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The Role of Filtration in Protecting Clinicians from Contagious Respiratory Pathogens

When a patient is brought into a hospital with respiratory distress, the safety of the patient, caregivers, other patients, and even family members is on the line. In busy hospital environments today, it’s vital that clinicians control the spread of contagious respiratory pathogens like SARS, influenza, tuberculosis, or the novel coronavirus (COVID-19).

Some patients who present with respiratory distress will require ventilation. Ventilator filters can play a key role in protecting the safety anyone entering the environment of patients on mechanical ventilation by reducing the risk of cross contamination. Filtration can also protect your hospital staff by helping to prevent the inhalation of harmful pathogens and the contamination with bacteria and viruses that can lead to the spread of infection.

Related: Learn how we are responding to the COVID-19 pandemic.


During a 2003 North American SARS outbreak, two hospitals in Canada housed SARS patients with very different results. In Vancouver, three imported cases developed into only one secondary case, while in Toronto, two imported cases developed in 245 secondary cases.1 What explains this striking difference? Protocols in the early management of the patients differed, but containment and filtration also likely had a role.

The Toronto index case presented to the emergency department with high fever, cough, shortness of breath, and no travel history. This patient was admitted alongside the general population unmasked. With impending respiratory failure, the patient was noninvasively ventilated using a single-limb, passive system.1,2

In contrast, the Vancouver index case presented to the emergency department with undiagnosed fever, respiratory infection, and recent travel history. The patient was promptly masked and isolated due to a “potentially airborne” infection flagging.1,2 Due to rapidly progressing symptoms, the patient was intubated within hours and placed on a ventilator — remaining ventilated for 63 days.1,2

During this time, the Vancouver hospital had no infection control policy regarding ventilator filtration and only about half of the intensive care ventilators had an integrated filter.2 By chance, the respiratory therapist decided to use a ventilator with N100-equivalent inspiratory and expiratory filters — the highest rated filtration level recognized by the National Institute of Occupational Safety and Health (NIOSH).3 Further, the expiratory filter was contained (by design) within a heated housing, meaning filter changes due to condensation buildup and subsequent resistant increases were unnecessary.2 When the filter does not have to be changed, breaks in the ventilator circuit can be significantly decreased, which helps contain respiratory pathogens almost completely.2

This may shed light on why three index cases in Vancouver resulted in only one secondary case (of which zero secondary cases came from the patient ventilated for 63 days), while two index cases developed into 245 secondary cases in Toronto.

Of the 247 SARS cases in Toronto that year, 43 percent were healthcare workers — with a mortality rate of 17 percent.1 After these events, many respiratory therapy departments across Canada mandated heated, N100-equivalent expiratory filters for any new critical care ventilator purchase.

Want to learn more about the ventilator used in Vancouver? Visit the Medtronic website to learn more about the Puritan Bennett™ 840 ventilator and other newer ventilation solutions.

Related: How does patient-ventilator asynchrony affect your patients? And what can you do to help prevent or lessen its adverse effects? Read our blog post for tips.


As we saw from the Vancouver hospital’s experience managing a SARS patient, filtration can play a key role in limiting the spread of certain pandemics. But not all filters are created equal. Two important considerations when choosing a ventilator filter are:

  • Filter efficiency
  • Whether the filter is integrated and housed in a warmed environment protected from cooler ambient air

Filter efficiency ratings are determined by how a filter performs at the most penetrating particle size (MPPS).1 MPPS refers to the most difficult particles to filter, and they are the particles of greatest concern that could penetrate a filter.1 These particles have become the standard particle size for NIOSH testing of breathing system filters.4

The NIOSH has established three levels of filter efficiency percentage that correspond with three filter classes — N95 (95 percent), N99 (99 percent), and N100 (99.97 percent).3 In filtration, penetration is the percentage of particles that completely pass through the filter. For example, if 1,000 particles hit a filter and five slip through, the penetration would be 0.5 percent.4 The efficiency is the percentage of particles that are caught by the filter — 99.5 percent in this example.4

Therefore, choosing an N100-rated equivalent filter with 99.97 percent efficiency against 0.3 µm particles like the one used in Vancouver provides the most protection from particles of greatest concern.4 The filtration efficiencies posted by many manufacturers include Bacterial Filtration Efficiency or Viral Filtration Efficiency results, often with ratings of 99.99 percent to 99.9999 percent.4  This testing is typically done using 3.0 µm droplets, which are ten times the diameter (and one thousand times the mass or volume) of the MPPS. This makes the results appear impressive but difficult to relate to real world filtration capabilities.4

Other important considerations when choosing a filter include:

  • Whether the filter is integrated
  • How frequently it needs to be changed due to condensation build up
  • The subsequent rise in resistance from condensation build-up

Heated and nonheated filters may have similar filtration performance.2 The difference is in the management of condensation. Nonheated expiratory filters and heat moisture exchanging filters (HMEFs) require routine changing, since accumulated condensation may increase the resistance of these filters.1 With every instance of a filter change, a break in containment — and therefore a risk of pathogen exposure — is introduced.1

A ventilator with an integrated filter in a housing designed to be heated (actively or passively) and protected from ambient air temperatures can help reduce condensation, while allowing any excess water to be accumulated in a collection vial. This water can be easily drained without breaking the patient circuit or exchanging the expiratory filter, and heated filters can be left in use for extended durations before needing to be replaced.5 Avoiding the need for routine circuit disconnections to manage water accumulation may enhance a hospital’s infection control efforts.

Related: Many studies support the use of daily sedation holidays for mechanical ventilation patients. Learn how these breaks from sedative infusions can benefit your patients.

Do you know which kind of ventilator filters your hospital uses? Learn more about filter efficiency ratings on the NIOSH website.

1. Thiessen RJ. The impact of severe acute respiratory syndrome on the use of and requirements for filters in Canada. Respir Care Clin N Am. 2006;12(2):287–306. 
2. Thiessen, RJ. Heated expiratory filtration: lessons from the SARS experience. Published 2007. Accessed Feb. 5, 2020.

3. Centers for Disease Control and Prevention. NIOSH guide to the selection and use of particulate respirators. Published Jan., 1996. Accessed Feb. 17, 2020.

4. Thiessen RJ. Filtration of respired gases: theoretical aspects. Respir Care Clin N Am. 2006;12(2):183–201. 
5. Puritan Bennett™ 980 Series Ventilator Operator’s Manual

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TOPICS: Puritan Bennett™ VentilatorsIntensive Care Unit