Evaluation of an electrical aerosol detector (EAD) for the aerosol integral parameter measurement

The performance of an electrical aerosol detector (EAD; TSI Model 3070A) was experimentally evaluated for measuring the integral parameters of particles (i.e., total length concentration of particles, and the total surface area concentrations of particles deposited in a human lung). The EAD consists of a unipolar diffusion charger with an ion trap, and aerosol electrometer. We first evaluated the performance of the EAD charger. Both polydisperse and monodisperse particles of Ag, NaCl, and oleic acid (with the dielectric constants of infinite, 6.1 and 2.5) were then generated to evaluate the particle material effect on the EAD readout.     

Rationale and principle of an instrument measuring lung deposited nanoparticle surface area

The risk of nanoparticles by inhalation for human health is still being debated but some evidences of risk on specific properties of particles <100 nm diameter exist. One of the nanoparticle parameters discussed by toxicologists is their surface area concentration as a relevant property for e.g. causing inflammation. Concentrations of these small particles (~ <100 nm) are currently not measured, since the mass concentrations of these small particles are normally low despite large surface area concentrations. Airborne particles will always be polydisperse and show a size distribution. Size is normally described by an equivalent diameter to include deviations in properties from ideal spherical particles. Here only nanoparticles below a certain size to be defined are of interest. Total concentration measures are determined by integration over the size range of interest. The ideal instrument should measure the particles according to the size weighting of the wanted quantity. Besides for the geometric surface area the wanted response function can be derived for the lung deposited surface area in the alveolar region. This can be obtained by weighting the geometric surface area as a function of particle size with the deposition efficiency for the alveolar region for e.g. a reference worker for work place exposure determination. The investigation of the performance of an Electrical Aerosol Detector (EAD) for nearly spherical particles showed that its response function is close to the lung deposited surface areas in different regions of the human respiratory system. By changing the ion trap voltage an even better agreement has been achieved. By determining the size dependent response of the instrument as a function of ion trap voltage the operating parameters can be optimized to give the smallest error possible. Since the concept of the instrument is based on spherical particles and idealized lung deposition curves have been used, in all other cases errors will occur, which still have to be defined. A method is now available which allows in principle the determination of the total deposited surface area in different regions of the lung in real time. It can easily be changed from one deposited region to another by varying the ion trap voltage. It has the potential to become a routine measurement technique for area measurements and personal control in e.g. work place environments.     

Conceptual limitations and extensions of lung-deposited Nanoparticle Surface Area Monitor (NSAM)

Nanoparticle Surface Area Monitor (NSAM, TSI model 3550 and Aerotrak 9000) is an instrument designed to measure airborne surface area concentrations that would deposit in the alveolar or tracheobronchial region of the lung. It was found that the instrument can only be reliably used for the size range of nanoparticles between 20 and 100 nm. The upper size range can be extended to 400 nm, where the minimum in the deposition curves occurs. While the fraction below 20 nm usually contributes only negligibly to the total surface area and is therefore not critical, a preseparator is needed to remove all particles above 400 nm in cases where the size distribution extends into the larger size range. Besides limitations in the particle size range, potential implications of extreme concentrations up to the coagulation limit, particle material (density and composition) and particle morphology are discussed. While concentration does not seem to pose any major constraints, the effect of different agglomerate shapes still has to be further investigated. Particle material has a noticeable impact neither on particle charging in NSAM nor on the deposition curves within the aforementioned size range, but particle hygroscopicity can cause the lung deposition curves to change significantly which currently cannot be mimicked with the instrument. Besides limitations, possible extensions are also discussed. It was found that the tendencies of the particle deposition curves of a reference worker for alveolar, tracheobronchial, total and nasal depositions share the same tendencies in the 20–400 nm size range and that their ratios are almost constant. This also seems to be the case for different individuals and under different breathing conditions. By means of appropriate calibration factors NSAM can be used to deliver the lung deposited surface area concentrations in all these regions, based on a single measurement.