The convention to be used is not always straightforward, but generally depends on what part of the respiratory system is affected by the aerosol particles. For example, if an aerosol (for example, silica) is expected to be hazardous mainly in the alveolar regions of the respiratory system, then the respirable convention applies. On the other hand, if an aerosol is extremely soluble (for example, KCN), then the inhalable convention should be used for monitoring or setting exposure limit standards. The conventions are often applied for approximating mass fractions, but they may also be used in the evaluation of total surface area or the number of particles in the collected material.

The conventions have now been adopted by the International Standards Organization (Technical Report ISO TR 7708), the Comit� Europ�en de Normalisation (CEN Standard EN 481), and the American Conference of Governmental Industrial Hygienists (ACGIH) (1). The definition of respirable aerosol is the basis for recommended exposure levels (REL) of respirable coal mine dust as promulgated by NIOSH (Criteria for a Recommended Standard, Occupational Exposure to Respirable Coal Mine Dust). The respirable aerosol definition also forms the basis of the NIOSH sampling method for respirable particulates not otherwise regulated (NIOSH Manual of Analytical Methods).

The conventions constitute a part of the performance characteristics required of aerosol samplers for collecting aerosol according to the relevant health effects. This guide therefore does not specify particular samplers for measuring the aerosol fractions defined here. Detailed guidelines for evaluating any given sampler relative to the conventions are under preparation. Several advantages over instrument specification can be attributed to the adoption of these performance-associated conventions:

The conventions have a recognized tie to health effects.

Performance criteria permit instrument designers to seek practical sampler improvements.

Performance criteria promote continued experimental testing of the samplers in use with the result that the significant variables (such as wind speed, particle charge, and so forth) affecting sampler operation become understood.

1.1 This guide defines conventions for personal samplers of specific particle-size-dependent fractions of any given non-fibrous airborne aerosol. Such samplers are used for assessing health effects and in the setting of and testing for compliance with permissible exposure limits in the workplace and ambient environment. The conventions have been adopted by the International Standards Organization (Technical Report ISO TR 7708), the Comit� Europ�en de Normalisation (CEN Standard EN 481), and the American Conference of Governmental Industrial Hygienists (ACGIH) (1). The conventions were developed (2) in part from health-effects studies reviewed (3) by the ACGIH and in part as a compromise between definitions proposed by the ACGIH (3) and by the British Medical Research Council (BMRC) (4). Conventions are given here for inhalable, thoracic, and respirable fractions.

1.2 This guide is complementary to Test Method D4532, which describes the performance of a particular instrument, the 10-mm cyclone, and operational procedures for use. The procedures, specifically the optimal flow rate, are still valid although the estimated accuracy differs somewhat from use with previous aerosol fraction definitions. Details on this instrument and also the Higgins-Dewell cyclone have recently been published (5-7).

1.3 Limitations:

1.3.1 The definitions given here were adopted by the agencies listed in 1.1 in part on the basis of expected health effects of the different size fractions, but in part allowing for available sampling equipment. The original adoption by CEN was, in fact, for the eventual setting of common standards by the EC countries while permitting the use of a variety of instrumentation. Deviations of the sampling conventions from health-related effects are as follows:

1.3.1.1 The inhalable fraction actually depends on the specific air speed and direction, on the breathing rate, and on whether breathing is by nose or mouth. The values given in the inhalable convention are for representative values of breathing rate and represent averages over all wind directions.

1.3.1.2 The respirable and thoracic fractions vary from individual to individual and with the breathing pattern. The conventions are approximations to the average case.

1.3.1.3 Each convention applies strictly to a fraction penetrating to a region, rather than depositing. Therefore, samples collected according to the conventions may only approximate correlations with biological effects. For example, the respirable convention overestimates the fraction of very small particles deposited in the alveolar region of the respiratory system because some of the particles are actually exhaled without being deposited (8). In many workplaces, these very small particles contribute insignificantly to the sampled mass. Furthermore, the large variability between individuals and the details of clearance may be as important as this type of effect.

1.3.1.4 The thoracic convention applies to mouth breathing, for which aerosol collection is greater than during nose breathing.

1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.