Ultrafine Particles in the Helsinki Metropolitan Area
Ultrafine particles (UFP) means particles smaller in diameter than 100 nanometers. They are measured as particle number concentration in air (PNC; unit: particles/cm³). HSY has been mapping ultrafine particles in the Helsinki metropolitan area since 2009. The highest concentrations occur near busy roads, streets, and the airport. More information is needed on concentrations and health impacts of ultrafine particles. There are currently no guidelines or limit values for them.
Traffic exhaust emissions have the greatest impact on ultrafine particle concentrations in urban air
In the Helsinki metropolitan area ultrafine particle concentrations are mainly influenced by exhaust emissions from vehicles, work machinery, aircraft, and ships. Emissions from wood burning and combustion emissions from energy production and industry also slightly increase concentrations. Regional background levels and long-range transport affect concentrations across the area.
Some ultrafine particles are emitted directly as particles, while others form from gases during atmospheric transport. Precursor gases also originate from natural sources such as vegetation, soil, and seas. Gas-to-particle formation is most intense in sunny spring and early summer weather.
High concentrations near major roads and the airport
Ultrafine particles have been measured at more than 20 indicative monitoring sites in the Helsinki metropolitan area. The highest annual average particle number concentrations have exceeded 10,000 particles/cm³ along busy ring roads and entry routes, in street canyons lined with apartment buildings, and near the airport. Airport and aircraft emissions raise concentrations several kilometers away when the wind blows from the airport.
In ports and their vicinity, emissions from ship and car traffic and work machines increase ultrafine particle concentrations. In detached-house areas, concentrations are generally low, but emissions from wood burning and traffic are clearly visible. Concentrations are low far from emission sources, such as in forests, by the sea, and in parks.
Temporal variation in emissions is also evident in measured concentrations. Road traffic raises concentrations most during weekday rush hours, and wood burning in the evenings in residential areas. The rhythm of air and sea traffic is reflected in concentrations near airports and ports.
Concentrations have decreased over the long term
HSY has measured ultrafine particles in Helsinki at Kallio (urban background station) and along the busy street Mäkelänkatu since 2015. Over ten years, concentrations have clearly decreased at Mäkelänkatu, but more slowly than emissions of nitrogen oxides (NOx) and black carbon (BC). Reductions are due to stricter exhaust regulations (e.g., particle filters and catalytic converters), electrification of traffic, and decreased traffic volumes. Concentrations have also slightly decreased at the Kallio urban background station.
Health impacts require further research
Ultrafine particles pose different health risks compared to larger particles because they can penetrate deep into the lungs and reach the bloodstream. They may also enter the brain via nasal inhalation and through the blood-brain barrier.
Ultrafine particles have a low mass but a high number and large surface area, which influence health effects. Risks also depend on the origin of the particles. For example, exhaust particles contain more harmful substances than particles from natural sources.
Studies have shown links between exposure to ultrafine particles and cardiovascular diseases, respiratory illnesses, and increased mortality risk. Health impacts are partly independent of other regulated air pollutants (e.g., PM2.5, NO₂). More research is needed on both short- and long-term exposure risks. WHO has not set guideline values for ultrafine particle concentrations but recommends measuring them as part of air quality monitoring to support health studies.
WHO has also provided preliminary thresholds for high concentrations to support decision-making on reduction: high hourly concentration is over 20,000 particles/cm³ and high daily concentration over 10,000 particles/cm³. These thresholds are frequently exceeded in busy traffic areas and near the airport.
European monitoring network develops and harmonizes
The EU’s new Air Quality Directive (2024) introduced measurement obligations for ultrafine particles at urban background and rural supersite monitoring stations and at locations likely to have high concentrations. The directive sets the lower size limit for particle number measurement at 10 nanometers with no restriction on the upper limit. The lower size limit significantly affects measurement results and must be considered when comparing studies.
Ultrafine particle measurement results are used to monitor the effectiveness of air protection work, in health impact studies, and in international comparisons. Concentrations measured in the Helsinki metropolitan area have been moderate compared to other European cities.