can both exacerbate existing health conditions and potentially lead to new respiratory illnesses’.
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