First results from the second national campaign on air quality in homes (CNL2)

Scope of the CNL2

While outdoor air quality is subject to continuous monitoring for numerous "regulated" pollutants, this is not the case with indoor air. The National Housing Campaigns therefore aim to monitor changes in the quality of indoor environments, where we spend much of our time. 

The CNL2, which ran from November 2020 to February 2023, followed on from the First National Housing Campaign (CNL1) carried out between 2003 and 2005. It combined measurements of air quality in participants' homes with a survey of the characteristics of homes, households and their equipment, a description of domestic activities (works, cleaning, cooking, smoking, etc.) and the occupants perceptions of comfort. This survey was carried out in 571 homes (both individual houses and flats) in 321 municipalities across mainland France, and involved interviews with 1516 individuals. The survey methodology then enabled the results to be extrapolated to all 30 million main residences in mainland France.

This monitoring of indoor air quality over time was made possible by financial contributions from the Directorate General for Risk Prevention (DGPR), Directorate General for Health (DGS), Directorate for Housing, Urban Planning and Landscapes (DHUP), French Agency for Ecological Transition (ADEME), Santé Publique France and ANSES, through its phytopharmacovigilance scheme.

Main findings

An overall improvement in indoor air quality in homes...

Generally speaking, over the 15 years since the CNL1, a fall in concentrations of VOCs, aldehydes and particulate matter in the air inside homes has been observed:

• A particularly sharp fall of over 80% in chlorinated VOCs (1,4-dichlorobenzene, trichloroethylene and tetrachloroethylene), which are often used as solvents or disinfectants in many different DIY, industrial and household products;
• Reductions of 28% for formaldehyde (found in emissions from wood products), 33% for fine particulate matter (PM2.5, associated with smoking, road traffic and other combustion activities) and 47% for benzene (road traffic and combustion activities);
• Falls in other measured VOCs, ranging from 30% to over 80%.

No change was observed for certain pollutants such as 1-methoxy-2-propyl acetate (detected very rarely in the two campaigns), hexaldehyde and radon. For the first time, concentrations of SVOCs and NO2 were measured in a representative panel of homes in mainland France.

... partly explained by changes in regulatory provisions:

• A ban on certain substances in construction products and on tetrachloroethylene in dry cleaning;
• Mandatory labelling of construction products to encourage users to choose products with lower emissions;
• Actions to reduce pollutant emissions into the ambient air, as they also have an impact on indoor air quality;
• Numerous initiatives since 2005 to raise awareness of indoor air quality among the general public and professionals, particularly through the National Environmental Health Plans (PNSEs): online publication of guides, leaflets, quizzes on indoor air quality, etc.;
• Incentives aimed at reducing the number of smokers in the population (with a fall of around 20% in the proportion of surveyed homes with smokers), which helps reduce indoor concentrations of fine particulate matter.

Lastly, compared with major international studies carried out in Europe and North America, the levels observed in the CNL2 are broadly similar, if not slightly lower. 

Pollution still high in some homes

In part of the housing stock, the concentrations observed in indoor air were still above the reference values used for management purposes or for protecting population health:

• Fine particulate matter, which exceeded the target objective [1] of 10 μg/m3 in more than 70% of homes;
• Radon, which exceeded the regulatory value [2] of 300 Bq/m3 in nearly 8% of homes;
• Formaldehyde, which exceeded the management value [3] of 30 μg/m3 in just over 6% of homes;
• Nitrogen dioxide, which exceeded the daily guideline value [4] of 25 μg/m3 in nearly 3% of homes;
• Benzene, which exceeded the benchmark value [5] of 6 μg/m3 in 1.4% of homes;
• Trichloroethylene, which exceeded the benchmark value [6] of 10 μg/m3 in 0.05% of homes.

In general, the concentrations measured in indoor air call for vigilance to be maintained and efforts to be stepped up to limit emissions of pollutants associated with carcinogenic effects, such as benzene, tetrachloroethylene and benzo(a)pyrene (from road traffic and combustion activities). The very presence of these pollutants is a cause for concern.

Lastly, the study highlighted the current lack of reference values for many of the pollutants detected in air, which would provide context for the results.

Next steps, outlook and options for exploiting the data

The data collected during this campaign continue to be analysed within the OQEI. They will be used to examine in greater depth a number of issues on which reviews or updates are planned in the coming months, such as:

• the state of ventilation in homes;
• research and assessment of the contribution of the determinants explaining the levels of pollutant concentrations in homes;
• problems of damp and mould in homes;
• revision of the assessment of the health and socio-economic impact of indoor air pollution.

ANSES has also funded specific measurements of pesticides in indoor air and dust, with the results due to be published in the second half of 2025.

"These initial results from the CNL2 clearly illustrate the aim of the OQEI, run by ANSES and the CSTB, to provide reliable knowledge on indoor environments to benefit public action, civic debate and the scientific community. Because reference values are lacking for many substances, exposure measurements cannot yet be interpreted in terms of the health risk. The broad spectrum of substances measured by the CNL2 also furthers our understanding of the chemical exposome, which is the combination of different exposures and their effects on health."

Éric Vial and Julien Rogé, OQEI leaders.
 

[1] Indoor air benchmark value set by the French High Council for Public Health for 2025. If this value is exceeded, corrective action (identification and reduction of particle sources) must be taken within one year.

[2] Article R.221-29 of the French Environment Code, 2022. Human exposure above this level is considered inappropriate, even if it does not constitute a limit not to be exceeded.

[3] Provisional management value proposed by the French High Council for Public Health in 2019. If this value is exceeded, corrective action (improvement of ventilation conditions, identification and reduction of emission sources) must be taken within one year.

[4] Recommendation of the World Health Organization in 2021. If this recommended value is exceeded, there are risks to public health.

[5] Management benchmark value proposed by the French High Council for Public Health in 2024. If this value is exceeded, corrective action (identification and reduction of indoor sources) must be taken within one year.

[6] Management benchmark value proposed by the French High Council for Public Health in 2020. If this value is exceeded, corrective action (identification and reduction of indoor sources, action on ventilation in the home) must be taken within five years.