In indoor air, we are dealing with a chemical stew of immense complexity with essentially unknown health consequences.
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Molecules are small. Very small. Very, very, very small. So small that with every breath you inhale several sextillion of them. That’s a 1 followed by 21 zeros! About 99% of these are molecules of oxygen and nitrogen, but that leaves plenty of room for others such as aromas from the kitchen and scents from personal-care products, cleaning agents, pine trees and flowers. Then there are numerous compounds with no odour at all.
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Let’s consider something as mundane as baking bread. The delightful smell generated is composed of dozens of compounds, 2-acetyl-1-pyrroline, (E)-2-nonenal, methional and maltol among them. Then there is acrylamide, a carcinogen that has no smell and forms when glucose and the amino acid asparagine, both of which occur naturally in flour, react at a high temperature. Toast that bread, and you get even more acrylamide. If you burn the toast, you will be sniffing furanones and polycyclic hydrocarbons (PAHs). They’re recognized carcinogens. And if you try to cover up the smell of the burned toast with an air freshener, you will be inhaling the likes of xylene, dichlorobenzene and limonene, each of which can be shown to have some sort of toxicity under certain conditions.
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Of course, just because these chemicals invade our body doesn’t mean they cause harm. But neither can we conclude that they don’t. The chemical complexity of the air we breathe and its possible consequences on health certainly merit investigation.
Such investigations have been carried out, but mostly on outdoor air. Numerous studies have found that air polluted with significant amounts of carbon monoxide, lead, nitrogen dioxide, ozone, sulphur dioxide, benzene, smoke or tiny particles from rubber tires raises the risk of heart disease, lung disease, cancer and even cognitive damage. People living near heavily trafficked areas are affected the most. However, much less is known about indoor air despite concerns about it having a long history.
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The effects of inhaled smoke were undoubtedly noted when humans began to heat their caves with fire. Benjamin Franklin was aware of the smoke problem and made sure that the “Franklin stove” he invented produced less smoke than a fireplace. He was also aware of the need for ventilation. John Adams, who once shared a room in a country inn with Franklin, preferred to sleep with windows closed. As Adams recorded in his diary, Franklin objected, and remarked that “The air within this chamber will soon be, and indeed is now worse than that without doors. Open the window and come to bed, and I will convince you.” We don’t know if Adams was convinced, but today we know that on occasion indoor air can be more polluted than outdoor air.
A series of experiments in 2018 under the framework, “House Observations of Microbial and Environmental Chemistry (HOMEChem),” organized by atmospheric chemist Delphine Farmer and mechanical engineering professor Marina Vance, tackled the indoor air issue. The “house” referred to is at the University of Texas in Austin and was specially built for conducting experiments. It is equipped with all sorts of monitoring equipment to measure chemicals in the air. These include ozone, nitrogen oxides, ammonia, carbon dioxide, particulate matter and a host of “volatile organic compounds (VOCs)” that include emissions from cooking, cleaning agents, plants and human bodies.
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Over the period of a month, the researchers and their students cleaned, cooked, sweated and accumulated data. One experiment involved cooking a Thanksgiving meal with all the trimmings. At one point, the instruments recorded fine particulate matter at a level that if found outdoors would trigger a warning from the Environmental Protection Agency about potentially serious damage to the heart and lungs. Some of these particles came from stir-frying, others from vapourizing deposits that had formed in the oven from previous use.
Indeed, self-cleaning ovens that jack up the temperature in an extreme fashion are known to release loads of fine particulate matter. Such particles can be so small, less than a nanometer, that they can even follow the olfactory nerve and pass into the brain. Carbon dioxide levels while the turkey was in the oven reached four thousand parts per million, levels that at least in the short term can impair cognitive function.
There were a number of other interesting findings. Emissions from stir-frying vegetables in teriyaki sauce reacted with vapours from a bleach solution used to mop the floor to produce chloramines, known respiratory tract irritants. When the frying was done on a gas stove, bleach vapours reacted with nitrogen oxides produced by the flames to form nitryl chloride, a substance implicated in smog formation.
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Another finding was the presence of hydroxyl radicals that are also involved in smog formation. These are normally produced outdoors when ozone in the air reacts with ultraviolet light in the presence of water vapour. Detecting hydroxyl radicals indoors was a surprise. Apparently, enough ultraviolet light sneaks through windows to produce these radicals that then can react with cooking vapours to produce yet more VOCs.
Human bodily emissions such as rectal gases and squalene, an oily substance produced by sebaceous glands in the skin that prevents the skin from drying out by locking in moisture, were also detected. Numerous chemicals from personal-care products such as decamethylcyclopentasiloxane used as a lubricant in creams and shampoos also appeared, as did diethyl phthalate added to perfumes to retard evaporation. Phthalates can also outgas from vinyl flooring where they are used to impart flexibility and are recognized as endocrine disruptors. And we haven’t even mentioned the hundreds of compounds found in coffee aroma, the fatty acids in foot odour, the numerous chemicals wafted into the air from the spices used in cooking or from mold in the bathroom. Furthermore, all these chemicals have the potential of engaging in chemical reactions among themselves, forming yet more substances. Obviously, in indoor air, we are dealing with a chemical stew of immense complexity with essentially unknown health consequences.
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Lea Hildebrandt Ruiz, one of the scientists involved in the HOMEChem study, also researches outdoor pollution and has monitored air in New Delhi, which has perhaps the world’s worst air. She noted that fine particulate matter that can reach 225 micrograms per cubic meter on Delhi’s worst days is still less than the 280 micrograms per cubic meter that was detected during the frenzied final hour of cooking the Thanksgiving meal.
What can we take away from all this? Low-temperature cooking on an electric rather than a gas stove under a well-functioning ventilation hood is the way to go. And if cooking smells begin to permeate the house, take advice from Benjamin Franklin and open a window. Ben, it seems, was quite an expert on indoor air, even penning an essay on flatulence with a rather provocative title that can be readily found on google.
Joe Schwarcz ([email protected]) is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.
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