Fact check: Will my child develop asthma because of our gas hob?

Stoves burning gas are as bad for kids as second-hand smoke, headlines said. Yet some experts say we still can’t be sure how big the problem is. Stuff explores the fiery debate.

For years, public health experts have been interested in the health effects of cooking with fossil fuels, including methane (better known as natural gas) and LPG. Homes with gas hobs have higher levels of pollutants.

Yet a research paper, quietly published over Christmas, suddenly sparked international headlines in January. One in every eight young asthmatics developed the condition from the pollution released from gas hobs, the public were warned.

A US product safety commissioner fanned the flames by stating his agency could even ban gas stoves. (As one of five commissioners, he doesn’t have the power to do this alone.)

A gas hob produces more than heat. In the flame, the nitrogen in the air reacts to create trace amounts of the air pollutant nitrogen dioxide. Large studies have established the gas (also produced by car engines and industry) results in cardiovascular and other respiratory diseases.

Gas appliances also release nasties such as formaldehyde and carbon monoxide. The flame also creates a lot of water vapour.

That’s less of a worry if it’s happening in a water heater on the side of your home, because the pollutants will be dispersed. But gas hobs and unflued heaters release these substances into enclosed kitchens and living areas, where they can build up.

Ventilation – such as using a range hood or opening windows – can reduce pollutant levels. But according to three North American studies, families use range hoods less than 40% of the time.

Whether indoor or outdoor, the combustion of fossil gas also releases additional carbon dioxide into the air, driving climate change. Drilling for gas also releases the potent planet-warmer methane into the air, further heating the planet.

Let’s say you gathered all the US children with asthma together in a (large) field. Each wears a coloured shirt, depending on the factor that brought on their asthma: red for the gas hob, yellow for secondhand smoke, green for household damp and mould, blue for cold room temperatures, pink for genetics, for example.

The research, published in the International Journal of Environmental Research and Public Health, asked the question: what proportion of kids are wearing red shirts?

Or put another way: if there wasn’t a single gas hob in the States, how many children would have never developed asthma?

But it’s impossible to gather every asthmatic child, and few would be able to accurately pick the colour they should wear.

READ MORE: * How electric stoves are poised to dethrone the mighty gas range * A culture war has ignited in the US over the future use of gas stoves * Gas stoves bad news for household health and the climate, researchers say

So scientists turn to surveys, asking a sample of families a whole range of things – including the diagnosis of asthma, asthma symptoms and the use of a gas cooker.

Surveys in the US and Europe had already been completed. Many found childhood asthma is associated with the use of gas stoves. Associated means that gas stoves and asthma are linked in some way – occurring together more than normal. It could be because gas pollution causes asthma, but the scientists aren’t ruling out that a third factor might be to blame.

Scientists – including the team that wrote this study – often combine the results of multiple studies in order to eliminate bias (a process known as meta-analysis).

Plugging the asthmatic risk from 10 studies into a formula, the study team had an answer: they’re very confident that somewhere between 6.3% and 19.3% of all asthmatic children are ‘red-shirters’ (gas hobs are to blame for their asthma).

The team also publicised the mid-range figure of 12.7% – or about one in eight young asthmatics. That’s “similar to the childhood asthma burden attributed to secondhand smoke exposure,” the authors said.

A lot of the media coverage focused on the 12.7% figure. But the advantage of the range is it conveys that scientists are still uncertain about the true fraction wearing red shirts. They are confident it is a decent proportion, however.

To complicate matters, many children wear striped shirts, because their condition has multiple factors. For example, an asthmatic girl lives with unflued gas appliances and mouldy rooms, so her top would be red and green.

University of Otago’s Lucy Telfar Barnard said two health risks can compound each other. “They work together to be worse than one thing on its own.”

Surveys can struggle to distinguish a kid with a full red shirt from one wearing stripes.

In turning the surveys into the 12.7%, the researchers may be effectively counting kids with full and partially red shirts.

If another group was counting the green shirts, our green-and-red-striped lass could be counted by both groups.

Scientists could end up accounting for more children than are actually on the field. That’s a known problem when you try to put a number on a public health risk such as indoor air pollution.

It’s another reason experts advise the 12.7% figure should be treated with caution.

Even so, we can be pretty certain there is red on the field. If a typical US child is exposed to the pollution from a gas stove they’re more at risk of asthma.

Better ventilation is a solution, but can only help so much, the researchers warned. “Ventilation is associated with the reduction – but not elimination – of childhood asthma risk,” the research authors said.

In addition, gas hobs have a double health impact. This study focuses on the cause of asthma: on how much disease could have been prevented if gas stoves didn’t exist. But once a person has the condition, nitrogen dioxide can exacerbate the condition, bringing on attacks.

Natural gas extracted in Aotearoa is similar to US gas: it’s predominantly made of methane – a potent greenhouse gas. When raw natural gas and oil are refined, propane and butane are similarly separated, bottled and sold as LPG.

Telfar Barnard said there’s “no doubt” that burning gas indoors increases pollutants and is detrimental to New Zealanders’ respiratory health.

“If your child has asthma, and you have unflued gas appliances, I would absolutely recommend exchanging them for flued or electric alternatives,” she said. “I would certainly encourage people to use their extractor fans.”

It’s highly likely that unflued gas pollutants are contributing to the number of Kiwi asthmatics, Telfar Barnard said.

But if we somehow gathered all the asthmatic kids in Aotearoa in a second field, would we spot exactly 12.7% wearing red shirts?

Probably not, she said.

Gas stoves and asthma risk haven’t been as well-studied in Aotearoa.

One project, surveying families in 2009, didn’t find an association between hobs and asthma.

Yet to more definitively prove that gas affects health, researchers might split households into two groups: one keeps their gas appliance, while the other group gets an electric replacement.

If the tamariki in the second group improve their health relative to the first, we have more compelling evidence that gas causes illness. A large New Zealand study tried this in hundreds of homes – and many respiratory symptoms (though not all) significantly improved.

While there’s insufficient evidence to estimate how much red is on New Zealand’s field, it’s probably a smaller percentage. For one thing, Kiwi families are less likely to use gas.

Just over one-third of US households cook with gas, the study noted.

According to gas industry data, a little less than a quarter of Kiwi homes have a gas connection. (There’s no breakdown for how many use gas for cooking, as some homes will have gas heaters or hot water only.)

At the same time, New Zealand has a crowded field for our population size.

One in seven Kiwi kids has asthma, compared to one in 12 in the US, Telfar Barnard said.

Her best guess is that the tamariki in red shirts are overwhelmed by the comparatively high numbers wearing blue (for cold houses) and green (for damp and mould).

“We have other big problems that the US doesn’t have.”

Gas NZ chief executive Janet Carson – who represents fossil gas extractors and appliance suppliers – was doubtful that the research applied to New Zealand considering the country’s “regulatory regime, quality of appliances and testing regimes”. But asked how regulations and appliance quality was superior to those in the US, Carson did not provide details.

The industry has been “clear and consistent in advice on suitable ventilation”, she said. Stuff requested examples, but these were not provided. “Most people would understand it was advisable to turn on the extractor when they cook.”

The Ministry of Business, Innovation and Employment oversees the safety standards for most consumer products, but gas appliances are regulated by WorkSafe.

Manufacturers must share with officials how much nitrogen dioxide and carbon monoxide is produced by their products. If they don’t meet regulated limits, the appliances won’t be approved for sale.

WorkSafe energy safety manager Mark Wogan said the health research is monitored, but there was no need for additional regulation of gas hobs “at this stage”.

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