Consumer Product Safety Commission is Concerned About Gas Stoves. Are Candles, Pets Next?

“Kill Your Gas Stove. It’s bad for you, and the environment. If you can afford to avoid it, you probably should." 

 The Atlantic

There are three issues here:

• Is gas cooking inherently unhealthy? Relevant studies go back about 50 years, with mixed findings.
• Which environment are we talking about, indoors or out? EPA has studiously ignored the indoors, where we spend 85% of our time.
• Controlling indoor air quality (IAQ) is now the overriding public health issue, but is controlled by individuals rather than institutions.

In researching these issues I drew on selections from the published literature comprising 56 peer-reviewed articles on health effects and indoor exposures, published from 1977 to 2022 and including the study that prompted the CPSC statement [1] as well as  previous ACSH articles. The NHS database [2] contains a variety of epidemiologic studies of “gas cooking” or “gas stoves” and air pollution.

• Comparisons of populations: The fractional population at risk is the product of the fraction of gas cookers and those with health problems. This design cannot account for more individual causes such as second-hand smoke or pet allergies. While it may be precise due to large populations, it cannot be accurate because the larger the population, the less individual information may be available.
• Cohort studies: Individuals are recruited and their exposures are compared with individual health status during a specific time period. This design allows potential confounding factors to be considered but cannot distinguish chronic health conditions from acute episodes. These studies may be more accurate but are less precise.
• Time-dependent studies:  Changes in health status are tracked as functions of exposure time while controlling for collinear factors.

• Intervention studies: Exposures are manipulated during the study and changes in health are noted. This design may be less relevant since chronic conditions cannot be evaluated.
• Occupational studies: Long-term health of domestic or professional cooks is evaluated by duration of exposure. Such studies may not be applicable to the general public.

The primary air pollutants of interest here are nitric oxide (NO), which is oxidized in the atmosphere to form NO₂, fine particles (PM_2.5), and the much finer particles (nanoparticles) that comprise smoke. Ambient concentration limits have been promulgated for outdoor NO₂ and PM_2.5, but no air pollutants are regulated indoors. Other potentially harmful indoor particles include second-hand smoke (SHS), candles, household dust and allergens, and pet dander. Exposure to combustion-generated species is inferred by the presence/absence of emission sources like gas stoves; such implications provide no information on specific personal exposure concentrations or duration. Determining associations with acute health responses like asthma attacks requires continuous monitoring of personal exposures that are unlikely to be available in numbers sufficient for appropriate statistical analysis. In summary, adequate indoor air pollution exposure data are sparse.

Findings on Gas Stoves

Early studies include a 1973 survey of 5758 children in the UK[3] where gas cooking was accompanied by cough, wheeze, and bronchitis, after controlling for outdoor air quality, age, social class, overcrowding, etc. Smoking was assumed to be at least partly controlled by social class. The Harvard Six City Study[4] measured personal exposures in 2 small Tennessee communities and noted that indoor PM exposures were more variable than outdoor, in large part due to differences in smoking habits.

Relevant studies published in 2022 include:

• National surveys were used to estimate the fraction of US asthmatics in households with gas stoves, but only 9 states had sufficient data.[5] This study was used to support the CPSC statement, but was poorly designed since no specific confounding variables were considered.

• A cohort of 5199 Danish adults [6] was characterized by long-term outdoor levels of NO₂ and PM. Lung function was measured and was inversely associated with outdoor NO₂ and “gas cookers without hood”, as were inflammatory respiratory markers.
• Numbers of asthma symptom-days after indoor air intervention by air cleaners and NO₂ scrubbing were studied in 126 New England asthmatic child
• A systematic review of 69 indoor air pollution studies from 1991-2021 [8] concluded that lung function, childhood asthma, and symptoms of chronic obstructive pulmonary disease were associated with PM, but NO₂ was not considered.

I conclude that evidence linking childhood asthma with gas stoves or indoor NO₂ is weak, but there may be other risks associated with indoor air pollution. Stronger evidence is provided by the experiences of professional cooks [9] including the female residents of a household and by intervention studies. An Australian intervention study[10] tested the NO₂ hypothesis directly by installing flues in 8 primary schools, after which NO2 decreased from 47 to 15 ppb. This 3-fold reduction was accompanied by 2.5-fold reductions in respiratory symptoms including asthma attacks. A US study[11] modified homes with unvented gas stoves and measured changes in indoor NO₂ levels. Switching from gas to electric cooking reduced kitchen and bedroom NO₂ in half, but installing a ventilation hood had little effect. Installing an air purifier also reduced NO₂ levels. Unfortunately, corresponding health data were not collected.

Alternatives To Gas Cooking.

The real question is not whether an individual can afford it, but whether the environment can “afford” it.   About 35% of US households cook with gas and would have to rely on electricity in its absence. Currently, an electric-powered kitchen requires about 1000 kilowatt-hr (Kwh) per year; supplying those 44 million kitchens at that rate would require about 13 new power plants, at least some of which would be using fossil fuels. Note that the energy required for gas cooking is supplied instantly with no warm-up at essentially 100% efficiency, while the efficiency of electric cooking is limited by the power plant that supplied it, resulting in additional pollutant emissions, some of which may affect climate change. Outdoor CO₂ can have direct effects while CO₂ deposits on indoor surfaces and thus does not.

At a recent Oregon county hearing [14, 15], the local supplier of natural gas presented a statement on gas stoves with which I agree; the media seem to have ignored it.

• Multiple studies demonstrate that proper ventilation when cooking is key to mitigating cooking related emissions, whether you are cooking with a gas or electric stove, because cooking activities themselves (e.g., grilling, frying, broiling, baking) are a source of indoor air emissions

The importance Of Indoor Air Quality.

Outdoor emissions have been regulated by institutions to limit adverse health effects in the general population, while indoor air pollution is solely in the hands of individuals. Outdoor air is now so clean in most of the US that IAQ is the dominant exposure medium. Ventilation by outdoor air is their nexus and the domain of residents and architects. This is a fundamental distinction; we can blame outdoor smog on “them” (i.e., others), while each of us is responsible for own indoor environment and protecting sensitive individuals. Banning gas stoves will eventually impact everyone while neglecting the myriad other sources of airborne toxics and creating a false sense of security.

A new comprehensive indoor air pollution research program is clearly needed. EPA recently announced a grant program on “Cooking and Reducing Household Energy Emissions” totaling all of $3 million, intended to “facilitate the identification, promotion, and implementation of effective approaches and government policy actions”. I would respond that it is high time to bring engineering into the kitchen, instead. The success of the environmental movement has been in reducing impacts while retaining function; catalytic converters were introduced to remove the toxic compounds in vehicle exhaust, air pollutant “scrubbers” have been mandated for large smokestacks, and deadly toxins (lead, mercury) have been banished outright.

Get rid of the air fresheners and any smoky appliances, turn on the kitchen exhaust fan, banish the smokers, and keep the severe asthmatics entertained elsewhere!

The gas stoves, pets, and candles can stay.

[1] Statement of Chair Alexander Hoehn-Saric Regarding Gas Stoves

[2] NIH Database Search Term Gas Stoves

[3] Association between gas cooking and respiratory disease in children. Br Med J. DOI: 10.1136/bmj.2.6080.149.

[4] Longitudinal and cross-sectional estimates of pulmonary function decline in never-smoking adults. Am J Epidemiol. DOI: 10.1093/oxfordjournals.aje.a115710.

[5] Population Attributable Fraction of Gas Stoves and Childhood Asthma in the United States

[6] Inflammatory markers and lung function in relation to indoor and ambient air pollution. Int J Hyg Environ Health. DOI: 10.1016/j.ijheh.2022.113944.

[7] Childhood asthma and household exposures to nitrogen dioxide and fine particles: a triple-crossover randomized intervention trial. J Asthma DOI: 10.1080/02770903.2022.2093219.

[8] Indoor Air Pollution and the Health of Vulnerable Groups: A Systematic Review Focused on Particulate Matter (PM), Volatile Organic Compounds (VOCs) and Their Effects on Children and People with Pre-Existing Lung Disease. Int J Environ Res Public Health. DOI: 10.3390/ijerph19148752.  

[9] Indoor NO2 air pollution and lung function of professional cooks. Braz J Med Biol Res. DOI:10.1590/s0100-879x2007000400011. 

[10] Respiratory health effects of exposure to low-NOx unflued gas heaters in the classroom: a double-blind, cluster-randomized, crossover study. Environ Health Perspect. 2010 Oct;118(10):1476-82. DOI: 10.1289/ehp.1002186.

[11] Lung function and incidence of chronic obstructive pulmonary disease after improved cooking fuels and kitchen ventilation: a 9-year prospective cohort study. PLoS Med. DOI: 10.1371/journal.pmed.1001621.

[12] Randomized controlled trial of unflued gas heater replacement on respiratory health of asthmatic schoolchildren. Int J Epidemiol. DOI:10.1093/ije/dyh018. 

[13] Home interventions are effective at decreasing indoor nitrogen dioxide concentrations. Indoor Air. DOI: 10.1111/ina.12085. 

[14] Oregon Municipalities Push Anti-Natural Gas Agenda Based On Flawed Research

[15] Board Commissioners briefed on the Public Health review of health risks posed by gas stoves