A breath of fresh air for indoor air quality

‘‘We are fairly well aware of outdoor air quality, but indoor air is often forgotten’’, laments Aurélie Joubert, professor of process engineering applied to air treatment at IMT Atlantique. Yet indoor air quality is a real public health concern, because indoor environments are the primary compartment of exposure to the many pollutants that accumulate there. These include chemical pollutants such as volatile organic compounds and fine particles, as well as micro-organisms such as bacteria, viruses and fungi. It is precisely on these invisible micro-organisms that the associate professor’s team has focused its research for their latest publication. It presents the results of research carried out as part of a thesis in partnership with the École des hautes études en santé publique (EHESP). Gaëtan Pavard, a former PhD student, studied a sampling method for monitoring the micro-organisms present in indoor air in buildings. This approach is based on the use of return filters from air handling units, and differs from traditional methods in its ability to collect data over long periods. ‘‘This makes it possible to quantify average contamination, which is more representative of reality,’’ explains Delphine Deshayes, a PhD student and the study's first author. ‘‘It is not just a photo at a given moment’’, she adds.

Traditional methods are often unsuitable

Traditional methods of active sampling of microbial aerosols in indoor environments - collection on an agar plate, collection in a liquid or membrane filtration - are rapidly showing their limitations. These devices only allow samples to be taken over a short period of time, just a few hours, with restricted volumes of air.
Faced with these constraints, the research team decided to use the return filters of air handling units as sampling media. The filters, which continuously treat the air in buildings, act as accumulators of biological particles. The scientists attached sampling discs to the filters to continuously collect the micro-organisms present in the air. Unlike traditional samplers, this method enables much larger volumes of air to be analysed, and therefore more representative results to be obtained. The discs also allow representative sampling over periods of up to a full year.

Aurélie Joubert and Delphine Deshayes

A year to reveal the invisible in indoor air

To test the effectiveness of this method, the team conducted a year-long study in an office building on the IMT Atlantique campus in Nantes. Each month, discs were taken and analysed for the presence of bacteria and fungi using culture methods and DNA sequencing. Tests for the detection of specific viruses were also carried out using molecular biology methods. As a result: the discs show a significant diversity of micro-organisms present in indoor air.
Among the bacteria identified, some were attributed to the external environment, while others came from the human microbiome of the building's occupants. As far as fungi are concerned, genera often present in the outdoor environment were detected, probably introduced into buildings by transfer or the movement of people. More unusual, because difficult to observe using traditional methods, the study also detected viruses. Seasonal coronaviruses and adenoviruses were identified during the periods when their circulation in the population was highest.

Better sampling to improve analysis

One of the main advantages of this method is its minimal impact on the operation of the filters, which guarantees stable energy consumption by the ventilation system. In addition, the discs can be collected easily without the need to remove the filters completely, which considerably simplifies the sampling process.
Thanks to this method, a wide range of tools for analysing samples is possible. By combining traditional culture methods with genomic techniques such as sequencing, it becomes possible to identify a wide range of microorganisms, including some that cannot be grown in the laboratory. ‘‘Our method makes it possible to accumulate, extract and identify micro-organisms that may be present in low concentrations in indoor air, and therefore presents fewer constraints in relation to the detection thresholds of analytical methods,’’ notes Delphine Deshayes.

Refining the method, extending applications

However, the two researchers agree that adjustments are still needed to optimise certain stages of the process. ‘‘There remains a real challenge in terms of analysis, to increase the reliability of the results,’’ explains Aurélie Joubert.  The extraction of DNA from the discs, for example, could be improved. Some genetic sequences remain difficult to identify because of the quality of the extraction or the incompleteness of current databases.
‘‘Now that we know how to quantify micro-organisms, we need to quantify the risks,’’ says Aurélie Joubert, glancing at Delphine Deshayes. This is precisely the aim of this young researcher's thesis, once again in partnership with EHESP. She aims to identify potentially health-threatening strains of bacteria present in indoor air and to analyse their antibiotic resistance. To achieve this, hospitals in Rennes and Brest are being used as a testing ground for the sampling method. See you in 2 years' time for the results of this new experiment.