They weigh very little. So little that ultrafine particles can contribute less than 5% to the overall mass of PM2.5. Yet, counted one by one, they arrive in the thousands in every cubic centimeter of air and can tell a different story from that given by the values with which we are used to judging pollution.
A study published in GeoHealth and coordinated by Francesca Costabile of the Institute of Atmospheric and Climate Sciences of the Cnr found, in high-income countries analyzed between 2010 and 2019, a relationship between greater exposure to ultrafine particles and a greater burden of cardiovascular diseases. The result concerns associations observed at a population level and does not demonstrate that these particles directly cause heart attacks, strokes or other pathologies. But it points to a rather interesting hole in the way we measure air: Meeting limits on the mass of PM2.5 may not tell the whole story.
Tiny enough to weigh little, numerous enough to count
The UFPs, from English Ultrafine Particleshave dimensions less than 100 nanometers. The study considered those between 10 and 100 nanometers: at the upper limit we are at one ten thousandth of a millimeter. They can penetrate deeply into the respiratory system and scientific literature has already indicated the possibility that some reach the bloodstream and other organs.
The difficulty also lies in measuring them. PM2.5 is regulated primarily by its mass, expressed in micrograms per cubic meter. Ultrafine particles, on the other hand, have an almost negligible mass and are described above all through their number. A control unit can therefore record very small variations in the overall mass of the particulate while the number of the smallest particles changes significantly.
To reconstruct the exposure, the researchers used a high-resolution global database of ultrafine particles, obtained by combining ground-based measurements and a machine learning model with data on urbanization, land use, emissions, PM2.5, nitrogen dioxide and black carbon. The estimates cover the period 2010-2019 with a spatial resolution of one kilometer. It is one of the first tools that allow us to try to follow these particles on such large geographical scales, precisely because homogeneous historical series of direct measurements are still rare.
What the researchers found
The authors cross-referenced the average exposure of the population to UFPs with data from Global Burden of Disease on cardiovascular diseases. They looked at three indicators: years of life lost due to premature death, years lived with disability and DALYs, which put the two components together.
Between 2010 and 2019, exposure to both UFPs and PM2.5 generally decreased in the countries studied. Years of life lost to cardiovascular disease also followed a downward trajectory, while years lived with disability increased slightly. Looking at the different countries together, changes in ultrafine particle exposure paralleled changes in cardiovascular burden.
There is a detail that helps understand why UFP and PM2.5 are not interchangeable. The average correlation between the two measures was quite weak, with an R² of 0.29, and varied quite a bit from country to country. They have some sources in common, but ultrafine particles are highly affected by sources such as traffic, cities, airports, ports and large road infrastructures.
In the model that considers pollutants separately, an increase of one standard deviation in exposure to UFPs, equal to approximately 1,536 particles per cubic centimeter, was associated with approximately 335 additional cardiovascular DALYs per 100,000 inhabitants. It is a population statistical result: the authors themselves point out that that number cannot be transformed into an individual function of the type “so many particles equals so many diseases”.
When UFP and PM2.5 are inserted together into the model, the reading becomes even less linear: the association between ultrafine particles and years lived with disability is positive after adjustment for PM2.5, while for years of life lost the evidence becomes insufficient. The authors themselves point out the statistical problems of models with multiple pollutants and invite them not to use these results to establish which particle is “more toxic”.
Europe has started counting them, but has not yet set a limit
Here there is also an important correction compared to the “standard air” formula. Today ultrafine particles do not have a European limit value comparable to those envisaged for PM2.5 and PM10. What can be respected is the limit based on the mass of PM2.5, while a high concentration of UFP can be much less visible within that same measurement.
However, the new European Directive 2024/2881 on air quality has changed something: it requires Member States to expand the monitoring of UFPs, including them in the so-called supersites and providing measurement points also in areas where high concentrations are expected, for example near roads, airports, ports, industrial areas and domestic heating sources. In short, the directive began to ask us to count them. For now, there is no concentration limit.
And this is precisely the interesting part of the study. According to the authors’ calculations, a relatively large change in the number of ultrafine particles can add as little as 0.15 micrograms per cubic meter to the mass of PM2.5—a tiny amount on the scales by which we judge much particulate pollution today. Yet that variation was associated, in the data analyzed, with changes in cardiovascular load.
A lot of caution is needed. The study is ecological: it compares aggregate data by country, has ten annual observations for each country and does not know exposure, age, previous pathologies and other risk factors of individual people. Furthermore, analyzes with lags of one, two or three years did not produce statistically significant associations. To establish a cause and effect relationship, specially constructed epidemiological cohort studies will be needed.
Meanwhile, a very real problem remains: the particles that weigh less are also those that our mass-based measurement risks seeing worse. Europe has just begun to put them permanently under the microscope. Before we decide how dangerous they are, we will at least need to know how many there are.
