Concerns over the health risks from ultrafine particulates (UFPs) released from incinerators and power stations have steadily grown in recent years. Paul Landau describes what UFPs are, explores their sources and impacts, describes how they are measured and controlled, and what we can expect in the future.
During the past year, an Italian team of researchers published the results of studies1,2 examining the effectiveness of fabric filters in controlling the potential releases of ultrafine particulates (UFPs) from incinerator stacks. In simple terms, the reports stated that fabric filters are very effective at capturing these so-called “nanoparticles”, which are the smallest category of particulate pollutants.
This research is controversial, because during the past five years there has been both a growing interest and concern over the health impacts of UFPs. For example, during the past two years, proposals for new industrial installations such as incinerators and biomass-fuelled power stations have been fiercely opposed on the grounds of alleged emissions of UFPs, as well as the more traditional concerns of protestors, such as dioxins. Indeed, there is established scientific evidence that the smaller the size of particles, the more potential damage they can do to human health3. There is also clear evidence that combustion is a significant source of UFPs.
Graded grains make finer particulates
While respirable particulate matter, or particulates that are not more than 10 microns in size (PM10), have received a great deal of attention during the past 20 years, UFPs in ambient air have not seen nearly as much publicity until recent times. Put simply, UFPs are a sub-set of respirable particulate matter. Officially, UFPs have diameters of 100 nanometre (nm) or less; however, many people also refer to sub-micron sized particles, ie those less than 1 micron, or PM1 — within the definition of UFPs.
Either way, any respirable particles are a potential health hazard, and in simple terms, the risk to health increases with decreasing particle size and increasing numbers of particles3. Scientists believe that there are two reasons for this. Firstly, smaller particles are able to penetrate deeper into the human lung; and secondly, smaller particles have a greater surface-to-area ratio than larger particles. In terms of mass, UFPs make up a very small proportion of the total mass of airborne, respirable particulate, although UFPs make a much larger contribution to the total number of particles and the total surface area; for example, 1 particle 10 microns in diameter would weigh as much as one million particles each 100nm in size. At the same time, these one million, 100nm particles would collectively have 100 times the surface area of a single particle of PM10. So if the relative numbers and large surface area of UFPs are a threat, then what are their origins, how are they measured, where are they present, and how effective are the controls on the sources which emit them?
Sources and measurements
UFPs can originate from two sources — human activities, and natural processes in the atmosphere. Studies looking at the composition of PM2.5 have found that around a third of these particles derive from natural sources and most of the rest from burning fossil fuels. During the past five years, research in Finland and Italy targeting PM1 and UFPs has found that between 50% and 70% of these smallest particles arise from combustion sources, thereby supporting the findings of the investigations on PM2.5.
However, we know far more about PM10 and PM2.5 in ambient air than we do about sub-micron particles, simply because it has been much easier to routinely measure the larger respirable particles, and because there has been a regulatory requirement to do so. Not only are there validated methods to monitor PM10 and PM2.5 for both continuous measurements and for short-term averages, but these measurements have been widespread and have taken place over many years. In contrast, measurements of UFPs in ambient air and from industrial sources have been scattered and focused on research rather than regulatory monitoring, because there is not yet a legal requirement to measure UFPs in ambient air or within industrial emissions.
Considering that UFPs make up a very small proportion of the mass of total suspended particulate, yet a large proportion of the total numbers of particles, technologists have developed techniques to measure particle numbers rather than mass. This approach has been essential, for example, for indoor air monitoring and clean-rooms for semi-conductor manufacturing, where a few microscopic specks can damage an integrated circuit during manufacturing. Techniques for clean-rooms have combined optical and electrostatic techniques for nanoparticle monitoring, such as Scanning Mobility Particle Sizers (SMPS) and Aerodynamic Particle Sizers, while research has added the use of other research and analysis tools such as scanning electron microscopes (SEMs) and X-Ray Spectroscopy to examine the physical and chemical characteristics of UFPs. Researchers have used such techniques, both individually and in combination, to examine UFPs in ambient air and from industrial sources.
The studies of ambient air have often found higher concentrations in urban rather than rural air, although there is not always a clear correlation between UFPs and larger respirable particles such as PM10. In cities, recent evidence strongly suggests that the dominant source of UFPs is traffic, because the highest concentrations are found downwind of major roads4. This in turn suggests that policies to manage traffic and vehicle emissions could significantly reduce airborne concentrations of UFPs, in addition to reducing other more mainstream pollutants.
On the other hand, NGOs have focused their attention on incinerators and the growing number of biomass-fuelled power-stations, alleging that these types of process could be responsible for emissions of UFPs at concentrations harmful to health. The research, however, does not support this, with teams in Finland and Italy finding relatively low concentrations of UFPs in incinerator emissions.
Even before the current interest in UFPs, in 1978, the Electric Power Research Institute (EPRI) in the USA compared the effectiveness of electrostatic-precipitators (ESPs) and fabric-filters for capturing particulate emissions from industrial processes. In simple terms, both were very effective, although fabric filters were far more effective than ESPs for PM1; fabric filters are commonly used on incinerators, for example. EPRI found that ESPs were effective at capturing up to 98% of PM1, whereas fabric filters captured not less than 99.5% of these sub-micron particles. Abatement technologies have improved during the past 35 years; for example, in 2007, a team from VTT in Finland examined respirable particulate emissions from a number of combustion plants and found that fabric filters were effective at curtailing 99.99% of the PM1 fraction.
Meanwhile, in Italy, a team led by Professor Georgio Buonanno from the University of Cassino in Italy focused on UFPs in both ambient air and in the emissions from incinerators1,2,4. Among other things, the team examined emissions of UFPs from five incinerator stacks from 2007 to 2010. One result, surprisingly, was that measured concentrations in the stacks were less than those found in ambient air. Buonanno’s team also looked at the removal efficiency for UFPs, by measuring particle numbers before and after the fabric filters. The team found that such filters had a collection efficiency from 99.99% and 99.995%.
Both the Finnish and Italian teams had to design and build the sampling and monitoring equipment for measuring PM10 and UFPs, adapting existing techniques and developing new approaches. The Italian team, for example, combined modern stack-sampling equipment with technology originally developed for monitoring the numbers of UFPs in clean rooms, and since converted for monitoring ambient air.
Worldwide, research on UFPs emitted from industrial sources and in ambient air is growing, with the EC, national governments and regulators taking an ever keener interest. This means that in the future, we can expect to see more investigations in this area; if the history of PM10 and PM2.5 is an indicator, then we could expect requirements to measure UFPs in ambient air, together with guideline target values (as we have for PM2.5), and potentially even limits.
“Chemical, Dimensional and Morphological Ultrafine Particle Characterisation from a Waste-to-energy Plant”, in Waste Management , 31, 2253–2262, Buonanno, G, et al, 2011.
“Ultrafine Particle Emission from Incinerators: The Role of the Fabric Filter”, in Journal of the Air and Waste Management Association, 62(1): 103–11, Buonanno, G, et al, 2012.
“Pulmonary Effects of Inhaled Ultrafine Particles”, in International Archive of Occupational Environmental Health, 74: 1–8, Oberdorster, G, 2001.
“Ultrafine Particle Apportionment and Exposure Assessment in Respect of Linear and Point Sources”, in Atmospheric Pollution Research 1, 36–43, Buonanno, G, et al, 2010.