FAQ's

Does the health hazard associated with airborne particles depend upon the particle size?
Yes it does. Both toxicity and the deposition efficiency in the respiratory tract are influenced by the particle size.

What is the deposition efficiency?
The deposition efficiency is determined as a ratio of the number of deposited particles in the respiratory system to the number of inhaled particles. It depends on the particle size.

How to evaluate the health risk caused by exposure to airborne particles?
The health risk is influenced by the toxicity of the particles and the dose accumulated by individuals during the exposure time.

Is it possible to reduce the accumulated dose and the health risk?
Yes, it is possible. Accumulated dose is influenced by the exposure time number concentrations and the size of particles. There are two ways of reducing the health risk:

•    The first – the total reduction of the number of particles in the air;
•    The second - a cost-effective way is to reduce the number of particles with the greatest deposition efficiency. Particles in different size ranges exhibit different deposition efficiency. An example of the cost-effective way is in the case studies.

What is the accumulated dose?
Accumulated dose is the amount of particulate matter deposited in the respiratory system during the exposure time. It can be measured in numbers or in the mass of particles or the mass of a particular chemical element (e.g. lead) or compound.

What is a nanoparticle?
•    1 nanometre (nm) is 1 millionth of a millimetre.
•    Nanoparticles are particles of less than 0.1 micrometer (µm) in diameter

How does particle shape affect distribution?
•    Particle size: For spherical particles particle size is the diameter of the particle. Real particles in the air often have complicated shapes. For non-spherical particles, the term "diameter" does not strictly applicable. For example, a flake or a fibre has different dimensions in different directions. Particles of identical shape can be composed of different chemical substances and have different densities. The differences in shape and density cause considerable confusion in defining particle size.
•    Aerodynamic diameter: The term "aerodynamic diameter" has been introduced in order to provide a single parameter of describing real non-spherical particles having of arbitrary shapes and densities. The aerodynamic diameter is the diameter of a spherical particle having a density of 1g/cm3 that has the same inertial property (terminal settling velocity) in the air (at standard temperature and pressure) as the particle of interest. Inertial sampling instruments like cascade impactors enable the aerodynamic diameter to be determined. The term "aerodynamic diameter" is convenient for all particles including clusters and aggregates of any forms and density. It is not a true geometric size because non-spherical particles usually have lesser terminal settling velocity than the spherical one

What is Brownian diffusion of particles?
•    Brownian diffusion is erratic random movements of particles in a gas medium. Brownian diffusion is the dominant collection mechanism for particles less than 0.3 micrometer. It is especially significant for nanoparticles. Nanoparticles in a gas medium deflect slightly when gas molecules strike them. It causes the diffusion.
•    Diffusivity is a measure to which molecular collisions influence small particles. The diffusion coefficient is influenced by the particle size and gas medium conditions

What is particle size terminology?
•    Since the range of particle sizes for air quality evaluation is quite broad it is convenient to divide this range into smaller sections. Defining different size sections is useful since particles of different sizes behave differently and are deposited in different parts of the respiratory system. Four size sections historically have been defined.

What is total suspended particulate matter?
•    Total suspended particulate matter (TSP) particles refers to the entire aerosol size range

What is PM10?
•    PM10 is particulate matter collected with a sampling collection device having a cut off aerodynamic diameter of 10 micrometers (with 50% collection efficiency at 10 micrometers aerodynamic diameter).

What is the benefit of size distribution measurement?
•    The particle size distribution can be very broad. It is influenced by the creation of particles during the industrial process and its evolution.
•    Here are some benefits of particle size distributions:
•    The efficiency of the particle collection depends on the particle size.
•    The particle size distribution determines the operating conditions necessary to collect the particles.
•    Particle size distributions are important in determining the behaviour of particles in the respiratory tract and they affect the exposure to the particulate matter.

What is the particle size distribution?
•    Particle size distribution is a particle frequency distribution that shows the fraction or number or mass of particles found in each size range. Frequency can be plotted (on the Y-axis) by percentage, in addition, number count, surface area, or mass can be plotted. A histogram is one way to display a particle size distribution.

Is the total mass measurement sufficient to evaluate the health risk associated with exposure to airborne particle pollution?
•    According to numerous studies it is apparent that measuring exposures against mass is not sufficient. Particles of different sizes cause different degree of health damage and hence responsible for different degree of health risk.

Who is responsible for providing safe environment at your company working places?
•    Every company has a legal obligation and a duty-of-care to protect its employees and people in the workplace. A responsible organisation prefers prevention to compensation, after all in the eyes of the law ignorance is negligence. Do not repeat the mistakes of the asbestos industry.

Are nanoparticles toxic?
•    Yes, some particles that are harmless in bulk or micro-size form become considerably more toxic in the nano-form. The toxicity of insoluble materials increases with decreasing particle size, on a mass for mass basis.

A few facts:

•    Nanoparticles can cross the blood-brain barrier in humans and gold nano-particles can move across the placenta from mother to foetus

•    Early studies with PTFE (polytetrafluoroethylene) particles around 20 nm in diameter showed that airborne concentrations of a supposedly inert insoluble material lower than 50 µg/m3 could be fatal to rats

•    Nano-tubes produce a more toxic response in rats than quartz dust

•    Engineered carbon molecules known as "buckyballs" cause brain damage in fish.

Can one extrapolate manufactured nanoparticle toxicity using existing micro-particle and bulk toxicological databases?
•    No, ability to extrapolate manufactured nanoparticle toxicity using existing particle and fibre toxicological databases are very limited. The toxicity of nanoparticles is influenced by the size and increases with decreasing particle size, on a mass for mass basis.

What is the most cost-effective way of improving air quality at working places and avoiding health related litigation?
Air quality engineering based upon revealing the hot spots of emission of the most dangerous fraction of the particulate matter is the best way of solving the problem. Health risk is influenced by the local micro-environment and the particle size distribution. Identification of the main source of the health risk and solving the problem on a local scale can reduce the costs considerably. Reducing the emission at the main spot can often be sufficient to comply with air quality standards. It is cheaper than reducing the emission of the all-possible sources across the production area.

Some relevant literature:

Hinds W.C. (1999) Aerosol technology. Properties, behaviour and measurement of airborne particles.
N.-Y.: J. Wiley and Sons, pp. 233-259.

Aerosol Measurement. Principles, Techniques and Applications. (2001) Ed. PA Baron and K Willeke.
N.-Y.: J. Wiley and Sons, pp. 99-116.