Cabin Air Q&A

According to the European air filter efficiency classification, a HEPA  (High Efficiency Particulate Air) filter can be any filter element rated between 85% and 99.995% removal efficiency.
However, for aircraft cabin air recirculation systems, this definition has been tightened and the current aerospace industry standard is 99.99% minimum removal efficiency by sodium flame test to British Standard BS.3928 or 99.97% minimum removal efficiency by DOP test to ASTM publication D 2986-95 (see Note 1 below) . This is the efficiency standard now specified by Airbus and Boeing for their new generation aircraft.

Note 1: The sodium chloride test consists of challenging the filter with an aerosol mist of sodium chloride (NaCl) particles, with a mean particle size of 0.58 micrometers. The DOP test consists of challenging the filter with an aerosol mist of oil droplets, with a mean size of 0.3 micrometers.  The removal efficiency, or penetration, is calculated as a percentage by measuring the aerosol concentrations upstream and downstream of the filter element under test.

Because of significant differences between microbes and chemical compounds (such as DOP and NaCl), the use of chemicals is not suitable for rating the microbial removal efficiency of air filters. Based on our decades of experience in our Life Sciences division, and following standard practices used in the healthcare and pharmaceutical industries, Pall filter media have been challenged with bacteria and viruses.
An independent test house has tested full size filter elements at their rated air flow and demonstrated that Pall cabin air filters have a microbial removal efficiency of >99.999% with bacteria and viruses.
Pall has also carried out microbial removal tests on both clean (unused) filters as well as used cabin air filters removed after C check maintenance. The results of this back to back testing were a pass in both cases.

In other words, "True HEPA" filter = microbially tested.

There are currently no airworthiness standards or regulations which specify the level of filtration removal efficiency which must be used on board aircraft.
The majority of modern, large, commercial aircraft, which use a recirculation type of cabin air system, utilize fine HEPA filtration, (99.99% minimum sodium flame efficiency/99.97& minimum DOP efficiency).  A small number of aircraft types have filters with lower efficiencies.  Some older aircraft have either total outside air ventilation, or a small amount of unfiltered recirculation combined with the outside air.

Using the definition of a HEPA filter as either 99.99% sodium flame or 99.97% DOP, the first Airbus HEPA filters were introduced in 1994 (ref Airbus SIL21-065).
In the Airbus SIL, it states that “the existing filter element of 98% efficiency with a mean particle size of 8 microns (when tested with gravimetric method) has been replaced in production by a new filter element with a 99.99% efficiency with a particle size of about 0.58 microns (when tested by sodium flame test)”

The first HEPA filters (99.97% DOP) for Boeing aircraft were introduced on the B747-400 in 1998 (ref Boeing SIL 747-SL-21-52-A)

Cabin air filters remove particulate contamination using three basic mechanisms of filtration;
Direct Interception, Inertial Impaction, Diffusional Interception
 
In practice, these three capture mechanisms will combine to produce a characteristic of efficiency versus particle size. This characteristic curve shows that the minimum capture efficiency occurs at about 0.1 to 0.3 micron particle size, which is defined as the "Most Penetrating Particle Size", or MPPS, for the filter medium and conditions applicable to a fine HEPA cabin air filter.  For particles both larger and smaller than the MPPS, the capture efficiency is higher. 

Yes, provided that they are ‘true HEPA’, microbially validated to 99.999% microbial removal. These are effective at removing viruses  (refer to Q2).

Following standard practices used in the healthcare and pharmaceutical industries, Pall has performed challenges of its HEPA cabin air filters using the bacteria Brevundimonas  diminuta and Bacillus Subtilis. The bacteria removal efficiency has been measured to be greater than 99.999%. Similarly, using the MS2 Coliphage virus, the virus removal efficiency has been measured to be greater than 99.9995%.

In response to recent airline requests, we can advise that the MS2 coliphage virus is approximately 4 times smaller than the corona virus which scientists believe is the cause of the disease, Severe Acute Respiratory Syndrome (SARS).  The corona virus particles are about 0.080 to 0.160 micron (80 to 160 nm) in size.
The Avian flu virus, H5N1 current strain, is approximately 0.1 micron (100 nm) in size

Also note that anti-microbial and biocide treatments that rely on direct surface contact are NOT EFFECTIVE on aircraft cabin air filter elements.

Viruses can typically range from about 0.01 to 0.2 micron in size and are removed by the mechanism of diffusional interception which is described below.
For very small particles (less than about 0.1 micron in size), Brownian motion causes particles to be collected on the individual cabin air fibers and pore walls. The smaller the particles, the greater are their random Brownian movements, and the higher the capture efficiency.

Bacteria can typically range from about 0.5 to 1.5 micron in size and are removed by the mechanism of inertial impaction which is described below.
Particles of higher density than air deviate from the air stream path by virtue of their inertia and impact on the solid surfaces or walls of the pores, where they adhere.

a) Cabin Air Recirculation System
The majority of cabin air recirculation filters will take out particulate contamination only.

However, to further enhance the comfort of passengers and crew, Pall Aerospace, in conjunction with Airbus Industrie, have developed a combined particulate and odor cabin air filter element to be installed in the recirculation line.  These combined particulate/odor filter elements have identical dimensions and interfaces to the existing cabin air filter elements, which means that no modifications are required to the existing aircraft systems to install this option.
The combined element design consists of two stages:
(i) High Efficiency Particulate Air (HEPA) filter first stage to remove dust and smoke particulate, bacteria and viruses. (99.99% NaCl efficiency to BS3928)
(ii) Odor adsorber second stage to remove odors and volatile organic compounds (VOC’s)

These combined particulate / odor elements are designed for the Airbus A320 family and A330/340 aircraft and are available to the operators as an alternative to the particulate filter elements.

b) Outside Air Supply into Environmental Control System (ECS)
Most aircraft have the filters or adsorbers located in the recirculation loop, where it is essential to have good filtration.  However, some particulate, odours and trace chemical VOCs may enter from the outside air system via the mixing chamber where filtration is not normally provided.  If the manufacturers require a filter system to be installed into the high temperature air supply line, then the current odor removal elements using adsorbent technology are not suitable because of their temperature limitations. Other solutions that can be installed in this high temperature part of the ECS are currently being considered.

There is no definitive time interval for changing out a cabin air filter element. The time interval varies between aircraft types. We recommend that airlines follow the guidelines provided by the manufacturers in the aircraft maintenance manuals (AMM's).
It is often the case that airlines will choose to replace cabin air filter elements at regular "hard time" intervals to fit in with routine scheduled maintenance periods, such as a C-check.  The definition of a C check varies between aircraft models and operators.

There are currently no airworthiness regulations to define the filter element replacement schedule.
However, the aircraft maintenance manual will define a recommended change-out interval, based on the aircraft system requirements.

Unfortunately, it is not possible to provide an average cost of a HEPA cabin air filter because the price varies between aircraft types and between filter element manufacturers.

If the elements are not changed at the specified interval, or if the "on-condition" warning of element clogging was ignored, there would be no immediate cause for alarm.  The filter pressure drop will gradually increase as the element clogs, and there will be some gradual reduction of the recirculation fan air flow as a result.  The filter element will not collapse or fail, because it is designed to accept the full pressure rise capacity of the recirculation fans.

One of the objectives of the EC Cabinair project was to establish the current in-service performance of filtration systems through the programme of Measurements in the Sky.
In order to evaluate the performance of the filtration systems installed on current commercial aircraft, Pall are continually monitoring the in-flight performance of their filtration equipment. Pall have been involved in cabin air filtration for over 20 years and in the last 5 years alone, more than 300 cabin air filter elements have been returned from service, either at the request of Pall or by the airlines themselves. Overall, this monitoring programme has confirmed that the cabin air filter elements are performing satisfactorily in-service.
The types of test performed include: weight measurement, differential pressure across the filter element, particulate removal efficiency, and analysis of contamination.

There is no more risk involved in replacing a cabin air filter than carrying out general maintenance on any aircraft part that has been in service for a number of years.  Maintenance staff who may be concerned about the general accumulation of dust and debris on cabin air filters should wear the same personal protective equipment as for other aircraft maintenance tasks.

In addition to the questions & answers above, both of the web sites from Airbus & Boeing contain further information regarding cabin air quality.

http://www.boeing.com/commercial/cabinair/facts.html