Increasing health and safety regulations require stringent controls on the amount of potentially harmful dust that workers can be exposed to during their time at work. Some substances present in the air are extremely harmful in the short term while many can be uncomfortable if ignored over the long term.
Particles are classified by how far they can travel into the respiratory system, with the smallest particles considered the most dangerous. Total Inhalable Fraction is the term given to the mass fraction of total airborne particles inhaled through the mouth and nose. The Thoracic Fraction is the mass fraction of inhaled particles that pass further than the larynx, representing particles with a mean aerodynamic diameter of less than 10 microns. Particle sizes smaller than about 4.5 microns are considered in the Respirable Fraction. These particles penetrate down to the lowest parts of the airways. The most minute may insinuate themselves into the smallest passageways of the lungs.
Using Personal Air Sampling
Personal air sampling allows individual employees to test surrounding air quality as they go through their workday. 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 run 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 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 may be used to trap just the particles of a certain size for examination. Filter cassettes have been developed to focus on specific types of particulates, such as the cyclone sampler, the (IOM) sampler, the respirable sampler, and the asbestos sampler.
It is also possible to sample for vapors in the atmosphere by drawing the air through a sorbent tube containing a chemical that reacts in a known way with a vapor under investigation. Again, a personal sampling pump is used to collect the sample. A pump fitted with a low flow adapter with a range of a few tens of mL/min is needed to give the sorbent material time to absorb the vapor.
Pump Characteristics
In general, personal sampling pumps should be lightweight, compact, and quiet since the pump must be worn by a worker. Efficient battery operation, quick recharge times, and a choice of power supply options are also important. In addition, 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 occur only for short periods of time, a flow rate of up to 5 L/min is an advantage.
Personal sampling pumps calculate only an average concentration level that is generated over the complete duration of the measurement period, leaving no evidence of when levels were high or when they were low. Collecting time history information about the temporal distribution of levels during a run is possible, however, by monitors equipped with datalogging. These real-time instruments log particulate concentrations at regular intervals and store these values in memory. Datalogging is invaluable for measurements of high concentrations that only last for short periods of time, such as in pharmaceutical plants that mix powders in open vats during a manufacturing process. Periods that show significant high values must be tackled first in any control scheme.
Balancing Safety vs Cost
Standards in the leading industrialized countries specify concentrations currently considered to represent an acceptable balance between worker safety and the cost of complete removal of these hazards. In the U.S. the main organizations that publish methods for monitoring dusts and particulates are OSHA and NIOSH. Overseas organizations are also actively involved in designing methods and techniques for sampling, including 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 Time Weighted Average (TWA) value plus the recommended volume or sample rate that should be used.