Particulate matter (PM) in the ambient air is a complex mixture of particles of different sizes and chemical composition. Ultrafine particles (UFP) are those very tiny particles defined by having a size below 0.1 µm (that is, below 100 nanometres) in diameter. Contrary to larger particle definitions, such as PM2.5 and PM10 (particles with a diameter below 2.5 and 10 µm, respectively; Figure 1), UFP are not regulated and, therefore, there is no limit value for the local and regional administrations to meet. However, many studies suggest that UFP might pose a higher threat to human health than PM2.5 and PM10. Let’s learn a bit more about ultrafine particles.
Figure 1. Image showing the relative size between a human hair, fine beach sand, PM10, PM2.5, and ultrafine particles (UFP, PM0.1). Source: Image from EPA modified by Bud Hixson, available at http://badwaterjournal.com/Bad_Water_Journal/Trans_emmissions.html.
What are the metrics for measuring ultrafine particles?
PM2.5 and PM10 are usually measured in terms of mass concentrations (that is, in µg/m3 of air). However, UFP contribute very little to the mass, but their contribution to particle number is huge. Therefore, they are usually measured in number concentration (number of particles/cm3 of air). To illustrate these differences, imagine that we have the same amount of mass and the same amount of air volume (Figure 2). In the right panel of Figure 2, we have a single particle of a size of 2.5 µm. In the left one, if we split this single particle into particles of a constant diameter of 100 nm (0.100 µm) and maintain mass (and particle volume, assuming all the particles have the same composition) we will get a huge number of particles in the same amount of air volume. In fact, the image is not on real scale, if we assume same composition, we will actually get more than 15,000 particles of size 0.1 µm in order to match the volume of a single particle of 2.5 µg of diameter.
Figure 2. These two panels are a representation of how many particles we would get for the same mass concentration (assuming same chemical composition). Left panel represents particles of 0.1µm and the right panel represents a particle of 2.5µm. Drawing is not to scale, the left panel should be showing a higher number of particles (more than 15,000 particles). Source: prepared by the authors.
Further difficulties when measuring UFP number concentrations arise since, contrary to the mass concentrations, the number of particles is not conservative (Figure 3). Therefore, the current number of particles evolves rapidly, increasing with new emissions of particulates and new particle formation from pre-existing gaseous pollutants (this process of forming a new particle is called nucleation; Kulmala, 2003). Particle number concentration will decrease when processes of coagulation of particles take place (particles stick to each other, thus, diminishing the number of particles). Ultrafine particle size is also constantly evolving, affected by coagulation as well as condensation (such as organic vapours condensing onto the particle surface).
Why are UFP suspected to be more dangerous?
Ultrafine particles show different physicochemical properties due to their size and the sources that emit them. Size is a determinant of how deep a particle will get inside the human body. The nose and bronchioles are not able to filter UFP efficiently. Figure 4 shows that UFP could reach the alveoli of the lungs and, in fact, be transferred to the circulatory system (Oberdörster et al., 2004). Once in the circulatory system, they may target different organs in the body, including the brain.
Figure 4. Image showing how deep particles enter in the body system according to their size. Source: Guarieiro and Guarieiro (2013).
Moreover, they have very little mass, but very high surface area per volume. They also have very high reactivity. The deposition on tissues for ultrafine particles has been found to be much higher than for larger ones (deposition efficiency of 50% for particles of 20nm, while for particles in the range from 0.1 – 2.5 µm it diminishes to 10-20%, Oberdörster et al. 2005). All these characteristics may account for higher toxicity of UFP (Kelly and Fussell, 2012).
Ultrafine particles are mainly emitted by anthropogenic sources, what defines their chemical composition. In urban areas, motor vehicle exhaust has been identified as a major contributor to UFP concentrations, particularly diesel vehicles. The contributions of the different types of motor vehicle emissions are changing rapidly, and due to regulation to reduce particle emissions from diesel and other vehicles (HEI Panel on Ultrafine Particles, 2013).
Why is not there any regulation for UFP?
The instruments for measuring UFP number concentrations were developed much later that the instruments for measuring particle mass concentrations. In the last few decades, an increasing number of studies have been focusing in ultrafine particles, either to study their concentrations and dynamics on ambient air, or, more recently, to study the effects on health of the exposure to UFP.
Given that UFP number concentration decrease rapidly with increasing distance from the source (such as roadways, Zhu et al. 2002), their spatial distribution is highly variable, and might importantly differ from one location to another within the same city. Since they are not regulated, UFP particles are not routinely measured in most locations. Therefore, it is difficult to evaluate the exposure of the population living within the city to perform epidemiological studies to assess the health impact of these particles.
From the studies that attempted to evaluate the impact on health, they showed some evidence for UFP effects. However, the current evidence is still not sufficiently strong to conclude that short-term exposures to UFP have effects that are different from those of larger particles (HEI Panel on Ultrafine Particles, 2013). More studies are needed in this field to disentangle the effect of the tiniest particles that we all breathe.
More evidence on the health effects from UFP will allow the corresponding authorities to make a decision on the necessity (or not) of establishing a legal limit value for those particles and to determine which should be this limit value.
HEI Panel on Ultrafine Particles, 2013. Understanding the Health Effects of Ambient Ultrafine Particles, HEI Perspectives 3. Boston, MA.
Kelly, F.J., Fussell, J.C., 2012. Size, source and chemical composition as determinants of toxicity attributable to ambient particulate matter. Atmos. Environ. 60, 504–526. doi:10.1016/j.atmosenv.2012.06.039
Kulmala, M., 2003. How particles nucleate and grow. Science (80-. ). 302, 1000–1001.
Oberdörster, G., Oberdörster, E., Oberdörster, J., 2005. Nanotoxicology: An emerging discipline evolving from studies of ultrafine particles. Environ. Health Perspect. 113, 823–839. doi:10.1289/ehp.7339
Oberdörster, G., Sharp, Z., Atudorei, V., Elder, a, Gelein, R., Kreyling, W., Cox, C., 2004. Translocation of inhaled ultrafine particles to the brain. Inhal. Toxicol. 16, 437–45. doi:10.1080/08958370490439597
Zhu, Y., Hinds, W.C., Kim, S., Shen, S., Sioutas, C., 2002. Study of ultrafine particles near a major highway with heavy-duty diesel traffic. Atmos. Environ. 36, 4323–4335. doi:10.1016/S1352-2310(02)00354-0