Outdoors, sunlight drives chemical reactions – photoreactions – in atmospheric gases and particles. Some of these chemical compounds, like ozone in the stratosphere, are beneficial. Others, like singlet oxygen, stress our lungs and, long term, are associated with an elevated risk of respiratory and cardiovascular diseases.
Atmospheric chemists historically thought photoreactions didn’t happen indoors because there wasn’t enough sunlight. Over the last decade, that thinking has begun to change and a recent experiment with aerosols from cooking tested for the extent of such reactions indoors.
Singlet oxygen, the first excited state of oxygen, is produced during incomplete combustion. The mechanism behind singlet oxygen’s creation is multi-step. “First you need light,” says UBC chemistry assistant professor Nadine Borduas-Dedekind. Second you need molecules that absorb light. “We call these chromophores,” she says. Finally, the chemical reaction requires oxygen.
As an atmospheric molecule of interest in terms of human health, singlet oxygen is considered a double-edged sword because it contributes to many physiological processes, yet also causes DNA and tissue damage. Circulating in the body, entering via exogenous sources like air pollution as well as produced endogenously as a metabolic by-product of exposure to radiation from the sun, singlet oxygen is a reactive oxygen species that plays a role in our immune system and aging as well as pathologies including inflammation and asthma, plus skin, eye and heart diseases. The presence of singlet oxygen in the atmosphere is poorly understood but its recent detection in wildfire smoke made researchers including Borduas-Dedekind wonder: is it also detectable indoors?
“The question that we had,” she says, was “is indoor light strong enough to generate this molecule?”
To find out, Borduas-Dedekind and a team of five others at UBC and ETH in Zürich, Switzerland, set out to test water-soluble aerosols generated from cooking meals typical of breakfast, lunch and dinner. The student member of their research team was vegetarian so they stuck to vegetarian options, says Borduas-Dedekind, choosing pancakes, pan-fried Brussels sprouts and vegetable stir-fry.
First, they tested for the presence of brown carbon-containing cooking organic aerosols (BrCOA) emitted by the cooked foods, comparing these levels with ambient aerosols in the room. After cooking, the collected aerosol samples were irradiated in one of three ways: in a photoreactor with UVA, under fluorescent lights, or under sunlight in a windowsill, representing a range of indoor light conditions within a home kitchen. Singlet oxygen concentrations were then measured for each light treatment.
BrCOA was detected from all dishes at similar concentrations. Analyses revealed that singlet oxygen was produced from BrCOA under all three light sources tested, with sunlit indoor areas having the highest concentration. This showed that light absorbing molecules from cooking emissions were able to absorb kitchen light. “We were pretty surprised that [even] the fluorescent lights were strong enough to produce it indoors,” says Borduas-Dedekind.
Exposure to industrial cooking emissions has previously been linked to chronic diseases in chefs. Based on the current study, it’s premature to link these diseases to singlet oxygen, says Borduas-Dedekind. “It could be a contributor in this cocktail that they’re breathing from all the cooking emissions,” she says, but “the dots are not fully joined.”
This new evidence that singlet oxygen can be produced indoors, especially in sunlit kitchens, but also in artificially lit ones, suggests that air filtration and venting air to the outside is important. One caveat, notes Borduas-Dedekind, is that ventilation assumes outdoor air is healthy. “During a wildfire smoke exposure event you might not want to open your window,” she says.
Next steps for the team include determining how this oxidant might affect humans and how much is breathed in during cooking. Borduas-Dedekind notes that cooking temperature and method might also play a role.
Elliott Gall, associate professor in mechanical and materials engineering at Portland State University, who was not involved in the study, highlights that this research “adds to a growing literature demonstrating indoor environments are complex – not just because of diverse sources of air pollution, mechanical systems, and human behaviours, but also because of the air and surface chemistry that happens indoors.
“Dr. Borduas-Dedekind and her team have done an impressive job developing a robust and clever approach for indirectly measuring singlet oxygen formed from indoor-relevant lighting and cooking emissions,” he says.
As for singlet oxygen, “Exposure to this molecule is not an acute problem,” Borduas-Dedekind explains. But chronic exposure to these oxidants and cooking aerosols, day in, day out, is something to mitigate. Singlet oxygen is “one more component of the cocktail that can cause respiratory issues,” she notes.
“One thing that was so fun and so unique about this project was that we actually ate the experiment,” says Borduas-Dedekind, who underlines that she wouldn’t want people to fear cooking as a result of their work.