Means of protecting workers from short and thin asbestos fibres

In February 2009 the Agency published a collective expert appraisal report and issued an opinion on short asbestos fibres (SAF) (fibre length < 5µm, diameter < 3µm and L/D ratio of ≥ 3), and thin asbestos fibres (TAF) (fibre length ≥ 5µm, diameter < 0.2 µm and L/D ratio of ≥ 3).

Concerning the occupational environment, the recommendations of this report primarily indicated the need to take into account TAFs whose carcinogenicity was confirmed in the expert appraisal and consequently to replace the method of counting fibres by optical phase contrast microscopy (PCM) with analytical transmission electron microscopy (TEM).

In its Opinion, the Agency also recommended a reassessment of collective (CPE) and personal protective equipment (PPE) in order to evaluate their effectiveness with respect to these two types of fibre.

As a result of this work, the Agency received a solicited request from the French Ministry of Labour in March 2009 to assess the effectiveness of collective and personal protective equipment as a means of protecting workers from exposure to short and thin asbestos fibres. This involved conducting three different types of study:

• the identification of existing collective protection methods and personal protective equipment against asbestos in the workplace;
• an assessment of their effectiveness against various asbestos fibres (TAF, SAF), by indicating their potential limitations; 
• a definition of types of personal protective equipment adapted according to:
  • duration of activities and wearing of this equipment;
  • dust accumulation levels;
  • techniques used;
  • nature of the operations;
  • new exposure limits to be observed;
  • PPE protective factors.

To the extent that standardised tests to assess the performance of personal protective equipment on the market are conducted with sodium chloride aerosols whose size is representative of that of the most penetrating particles (most penetrating particle size [MPPS]), it was deemed essential, prior to any assessment work, to determine whether the asbestos fibres (all fibres taken together: TAF, SAF and WHO-defined– fibre length ≥ 5µm, diameter ranging between 0.2 and 3 µm and a L/D ratio of ≥ 3 [AFSSET, 2009]) could also be considered together with these sodium chloride aerosols, with respect to their filtration behaviour.

Furthermore, the issue of the effectiveness of PPE against asbestos needed to be better understood by identifying the various factors that can influence their effectiveness.

Work method

It was therefore proposed to conduct a technical overview of the knowledge and requirements relating to collective and personal protective equipment. This work included:

• the identification of regulatory requirements for collective and personal protection with respect to asbestos;
• the identification of standards for this personal protection equipment against asbestos;
• an inventory of collective protective equipment and personal protective equipment for use against occupational exposure to asbestos;
• a review of the literature on the effectiveness of protective equipment (CPE and PPE) with respect to asbestos, in order to identify:
• the parameters that play a major role in the effectiveness of personal protective equipment (leak penetration, filter penetration, etc.);
• the various factors that can influence the effectiveness of CPE and PPE;
• the studies conducted specifically to assess the effectiveness of PPE against asbestos (standards testing and field testing); publications dealing with the relevance of considering asbestos fibres together with MPPS particles, i.e., dealing with the comparison of levels of effectiveness of protection determined using asbestos fibres compared to protection levels obtained for an aerosol of MPPS particles.

Results

This report, which identified the regulatory requirements for asbestos in terms of collective protection methods and personal protective equipment in the workplace, as well as standards covering all these protection systems, emphasised the followingpoints:

• there is in 2011 a distinction between regulations concerning the removal of materials containing friable asbestos and the removal of materials containing non-friable asbestos, particularly with respect to measures to be implemented (in terms of containment and protection methods). The relevance of this distinction merits consideration upon completion of the report, but to do so requires knowledge of exposure levels resulting from work entailing exposure to these two types of asbestos; 
• there are differences in the methods used in testing to assess filter penetration: one for the very high efficiency particle filters used in ventilation systems and the other for respiratory protection equipment filters. The issue of the application of the approach, which is to look for the maximum particle size for the penetration first, before conducting the penetration assessment tests, should be examined within the context of certification testing for respiratory protective equipment; 
• certification standards testing for the placing on the market of respiratory protective equipment use a sodium chloride aerosol and apply flame photometry as an analytical measurement technique. This technique, which consists in measuring concentration by particle weight, therefore underestimates the contribution of the finest particles in the assessment of penetration of the filters used on respirators. Thus, for asbestos fibres, whose pathogenicity is related to the number rather than the weight of inhaled fibres, it may be more appropriate to adopt an approach of determining the number of particles instead of the weight.

With respect to the relevance of inferring effective protection under actual conditions of use from conditions established by standards testing, it appears that although the standards tests take into consideration the dominant factors in the effectiveness of collective protective equipment and respiratory protective equipment, some reports recommend that filter penetration tests be conducted at higher air intake rates, to better simulate the breathing rate during a heavy workload.

It should also be remembered that filter penetration is one component among the possible sources of leak penetration in respirators and that it only contributes very slightly to total inward leakage (the main leakage being face seal leaks). Thus, even though filter performance is improving, the fact remains that the problem of facial sealing continues and that this phenomenon is still rarely studied in real life situations at the present time (several authors are studying induced leakage [piercing of the facepiece, faulty valve, etc.]) but are not concentrating on the study of characteristics of face seal leaks. The protection level provided by a respirator that requires adjustment at the face (mask, half-mask, etc.) could be improved by checking the fit using the adjustment factor. A number of studies emphasise the improvements brought about by this prior verification and by establishing a respiratory protection programme as described in the French Standard NF EN 529.

Thus, all the studies designed to determine the effectiveness of protection under actual-use conditions (exposure to asbestos or other particles) demonstrate that the level of protection offered by respirators is below that stipulated by the standards tests. Nominal protection factors, deduced from standards testing, are therefore not representative of the level of protection offered by respiration equipment in real life situations.

With respect to verifying the effectiveness of CPE and PPE against thin asbestos fibres and short asbestos fibres, it appears that the protection efficiency of respiratory protective equipment depends on the size of the aerosol particle, rather than the fibrous form of the aerosol. The relevance of reassessing the protection factors of respirators against an aerosol with particle sizes similar to that of thin asbestos fibres and short asbestos fibres should therefore be examined.