What are you breathing inside your home? The pollutants nobody measures

The figure is counterintuitive and, once accepted, uncomfortable: we spend roughly 87-90% of our time inside buildings (EPA TEAM studies and the National Human Activity Pattern Survey), and indoor concentrations of several pollutants run two to five times higher than outdoor levels. "Open the window" has been sold as a universal answer and solves almost nothing when the source is inside: the kitchen, the new piece of furniture, the particleboard, the vinyl floor, the dirty filter on the heat-recovery unit. The question is not whether indoor air is contaminated. It is why a country that regulates thermal envelopes to four decimal places still does not require measuring what the occupant breathes.

Between the 1980s and 1990s, the EPA published the Total Exposure Assessment Methodology (TEAM) with a finding that still holds: levels of about a dozen common organic pollutants were 2-5 times higher indoors than outdoors, and this happened both in rural and industrial settings. In 2009 the WHO published its Guidelines for Indoor Air Quality: Dampness and Mould, linking humidity and mould to asthma and rhinitis, followed in 2010 by the guidelines on selected organic compounds. Neither source is fringe and neither is recent.

The industry knows this. The revised EPBD directive talks about indoor air quality, but the Spanish transposition has focused on energy efficiency. CTE DB-HS 3 sets minimum ventilation flows; they are rarely verified once the building is finished. Passivhaus certification contractually requires pressurisation tests; the rest of the building stock does not. The renovation paradox is well known: lower the consumption, raise the relative humidity, mould appears and someone paints over it.

Royal Decree 732/2019 added the Basic Document HS 6 to the Spanish Building Code, in force since 16 September 2020. It sets a reference level of 300 Bq/m³ as an annual average and requires constructive measures in new builds, extensions, refurbishments and changes of use within municipalities classified as risk zones. Galicia, parts of Extremadura, mountain ranges of Madrid and Castilla y León and other mountainous areas concentrate the zones with the highest probability of exceeding the threshold. The Spanish Nuclear Safety Council (CSN) published the action map.

The detail the regulation does not close: existing housing without works. The Building Code only kicks in when there is a permit; without a permit, the occupant has no obligation to measure and the technician has no obligation to inform them. A passive dosimeter (charcoal or alpha-track) costs between 30 and 50 euros, is exposed for three months and returns a quantitative figure. The asymmetry is obvious: the cost of the test is trivial, the cost of not running it is radiological and silent.

Satish, Mendell and colleagues published in Environmental Health Perspectives in 2012 a controlled-exposure study at 600, 1,000 and 2,500 ppm of CO₂. At 1,000 ppm, significant decreases were observed on six of nine decision-making scales; at 2,500 ppm, large decreases on seven scales, with subjects rated as dysfunctional in initiative-taking and strategic thinking. Allen and colleagues replicated the effect in 2015 with office workers in green versus conventional offices. The associative threshold has been confirmed again in classroom samplings.

The SINPHONIE project (EU, 2014) measured classrooms in 25 countries and found that a significant percentage routinely exceeded 1,000 ppm of CO₂. Spain participated in the sampling and did not publish a follow-up action plan. An NDIR sensor for CO₂ costs between 60 and 150 euros and settles the question without debate: if the CO₂ curve plateaus at 1,500 ppm in the classroom, it is not a matter of opinion, it is insufficient ventilation. After the pandemic, part of the school estate reduced flows to save heating. The consequence shows up in the millimetres of ink on the pupils' tests.

PM2.5 particles generated while cooking (especially with gas and at high temperature) can raise indoor concentrations above outdoor urban levels, even in cities with heavy traffic. The review by Logue and colleagues at Lawrence Berkeley National Laboratory documented emission rates by burner type, pan and recipe. A range hood with measured actual flow (not catalogue nominal flow) and an outdoor exhaust is the difference between episodic exposure and accumulated exposure.

The natural-ventilation trap: an open window does not compete with a concentrated local emission. The recirculating-hood-with-charcoal-filter trap: it captures odours and not much for PM2.5. The developer's trap: the open kitchen onto the living room is sold without outdoor exhaust because routing the duct increases costs. The question the buyer rarely asks is whether the hood vents outside or simply blows the wok air back at the baby three metres away.

IARC has classified formaldehyde as a Group 1 carcinogen since 2004. It remains common in urea-formaldehyde resins for particleboard, MDF and certain varnishes. Directive 2004/42/EC has limited interior decorative paint to 30 g/L of VOCs since 2010. Many products are labelled "0 VOC" when the value is below the analytical detection limit, not below true zero — and that is perfectly legal.

Labels that do guarantee something: EU Ecolabel, Blue Angel (Germany), AgBB (Germany), M1 (Finland), GreenGuard Gold. Generic labels with no third-party verification: questionable. The information asymmetry between manufacturer and end customer would justify a clear label. Meanwhile, on site, furniture is still bought by the lot without knowing whether it is E0, E1 or CARB Phase 2 equivalent, and it is installed in rooms that are occupied 24 hours later.

A basic domestic kit covers most of the problem without turning the living room into a laboratory:

• CO₂: NDIR sensor, €60-150. Robust indicator of ventilation and occupancy load.
• Radon: passive dosimeter (alpha-track or charcoal), €30-50 for a 3-month exposure.
• PM2.5: domestic laser sensor, €80-200. Useful to detect cooking and combustion peaks.
• Relative humidity: thermohygrometer, €10-30. Above 60% sustained, mould risk.
• Total VOCs: PID or MOX sensor, €100-300. Indicative, not quantitative; useful after painting or installing new furniture.

Professional sampling with pumps and chromatography costs several hundred euros and only makes sense when there is a specific clinical suspicion or a contractual dispute. For everything else, consumer sensors are enough to decide when to open, when to extract and when to change the filter. The boundary between data and decision costs less than dinner for two.

The same country that has paved the floor with carbon taxes and EPBD obligations has spent years avoiding any duty to measure what the occupant breathes. The regulation moves on the carbon footprint, health is left to the informed owner. Meanwhile, data is available, sensors cost less than a pair of trainers and the EPA and the WHO have been repeating the same message for twenty years. The question is not why no one measures it. It is who finds it inconvenient to measure.

1. Klepeis, N. E., Nelson, W. C., Ott, W. R., Robinson, J. P., Tsang, A. M., Switzer, P., Behar, J. V., Hern, S. C., & Engelmann, W. H. (2001). The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants. Journal of Exposure Analysis and Environmental Epidemiology, 11(3), 231-252. DOI: 10.1038/sj.jea.7500165
2. Satish, U., Mendell, M. J., Shekhar, K., Hotchi, T., Sullivan, D., Streufert, S., & Fisk, W. J. (2012). Is CO₂ an indoor pollutant? Direct effects of low-to-moderate CO₂ concentrations on human decision-making performance. Environmental Health Perspectives, 120(12), 1671-1677. DOI: 10.1289/ehp.1104789
3. Allen, J. G., MacNaughton, P., Satish, U., Santanam, S., Vallarino, J., & Spengler, J. D. (2016). Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic compound exposures in office workers: a controlled exposure study of green and conventional office environments. Environmental Health Perspectives, 124(6), 805-812. DOI: 10.1289/ehp.1510037
4. World Health Organization. (2009). WHO guidelines for indoor air quality: dampness and mould. WHO Regional Office for Europe.
5. Logue, J. M., McKone, T. E., Sherman, M. H., & Singer, B. C. (2011). Hazard assessment of chemical air contaminants measured in residences. Indoor Air, 21(2), 92-109. DOI: 10.1111/j.1600-0668.2010.00683.x
6. Royal Decree 732/2019, of 20 December, amending the Spanish Building Code and approving Basic Document DB-HS 6 «Protection against radon exposure». Official State Gazette, no. 311, 27 December 2019.