Indoor Air: under-estimated pollutants

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Indoor air

While some sources of indoor air pollution are well known, there are others that researchers do not yet fully understand. This is the case for cleaning products and essential oils. The volatile organic compounds (VOCs) they become and their dynamics within buildings are being studied by chemists at IMT Lille Douai.

When it comes to air quality, staying indoors does not keep us safe from pollution. “In addition to outdoor pollutants, which enter buildings, there are the added pollutants from the indoor environment! A wide variety of volatile organic compounds are emitted by building materials, paint and even furniture,” explains Marie Verriele Duncianu, researcher in atmospheric chemistry at IMT Lille Douai. Compressed wood combined with resin, which is often used to make indoor furniture, is one of the leading sources of formaldehyde. In fact, indoor air is generally more polluted than outdoor air. This observation is not new, it has been the focus of numerous information campaigns by environmental agencies, including ADEME and the OQAI, the monitoring center for the quality of indoor air. However, the recent results of much academic research tend to show that the sources of indoor pollutants are still underestimated, and the emissions are poorly known.

In addition to sources from construction and interior design, many compounds are emitted by the occupants’ activities,” the researcher explains. Little research has been conducted on sources of volatile organic compounds such as cleaning products, cooking activities, and hygiene and personal care products. Unlike their counterparts produced by furniture and building materials, these pollutants originating from resident’s products are much more dynamic. While a wall constantly emits small quantiles of VOCs, a cleaning product spontaneously emits a quantity up to ten times more concentrated. This rapid emission makes the task of measuring the concentrations and defining the sources much more complex.

Since they are not as well known, these pollutants linked to users are also less controlled. “They are not taken into account in regulations at all,” explains Marie Verriele Duncianu. “The only legislation related to this issue is legislation for nursery schools and schools, and legislation requiring a label for construction materials.” Since 1st January 2018, institutions receiving children and young people are required to monitor the concentrations of formaldehyde and benzene in their indoor air. However, no actions have been imposed regarding the sources of these pollutants. Meanwhile, ADEME has issued a series of recommendations that advocate the use of green cleaning products for cleaning floors and buildings.

The green product paradox

These recommendations come at a time when consumers are becoming increasingly responsible in terms of their purchases, including for cleaning products. Certain cleaning products benefit from an Ecolabel, for example, guaranteeing a smaller environmental footprint. However, the impacts of these environmentally friendly products in terms of pollutant emissions has not been studied any more than it has for their label-free counterparts. Supported by marketing arguments alone, products featuring essential oils are being hailed as beneficial, without any evidence to back them up. Simply put, Researchers do not yet have a good understanding of indoor pollution, traditional cleaning products or those presented as green products. However, it is fairly easy to find false information claiming the opposite.

In fact, it was upon observing received ideas and “miracle” properties on consumer websites that Marie Verriele Duncianu decided to start a new project called ESSENTIEL.  “My fellow researchers and I saw statements claiming that essential oils purified the indoor air,” the researcher recalls. “On some blogs, we even read consumer testimonials of how essential oils eliminate pollutants. It’s not true: while they do have the ability to clean the environment in terms of bacteria, they definitely do not eliminate all air pollutants. On the contrary, they add more!”

In the laboratory, the researchers are studying the behavior of products featuring essential oils. What VOCs do they release? How are they distributed in indoor air?


Essential oils are in fact high in terpenes. These molecules are allergenic, particularly for the skin. They can also interact with ozone to form fine particles or formaldehyde. In focusing on essential oils and the molecules they release into the air; the ESSENTIAL project wants to help remedy this lack of knowledge about indoor pollutants. Therefore, the researchers are pursuing two objectives: understand how emissions from essential oil volatile organic compounds behave, and determine the risks related to these emissions.

The initial results show unusual emission dynamics. For floor cleaners, “there is a peak concentration of terpenes during the first half-hour following use,” explains Shadia Angulo Milhem, PhD student participating in the project with Marie Verriele Duncianu’s team. “Furthermore, the concentration of formaldehyde begins to regularly increase four hours after the cleaning activity.” Formaldehyde is a very controlled substance because it is an irritant and is carcinogenic in cases of high and repeated exposure. The concentrations measured up to several hours after the use of the cleaning products containing essential oils can be attributed to two factors. First of all, terpenes react with the ozone to create formaldehyde. Secondly, the decomposition of formaldehyde donors, used as preservatives, and biocide contained in the cleaning products.

A move towards regulatory thresholds?

In the framework of the ESSENTIAL project, researchers have not only measured cleaning products containing essential oils. They also studied diffusion devices for essential oils. The results show characteristic emissions for each device. “Reed diffusers, which are small bottles containing wooden sticks, take several hours to reach full capacity” Shadia Angulo Milhem explains. “The terpene concentrations then stabilize and remain constant for several days.” Vaporizing devices, on the other hand, which heat the oils, have a more spontaneous emission, resulting in terpene concentrations that are less permanent in the home.

In addition to the measurements of the concentrations, the dynamics of the volatile organic compounds that are released is difficult to determine. In some buildings, they can be trapped in porous materials, then released later due to changes in humidity and temperature. One of the areas the researchers want to explore in the future is how they are absorbed by indoor surfaces. Understanding the behavior of pollutants is essential in establishing the risks they present. How dangerous a compound is depends on whether it is dispersed quickly in the air or accumulates for several days in paint or in drop ceilings.

Currently, there are no regulatory thresholds for terpene concentrations in the air, due to a lack of knowledge about the public’s exposure and about long and short-term toxicity. We must keep in mind that the risk associated with exposure to a pollutant depends on the toxicity of the compound, its concentration in the air and the duration of contact. Upon completion of the ESSENTIAL project, anticipated for 2020, the project team will provide ADEME with a technical and scientific report. While waiting for legislation to be introduced, the results should at least offer recommendation sheets on the use of products containing essential oils. This will provide consumers with real information regarding the benefits as well as the potentially harmful effects of the products they purchase, a far cry from pseudo-scientific marketing arguments.

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