People in the industrialized world now spend the overwhelming majority of their lives inside buildings, yet most of what we know about air pollution and health comes from measurements taken outdoors. A new study from Finland tackles this gap head-on by asking a deceptively simple question: when polluted air arrives at a building, what actually makes it inside? The answer, it turns out, depends heavily on what kind of pollution is knocking at the door, how big the particles are, and whether the pollution traveled thousands of kilometers or was emitted just down the runway.
The research, led by Sami Harni of the Finnish Meteorological Institute and colleagues from Tampere University, VTT Technical Research Centre of Finland, and the Helsinki Region Environmental Services Authority, was conducted in an unoccupied, mechanically ventilated building located in the immediate vicinity of Helsinki-Vantaa International Airport, one of the busiest aviation hubs in Northern Europe. The team equipped the building with a clever switching valve system that alternated sampling every fifteen minutes between the building’s air intake, upstream of its filtration system, and the exhaust air vent. This design allowed near-simultaneous measurement of outdoor and indoor pollutant concentrations, dramatically reducing the uncertainty that plagues studies relying on separate instruments placed at different times and locations.
The measurement campaign ran from February 13 to February 27, 2025, and fortune smiled on the researchers: a strong long-range transport episode swept into the region during the study window. These events occur when weather systems carry polluted air masses from distant industrial regions across national borders, loading the atmosphere with sulfate, nitrate, and other secondary pollutants that formed far from where they are eventually deposited. Having both a normal baseline period and a pronounced transport event within a single campaign gave the team a rare natural experiment for testing how the chemical character of outdoor air changes the way pollution penetrates indoors.
The instrument suite was comprehensive. The researchers measured the chemical composition of aerosol particles using a soot-particle aerosol mass spectrometer, quantified black carbon with wavelength-dependent light absorption, and captured particle number concentrations, particle mass, and full size distributions. This combination matters because different pollutants behave very differently when air passes through a ventilation system. Large particles are efficiently removed by filters and deposit on surfaces, while ultrafine particles, those smaller than about 100 nanometers, are notoriously difficult to capture and can slip through filtration media that appears effective on paper.
One of the clearest findings concerned the airport itself. The team found that the nearby airport had a large effect on particle number concentrations but almost no effect on particle mass or black carbon concentrations. This pattern is characteristic of aircraft and airport operations, which emit enormous quantities of tiny ultrafine particles during taxiing, takeoff, and idling, particles so small that they contribute negligibly to total mass even while dominating the count of individual particles. It is a striking illustration of why mass-based air quality metrics alone can miss important exposure risks near airports: a cubic meter of air can pass a mass test while carrying millions of nanoscale particles that penetrate deep into human lungs.
Chemically, the story indoors and outdoors diverged in an unexpected way. Organic matter was the most abundant component of aerosol particles both inside and outside the building, which is typical for European winter aerosol. But the second-place finisher differed by environment. Outdoors, nitrate took second position, reflecting the contribution of nitrogen oxides transformed into particulate form during cold winter conditions. Indoors, however, sulfate outranked nitrate. The researchers suggest that nitrate’s poor showing indoors may stem from its semi-volatile nature: as particles are transported through the ventilation system and warmed, ammonium nitrate can evaporate back into gas, effectively stripping it from the particle phase before it reaches the indoor environment. The lowest indoor-to-outdoor ratio recorded in the study, 0.20 plus or minus 0.08, was observed for nitrate during the long-range transport period, consistent with this evaporation hypothesis.
Most indoor-to-outdoor ratios remained stable between the normal and long-range transport conditions, but there were telling exceptions. Particulate matter and black carbon showed higher indoor-to-outdoor ratios during normal conditions, while particle number concentration behaved in the opposite way, with notably higher ratios during the transport episode. The most likely explanation lies in particle size. During long-range transport, the geometric mean diameter of the particles increases, and larger particles are both more effectively captured by the building’s filtration system and more prone to deposition. Smaller particles, by contrast, penetrate more readily, so when the outdoor aerosol shifts toward smaller sizes, the indoor fraction rises even if the filters remain unchanged.
Perhaps the most sobering result involved black carbon, the sooty combustion residue linked to cardiovascular and respiratory disease. Under long-range transport conditions, indoor black carbon concentrations rose to levels comparable to those measured at street canyon sites in Finland, locations where pedestrians walk alongside dense urban traffic. In other words, a building sitting well away from any road, protected by mechanical ventilation and filtration, can still accumulate indoor soot at urban street-level concentrations simply because the regional air mass itself became polluted. The study highlights the significance of the intake filtration system in managing indoor pollutant concentrations, but it also makes clear that no filter is a perfect shield when the outdoor burden grows severe enough.
The work, published as a preprint in Aerosol Research and currently under peer review, comes with appropriate scientific caveats. An independent referee commended the dataset as valuable while recommending clarifications on several methodological points, including the representativeness of sampling from the exhaust duct, the treatment of transition periods in the alternating sampling scheme, and the distinction between filtered supply air and uncontrolled infiltration through the building envelope. The referee also noted that the wind sector associated with elevated particle counts is not unique to the airport and may include roads and other local sources, and that the study’s conclusions should be framed as a case study of one building, one winter campaign, and one ventilation configuration rather than a universal rule for buildings near airports.
Those caveats do little to diminish the broader significance of the findings. As aviation grows and urban development pushes housing closer to airport corridors, understanding which pollutants cross the threshold of a well-ventilated building becomes a matter of public health, not just building science. This study shows that the threat is not uniform: mass-based metrics may look reassuring while particle counts tell a different story, and a pollution event occurring hundreds of kilometers away can quietly raise the soot load inside a Finnish office to street-canyon levels. For building engineers, the message is that filtration effectiveness must be evaluated against particle number and composition, not mass alone. For regulators, it is that indoor exposure near airports cannot be inferred from outdoor monitoring stations. And for the rest of us, it is a reminder that the air we breathe indoors is never fully our own; it carries the chemical fingerprints of distant smokestacks, busy roads, and runways, filtered but never entirely erased.
Subject of Research: Indoor and outdoor aerosol chemical composition and particle penetration in a mechanically ventilated building near an international airport
Article Title: Chemical composition of indoor and outdoor particles in a building near an international airport: Influence of local and long-range transported air pollution
Article References: Harni, S. D., Li, D., Silvonen, V., Salo, L., Lepistö, T., Tykkä, T., Elsayed, M., Barreira, L., Saarikoski, S., Kulmala, I., Niemi, J. V., Hellén, H., Säämänen, A., Rönkkö, T., & Timonen, H. (2026). Chemical composition of indoor and outdoor particles in a building near an international airport: Influence of local and long-range transported air pollution. https://doi.org/10.5194/ar-2026-32
Image Credits: AI Generated
DOI: 10.5194/ar-2026-32
Keywords: indoor air quality, aerosol particles, airport emissions, ultrafine particles, black carbon, long-range transport, ventilation filtration, particle number concentration, nitrate, sulfate, Helsinki-Vantaa airport, aerosol mass spectrometry
News Source: Russell Cooper. (October 8, 2026). Airport Ultrafine Particles Slip Indoors While Filters Block the Heavier Pollution. Scienmag.
