Effectiveness of respirator protective equipment for nanoparticle exposure control (2000 - 2016) : a systematic review and meta-analysis

Abstract

More applications for engineered nanoparticles are being explored. In addition to incidental nanoparticles, workers in some industries are potentially exposed, mostly via inhalation. While the toxicity of the various nanoparticles found in the workplace air is inconclusive, some studies have reported toxicity at the cellular level and therefore respiratory protection as a control measure is recommended. Respirators, also referred to as respiratory protective equipment, particularly those with electret filters, are however certified for protection against micro-sized particles, where a most penetrating particle size (MPPS) of 100 – 400 nm is assumed, may not be effective for nanoparticles. Commonly used N95 particulate respirators are certified by the National Institute for Occupational Safety and Health (NIOSH) after passing a 95% collection efficiency test. The test uses a charge neutralised polydispersed sodium chloride (NaCl) with a 347 nm mass median aerodynamic diameter at 85 l/min. Respirators used for particulates in South Africa are certified by European Notification Bodies following the European Norm (EN) 149:2001 standard. The filters are also tested with non-neutralised polydispersed NaCl with a mass median particle size diameter of 600 nm at 95 l/min. The respirators are tested against an ability to remove either 80%, 94% or 99% of the particles. Both the NIOSH and EN tests use a light scattering photometer that measures the particle concentration upstream and then downstream and respirator efficiency reported as a function of mass load. Experimental studies have been conducted to assess N95 respirator penetration specifically by nanoparticles. These have used particles in the size range that is lower than what is prescribed in the testing standards and analysing particle counters, which are more sensitive to nanoparticles. This systematic review and meta-analysis was aimed at systematically reviewing studies that tested the effectiveness of N95 respirators against nanoparticle exposure and meta-analysing the average penetration outcome. The review was conducted following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Published and grey literature from 2000 to 2016, were searched by one reviewer from online databases and related conferences proceedings using specific key words. The title and abstract of the studies was screened for relevance by the reviewer. Full texts of the studies that met the criteria for the nature of the study, the type of respirators and the size of the particles used and those whose eligibility during the screening was unclear, were obtained. A total of 14 studies were reviewed qualitatively, while 13 of these with 29 data points were included in the meta-analysis. Sensitivity analysis was performed for only studies that used a manikin-based protocol, to assess whether this influenced the overall outcome. Subgroup analysis was also performed for those studies that used polydispersed vs monodispersed aerosols and manikin-based vs non- manikin protocol studies. The MPPS for all studies was in the nano-sized particle range, with the lowest at approximately 36 nm. In four of the studies the estimated mean penetration percentage (PP) was between was 1– 6%, exceeding the 5% threshold set by NIOSH for N95 respirators. The meta-analysis and weighted mean results were however, below the 5% criterion. Mean PP for sensitivity and all subgroup analyses were also below the NIOSH threshold reference. This means that commonly used N95 respirators may be effective for nanoparticles in workplaces, but subject to factors including characteristics of respirators and the dispersity of particles. Standard respirator testing protocols for nanoparticles, which should include the use of large sample sizes, are essential to limit inconclusive findings. More studies that test European Norm (EN) - certified respirators for nanoparticles are needed to assist in policies towards control measures of occupational nanoparticles exposure in South African workplaces.