Respiratory tract deposition of nano-particles from Biomass Combustion and Traffic;
impact of size-distribution and hygroscopicity on the deposited dose
Jakob Löndahl1, Erik Swietlicki1, Joakim Pagels2, Andreas Massling1, Christoffer Boman4, Jenny Rissler1, Elvira Vaclavik-BRäuner3, Steffen Loft3, Anders Blomberg5 and Thomas Sandström5
1Department of Physics, Lund University, PO Box 118, SE-221 00, Lund, Sweden
2Division of Aerosol Technology (EAT), Lund Institute of Technology, PO Box 118, SE-22100, Lund, Sweden
3Energy Technology and Thermal Process Chemistry, Umeå University, SE-901 87 Umeå, Sweden
4Department of Environmental and Occupational Health, Institute of Public Health, Copenhagen, Denmark
5Department of Respiratory Medicine and Allergy, University Hospital, SE-901 85 Umeå, Sweden
Keywords: ultrafine particles, respiratory deposition, health effects
INTRODUCTION
There is still an ongoing discussion about the relative importance of particle number, surface and mass concentration in health effects of an aerosol. Air quality guidelines usually regulate the mass concentration. However, several studies show a better correlation for dose-response when comparing with particle surface area. Furthermore, some studies indicate more severe effects for aerosols with small particles and thus a high number concentration per unit mass.
It is possible that the differences in health effects of various aerosols could be partly explained by a potentially large variation in the respiratory tract deposited dose of the particles. In this study the dose has been measured and calculated for particles from traffic and biomass combustion.
METHODS
The respiratory tract deposition of particles from traffic in a street canyon in Copenhagen, two types of biomass combustion and a hydrophobic reference aerosol was measured for healthy human subjects with a novel instrument named RESPI (Löndahl et al. 2006). Data which fulfilled our quality assurance protocol was obtained for 9 subjects (4f/5m) for the traffic aerosol and 10 subjects (4f/6m) for the two biomass combustion aerosols.
Hygroscopic growth at 90% relative humidity (RH) was measured with an H-TDMA. Based on these data and on the measured chemical composition, the particle hygroscopic growth factors at the RH prevailing in the lung (99.5%) were estimated. Thereby, the well-established ICRP dose model could be adjusted for particle water absorption. This was done in order to show consistency between measured deposition and theory. Finally, the total deposited dose was calculated based on the ICRP model for a normal healthy adult during relaxed breathing, exposed for 1 hour to 100µg/m3 of biomass combustion and traffic aerosols. The size distribution of the traffic aerosol is described by Kristensson et al. (2004) and was assumed to have a density according to Park et al. (2003).
Results and Conclusions
Figure 1 shows the measured size-resolved deposition fractions of the aerosols. Note that two different groups of subjects were exposed to the biomass combustion and traffic aerosols respectively. However, the lower deposition of the ultrafine (<100nm) biomass combustion particles is due their high hygroscopic growth factor. The ICRP model, adjusted for hygroscopic growth, and the experimental values yielded similar results.
Figure 1. Measured deposited fraction of the aerosols.
Table 1 shows the calculated deposited dose of the aerosol particles. The number concentration of the traffic aerosol per unit mass was substantially higher than for the biomass combustion particles. This, in combination with the higher probability of deposition resulted in a considerable difference in dose.
Table 1. Deposited dose for a healthy adult exposed to 100µg/m3 during 1 hour.
Number(x109) / Surface
(mm2) / Mass
(µg)
Efficient combustion / 7.2 / 170 / 3.0
Low temperature combustion / 2.4 / 180 / 6.7
Traffic / 6200 / 18800 / 14.3
This study represents the first evidence that hygroscopicity alters the respiratory tract deposition of real-world aerosols. It also shows the first measurements of respiratory tract deposition of biomass combustion and traffic aerosols. The findings demonstrate; a) that the respiratory tract deposition of biomass combustion aerosols is low compared to traffic, b) that it is essential to include particle water absorption in dose models and c) that the deposited dose has to be carefully calculated when estimating the toxicological potential of air pollution particles for a given exposure.
ACKNOWLEDGEMENTS
This work was supported by the “Swedish National Air Pollution and Health Effects Program” (SNAP) and the Swedish research council FORMAS.
REFERENCES
Kristensson A, Johansson C, Westerholm R, Swietlicki E, Gidhagen L, Wideqvist U, Vesely V. 2004. Real-world traffic emission factors of gases and particles measured in a road tunnel in Stockholm, Sweden. Atmospheric Environment 38:657-673.
Löndahl J, Pagels J, Swietlicki E, Zhou JC, Ketzel M, Massling A, Bohgard M. 2006. A set-up for field studies of respiratory tract deposition of fine and ultrafine particles in humans. Journal of Aerosol Science 37:1152-1163.
Park K, Cao F, Kittelson DB, McMurry PH. 2003. Relationship between particle mass and mobility for diesel exhaust particles. Environmental Science & Technology 37:577-583.