There are two main operational systems for conducting BSL-4 work throughout the world; manipulations within a totally enclosed cabinet line systems and a suited system, where workers wear a protective suit while manipulating Risk Group 4 Pathogens (RG4) in combination with other engineered controls such as class II cabinets or laminar flow. Cabinet line systems protect the worker and the immediate environment through a combination of good microbiological techniques and the use of rigid animal isolators (for high containment animal work) or a cabinet line consisting of Class III microbiological safety cabinets (for in vitro work). Positive pressure air-fed suits are designed for positive pressure to prevent contamination to the wearer even in the event of damage to the suit.
Over the last 30 years within the UK, cabinet line systems have been routinely used and the Defence Science and Technology Laboratory (Dstl) Porton Down, UK has been working with Advisory Committee on Dangerous Pathogens (ACDP) containment level 4 (equivalent to BSL-4) rigid half suit isolators and microbiological safety cabinet line (CL III cabinets) for 10 years with Risk Group 4 pathogens [1
]. Throughout the US, Canada and Europe the use of positive pressure suits have been used.
There are advantages and disadvantages associated with each system, and is very much dependent on the tasks to be performed. For example, if the majority of work performed is the routine handling of diagnostic samples, then cabinet lines may offer a simpler and more efficient system within which to work. Alternatively, if the work involves large non-human primates housed within high containment conditions or the use of large items of equipment, then suited systems offer a more appropriate alternative. Unlike microbiological safety cabinets, there is limited published data on the operator protection factor (OPF) afforded by positive pressure suits [2
]; therefore, tests were devised by the Physical Sciences Department at Dstl to evaluate these suits. The operator protection factor is defined as the ratio of exposure to airborne contamination generated on the open bench to the exposure resulting from the same disposal of airborne contamination generated within the cabinet. When tested in accordance with the British Standard all cabinets in use should have an operator protection factor of at least 1.0 × 105
. The authors are unaware of any equivalent standard applicable to positive pressure suits.
The aim of this study was to assess the operator protection factor of positive pressure suits against an airborne microbiological challenge.
The objective of this study was to assess the protection afforded by full positive-pressure suits against an aerosol challenge under normal and realistic accident scenarios. There have been few studies performed previously to assess the operator protection factors of suits with which to compare the current data. The operator protection tests demonstrated that an exceptionally high level of protection was provided by the suited system under normal working conditions. Tests demonstrated that the OPF was significantly higher than the minimum required OPF of 105
, recommended for microbiological safety cabinets. Such levels of protection are equivalent to those reported previously within a cabinet line system [1
When the integrity of the suit was compromised by a small cut in the glove the protection of the suit was reduced by approximately ten-fold, however this was still above the minimum recommended protection level (105). It is likely that the ingress seen at the glove is due to the lowered airflow in the gloves of the suit, due to constriction of airflow under the suit at the wrists. As long as the suit remained at positive pressure, relative to the external environment however, protection levels greater than 105 were maintained.
When compromised by a cut in the leg the positive pressure inside the suit prevented ingress of the challenge micro-organisms. It is likely that the air-flow within the leg of the suit was greater than that seen at the wrist and was sufficient to maintain positive pressure to prevent ingress of micro‑organisms. In all of the suits tested there are no filters or air pipes positioned near any of the compromised areas. In all cases, no B. atrophaeus penetration of the respiratory tract was observed.
The operator protection factor provided by full positive pressure suits in this study was comparable to the protection factors calculated for a rigid half-suited isolator system tested under similar accident scenarios (in-house data). It was not possible to test the protection of the suits under loss of positive pressure in this experimental set-up as the respiratory tract sampling air caused a negative pressure within the suit once supply air had been turned-off.
There is very little data published previously on protection factors of positive pressure suits. Kumin et al.
evaluated three different types of BSL-4 suits for their material compatibility against decontamination disinfectants, protection factors against chemicals and comfort for users [2
]. Chemical protection factor tests included simulating movement whilst in the laboratory and chemical protection of intact suits. Direct comparison between the current study and that of Kumin et al.
however, is difficult, as the current study looked at protection against biological aerosols whereas Kumin et al.
assessed chemical protection against VX gas.
The tests in this current study used an aerosol microbiological challenge to measure the protective efficacy of the suited system. Published aerosol infective doses for Ebola virus in rhesus monkeys are as low as 400 plaque-forming units (pfu) [4
] and Marburg virus has been shown to be lethal by the aerosol route in rhesus monkeys [6
], grivets [7
], and has an aerosol infective dose of less than 10 tissue culture infective doses (TCID50
) in marmosets (manuscript submitted) and mice [8
]. It was shown that 1 pfu was approximately equivalent to 25–30 virions [9
]. Therefore, the infectious dose for rhesus monkeys could be between 10,000–12,000 virions. Filovirus-infected non-human primates represent an appropriate animal model for predicting human infectious doses of filoviruses [10
], therefore the aerosol-infectious dose for humans may be reasonably assumed to be similar. In a monodispersed aerosol (of size range 1–3 µM) of bacterial spores (as used in the current study), each particle would usually contain one bacterium. A suspension of virus at an equivalent starting concentration would generate particles each reasonably expected to contain a number of virions (up to ten-fold more based on average filovirus dimensions of approximately 80 nm × 800 nm). Based on these theoretical calculations therefore, a damaged suit with a ten-fold reduction in protection, challenged with an aerosol of approximately 1.0 × 108
virions, would theoretically let in approximately 100 virions. This is equivalent to approximately 4 pfu therefore even in the worst case scenario this would be approximately 100 times less than the theoretical infectious dose for rhesus monkeys (and by extrapolation humans).
Many laboratory-acquired infections have been reported in the scientific literature [11
]. Historically, procedures such as centrifugation or needle stick injuries are the main cause of laboratory-acquired infections [13
]. Clearly, for needle stick injuries, positive pressure suits offer no greater protection than safety cabinets. In summary, positive pressure suited systems provide exceptionally good operator protection, even under accident scenarios, and potentially provide greater flexibility and ergonomic benefits to the user.