Air pollution is the greatest risk factor for both the environment and human health, according to the World Health Organization.

The statistics are alarming: 9 out of 10 people worldwide breathe polluted air, and 1 in 8 will die from it. This means that about 7 million deaths per year are caused by aggravated pollution diseases. Curiously, around 4 million of these deaths are caused by indoor pollution, in our own homes, although this is rarely talked about.

Africa and Asia are the regions most affected, as technologies and methods that cause pollution are still mostly used there for cooking, heating, and lighting, on a daily basis. Women and children, who spend most of their time indoors, are particularly affected. But internal pollution is not just a problem that affects developing countries.

Researchers in the developed world point out that people spend more than 90% of their time indoors. Studies conducted in the UK show that almost 75% of children spend less time outside than prisoners, which means less than 60 minutes a day. “So there is a big question here,” Marina Vance, environmental engineer at the University of Colorado, has pointed out. “If all these studies have found an association between outdoor air pollution and a decrease in quality of life and life expectancy, but we’re not outside, how does that relationship still hold?” It would hold if it was found that either a very short time spent outside has a totally disproportionate impact on human health, or that disproportionate amounts of external pollutants enter our bodies.

A study conducted in 2016 by the University of California, Berkeley suggests that there could be another explanation. The study showed that, at that time, the dominant source of volatile organic compounds (VOCs) in Los Angeles was from emissions from consumer products used in every home, such as perfumes and cleaners, and not chemicals transported from the outside. This could indicate that indoor pollution is at least as serious a problem as outdoor pollution.

A problem that has historically been ignored

The average American can expect to live for 79 years, of which about 70 will be spent between four walls, breathing about 17 kg of air a day, according to the Berkeley study. From the point of view of human health it is therefore crucial to understand indoor air quality, although, historically speaking, this topic has been massively ignored, while most research grants have been dedicated to the study of outdoor air quality. These financial and research efforts have led to important legislation to regulate the chemical composition of outdoor air, leading to a decrease in emissions of hazardous gases such as carbon monoxide and sulfur dioxide, but also the amount of particulate matter in suspension.

But the quality and composition of indoor air has mostly been ignored by researchers, and is not subject to any legislation. However, this is about to change with the establishment of HOMEChem—the House for the Observation of Microbial and Environmental Chemistry—a house assembled on the “JJ Pickle” Research Campus of the University of Texas. Here, researchers renowned for their work in measuring and analyzing the chemical composition of the external environment make great efforts to apply their studies and experience to the internal environment.

Nicola Twilley has written an in-depth article for The New Yorker about the tests currently underway at HOMEChem. Researchers there are trying to replicate events from everyday life—from preparing breakfast to cleaning to cooking a lavish holiday meal—to see what organic compounds are released into the air, what their sources are, and how they interact with each other. After the emissions generated by each activity were noted, the researchers began to group the activities. Thus, a group of student volunteers spent a day at home, cooking meals, cleaning, using the computer, operating the dishwasher, and so on. The effects of the pollutants generated by these activities on human health have not currently been studied.

Worrying findings

After a month of cooking meals, toasting bread, washing dishes, using deodorant, and other such household activities, even the most skeptical researchers were surprised by the results. Preliminary data indicate temporary peaks in chloramine, a chemical known to be inflammatory for airway membranes. Researchers also found unexpected chemical compounds in household air, such as sodium chloride, a compound known to chemists for its role in smog formation in coastal areas and formed as a result of the reaction between compounds released by cleaning products with a chlorine base and compounds released during the combustion of methane gas at the stove.

The researchers were also surprised to find hydroxyl radicals, present in atmospheric chemistry, where they interact with nitrogen oxides to produce ozone and facilitate the formation of smog. The production of these radicals is dependent on sunlight, which is why researchers did not expect them to form indoors. But measurements show that natural light, filtered through a window, combined with methane gas from the stove, is enough to produce chemical reactions “similar to what you’ll find in a city on a smoggy day,” says Philip Stevens, an atmospheric chemist at Indiana University. Researcher Lea Hildebrandt Ruiz also said that at one point “the conditions inside the house exceeded the levels encountered in the most polluted cities and I can say this because in parallel I also deal with an air quality monitoring program in New Delhi.” According to the WHO, the capital of India is the most polluted city in the world.

In addition to the chemicals released by all the sprays, perfumes, and lotions we use, and in addition to the organic chemicals released by our bodies, such as isoprene or acetaldehyde, cooking and cleaning are expected to be the main activities which add chemicals to the indoor environment. Cooking on the stove produces more emissions than cooking on an electric hub, especially when frying, but researchers have found that it may also matter what we cook.

Experiments indicate that the cooking of meat produces an atmospheric chemistry that is different from the chemistry of vegetarian food and could be the main source of ammonia concentrations detected in the air after cooking, say, turkey meat. At the same time, various spices have different levels of ozone reactivity. For example, star anise, which contains high levels of volatile sesquiterpenes, reduces ozone levels, and improves air quality.

Even scientists who were not involved in the study were alarmed by the preliminary data. Francesca Dominici, a biostatistician at Harvard University, says the level of microparticles released into the air during the preparation of a holiday meal, which can reach values ​​of 285 micrograms per cubic meter, is downright shocking. “Even a short-term increase of just 10 micrograms per cubic meter from one day to the next would increase the number of hospitalisations and the mortality rate among people over 65,” she says.

John Balmes, a pulmonologist at the University of California, says that the level of CO2 recorded during the preparation of a holiday meal, which reached 4000 ppmv, “decreases cognitive functions in the short term. Whether it has any long-term effect, we don’t know.” But he draws attention to the panic. “Simply measuring concentrations of a chemical in a test house is not enough to infer potential exposure. Going from chemistry to epidemiology is a big leap,” he says.

Outside, or at home?

What can be said with certainty about the preliminary data of this experiment is that the level of many traditional pollutants is lower indoors than outdoors, at least until we start frying food, at which time certain compounds will exceed almost 10 times the concentration detected outdoors. Other more complex organic compounds appear to be at a higher level indoors, regardless of activity. There are also clues that particles entering from the outside are lined with gases that facilitate another way of penetrating the lungs.

The good news is that once major risk factors are identified, indoor pollution should be easier to combat than outdoor pollution, because it is contained within a limited and confined space. Already, some steps which have been put into practice a long time ago have proven to have scientific validity, such as avoiding frying, cooking with the hood on, opening the window when toasting bread in the toaster, and daily ventilation of the house—especially when cleaning, and especially if we use bleach or other aggressive chemicals.

Houseplants are also useful, as they purify the air. And those of us who are not blessed with green thumbs and are expert “plant killers” will be happy to know that the Swedish retailer IKEA has been working for several years, in collaboration with a number of universities in Europe and Asia, on a technology for a type of curtain that purifies the air inside. This technology mimics photosynthesis. The textile material is coated with a mineral-based photocatalyst, which reacts to indoor ambient light and produces “photosynthesis”, destroying chemicals in the air, such as formaldehyde from cooking, which is a carcinogen. The product still needs to be tested for effectiveness before being made available in stores.