An invisible threat
Rick Gould asks, are ultrafine particles the new pariah of pollutants?
Earlier this year, a group of international scientists published the results of research that determined the sources of airborne ultrafine particulates (UFPs) in four major European cities. This work illustrates a rapidly growing interest in these tiny particles, which are currently not directly regulated, but can have significant impacts on human health. What are UFPs, where do they originate, and why are they generating so much discussion?
UFPs are particles that are less than 0.1µm across, and are also known as PM0.1. These microscopic particles are a small component of PM2.5, itself regulated in ambient air. While the overall mass of PM0.1 is small, they make up for this in their numbers, and ability to reach deep into the human body. “The sheer number of particles in our air is astonishing. Air in our cities typically contains up to 10,000 particles in each cubic centimetre,” explains Dr Gary Fuller, researcher at Kings College London and member of the Air Quality Expert Group (AQEG), which advises Defra.
The science of counting particles, instead of measuring their mass concentrations for regulation, is not new. In 1880, for example, Scottish meteorologist John Aitken published a paper in the Proceedings of the Royal Society of Edinburgh called ‘On dust, fogs, and clouds’. Aitken determined that aerosols form around the nucleus of a dust particle. He had invented a device known as a koniscope, the first instrument that could count the number of particles in the air, and found that the air contains a huge number of particles, all originating from many sources.
During the 20th century, scientists and legislators focused on mass concentrations of airborne particulate matter. Along with epidemiological studies examining the health impacts of this pollutant, this led to legislation to control their emissions and concentrations in ambient air, especially for respirable particulate matter – that is, PM10 and PM2.5.
However, during the past 20 years, there has been renewed interest in the numbers of airborne particles, especially UFPs. “There was growing excitement and concern about the use of engineered nanoparticles, and hazards and risks with them,” describes Fuller. “In the ambient air, Professor Anthony Seaton proposed a new hypothesis of the mechanism by which particles affected our health, through their number and the number of locations where the body has to mount a response. Both these factors led to renewed interest in counting the tiny particles in our air. Since this time there has been an increasing number of epidemiological studies that have looked at the association between UFPs and health.”
Fuller was part of a team carrying out one of these investigations in London; it found that the number of particles in the air was associated with heart attacks, while other metrics concerning the mass of particles were linked to hospital admissions and deaths associated with respiratory illnesses. Other investigations produced similar results. So, what did the recent research in four European cities reveal?
A tale of four cities
“This project began as an idea with researchers in Barcelona, who identified key European cities that were building large datasets. By understanding the sources in these cities, it would open up the possibility for health studies in the future, and for control policies, too”, says Fuller. The researchers measured UFP numbers in Helsinki, London, Barcelona and Zurich under comparable conditions.
Overall, they discovered that between 74% and 94% of UFP measured came from road traffic, confirming findings from similar investigations. The overwhelming results of several studies on UFP sources point to road-traffic emissions as the main culprit, especially from vehicles with diesel engines. The other major sources are: all types of combustion plants, notably those fuelled by coal, oil, and biomass; aircraft at airports; shipping ports; and municipal waste incinerators. The latter has the smallest emissions of these sources.
Incinerators themselves have drawn a lot of attention from those concerned with their contributions to ambient levels of UFPs. One pressure group, for example, has repeatedly asserted that incinerators are a significant source of UFPs, although peer-reviewed evidence from measurements does not support this assertion. One detailed investigation in Milan, for example, determined through measurements that UFP numbers in the stack gases of a waste incinerator were equivalent to those measured in ambient air, as evidence shows the abatement techniques for removing UFPs from flue gases are extremely effective. This is because legislation, such as that in the UK and the EU, specifies that incinerators must have best available techniques (BAT), such as fabric filters, to control particulate emissions. Investigations report that fabric filters remove up to 99.99% of UFPs, therefore making incinerators one of the smaller sources of this pollutant. That said, if UFPs in ambient air are clearly a significant health risk, then what are the challenges in monitoring and controlling them?
A secondary challenge
Larger particles, such as PM10 and PM2.5, are now relatively easy to measure and control from many sources. “However, there are some sources of UFPs that are largely outside of any current legislation. And we may need to devise new control technologies and policies for these,” explains Fuller.
UFPs, like PM10, can be primary pollutants – they are emitted directly from a source, such as car exhausts, biomass boilers and aircraft. However, under the right conditions, semi-volatile compounds in the air can condense and react with other chemicals to form secondary UFPs. Emissions of volatile organic compounds and sulphur dioxide from road traffic, for example, can promote the formation of secondary UFPs.
Secondary UFPs are much more challenging to control. The investigation into UFPs in European cities, for example, found that the concentrations of certain pollutants such as sulphur dioxide and ammonia, temperature and degree of sunlight have a strong influence on forming secondary UFPs. This, in turn, makes them harder to control.
Another challenge is detecting particles that are so tiny, especially if they are smaller than the wavelength of light. The solution is normally to grow them first by condensing butanol onto each tiny particle to enlarge it, so that the particles can then be detected. Monitoring locations are also sparse when compared with those for measuring PM10 and PM2.5, although the AQEG has recommended expanding the number of monitoring stations for UFP so we can learn more about their abundance and behaviour.
Then there is the increasing body of research pointing to the health impacts of UFPs. “A group of prominent European air pollution health-scientists have recently proposed that we include them in air pollution legislation,” Fuller says.
Tiny but toxic
UFPs are particles that are less than 0.1µm across, and are also known as PM0.1
Air in our cities typically contains up to 10,000 particles in each cubic centimetre
Researchers discovered that between 74% and 94% of UFPs measured came from road traffic
Rick Gould, MIEMA Cenv is a technical advisor at the Environment Agency. He is writing in a personal capacity.