Personal dust and vapor sampling
An overview of individual particulate monitoring  
Sampling conventions
Many people work in dusty environments where there may exist a certain level of risk that they will breathe toxic chemicals or particles that will have a detrimental effect on their health. Working in dirty or dusty environments in the workplace exposes a worker to the risk of inhalation into the air passages and ultimately the lungs. The size convention of particulates varies and is classified by how far they can travel into the respiratory system. Total inhalable fraction is the name given to the mass fraction of total airborne particles that are inhaled through the mouth and nose. Larger particles are filtered out by the nose and throat. These are typically just visible and have a mean diameter of about 10 microns (10-6 m). The thoracic fraction is the mass fraction of inhaled particles that pass further than the larynx. The respirable fraction is the mass fraction that penetrates down to the lowest parts of the unciliated airways. The smaller particulates will find their way into the throat and thoracic region whilst the very smallest may insinuate themselves into the smallest passageways of the lungs and are considered the most dangerous. Particle sizes smaller than about 4.5 microns are considered in this respirable category.
 
schematic of human lungs showing total, thoracic and respirable regions
What do we measure
Personal air sampling consists of drawing air through a suitable filter fitted in a cassette and measuring the mass increase as the "dirty air" passes through it. A personal sampling pump running at a constant flow rate is used to collect the sample which is then analyzed by a gravimetric process to obtain the mass of particles on the filter. This work involves weighing the filter "clean" at the start of the work and then again "dirty" at the end of the run. The difference in weight between dirty and clean in milligrams (mg) gives the mass needed. The pump must run at a constant flow rate in liters per minute (L/m) so that if the run duration is known in minutes (min) the total sampled volume can be calculated (L/min x min) and expressed in liters or cubic meters (L or m3). The concentration of particles is then expressed as mass per unit volume (mg/m3). An additional mesh or size selective filter is often used to trap just the particles of a certain size for examination by gravimetric methods outlined here. These Polyurethane Foam (PUF) Filters are inserted into the cassette before the filter that has been weighed so that the most dangerous particles are captured and analyzed.
 
example of a sampling head and various PUF filters
Limits on exposure
There are standards in the leading industrialized countries to specify concentrations that are currently considered to represent an acceptable balance between worker safety and the cost of complete removal of these hazards. In the USA some of the main organizations that publish methods for monitoring dusts and particulates are OSHA and NIOSH. Other overseas organizations are also actively involved in designing methods and techniques for sampling. These include the Institute of Occupational Medicine (IOM) in the UK and the Institut de Recherché en Santé et en Sécurité du Travail (IRSST) in Canada. Limits are specified for many substances in terms of their Time Weighted Average (TWA) value plus the recommended volume or sample rate that should be used. Different filter cassettes have been developed over the years to focus on certain types of particulates. These include the cyclone sampler, the (IOM) sampler, the respirable sampler, the asbestos sampler etc. It is also possible to sample for vapors in the atmosphere by drawing the air through a sorbent tube that contains a chemical that reacts in a known way with the vapor under investigation. Again a personal sampling pump is used to collect the sample. Usually a lower flow range in the order of a few tens of mL/min is required to give the sorbent material time to absorb the vapor. A personal sampling pump fitted with a low flow adapter in the inlet line is needed to accomplish this type of monitoring.
 
example of a filter fitted into a standard 37mm cassette
Hardware used
Key features of suitable sampling pumps used for these types of monitoring are light weight since it must be worn for extended periods by the worker. Efficient battery operation to prolong the useful working time on the worker. Quick recharge times and a choice of power supply options to get the pump back up and running after each use are important too. Small physical size and a low noise pump help the wearer to accept wearing the pump. The pump must hold the selected flow rate constant over the duration of the run and have a linear flow rate over a wide range to give the user the chance to collect samplers at a faster rate where circumstances dictate. For exposures that are only likely to occur for short periods a flow rate of up to 5 L/min is a distinct advantage. The Casella Apex range of personal sampling pumps are ideal for all the applications listed above and can be used in a very wide range of situations. The Casella Microdust pro can also be used for real time particulate monitoring as part of a sampling regime.
example of an Apex pro personal sampling pump

page last updated - Wednesday May 31, 2006