Manufacturers of air filters for respiratory devices believe they offer cheap and effective products that reduce the spread of infection through recognised pathways. With public interest in hospital-acquired infections at an all-time high, such products seem to meet a vital need within the medical sector.

Over the last decade, the use of air filters has become more common in anaesthesia and intensive care, following detailed research from Australia in 1994. This evidence showed a clear pathway of contamination through ventilators and other respiratory devices. The performance of air filters has improved greatly in that time, but there are still concerns throughout the medical industry that they are not being used to maxiumum effect.

This view is backed up by Sean Duggan, business development director for the Fenchurch Environmental Group, which owns filter design and manufacturing company Air Safety Limited, and Professor Tobias Welte, who heads the Respiratory Intensive Care Assembly for the European Respiratory Society, about the role of air filters in infection control.

“Until two or three years ago there was no international or European standard to measure the effectiveness of air filters.”

“Anaesthetists were split over the need for air filters and some argued strongly that they were unnecessary,” says Duggan. “But the paper from Australia identified a cross-contamination pathway. There were 413 patients in an operation list. The first patient on that list had Hepatitis C and some later patients on the list were then found to have contracted it. The pathway became clear for the first time and the market moved to 100% acceptance of the need to use air filters.”

Following the report, the Association of Anaesthetists of Great Britain and Ireland (AAGBI) recommended that breathing system filters or disposable breathing systems should be used for each patient. The use of Entonox by up to 70% of pregnant women in the UK when in labour meant this was identified as a key application.

“It has been a hot issue for some time, but it is a difficult one to tackle,” says Welte. “Infection in the OR and the intensive therapy unit [ITU] has been a hot topic for decades, but the lack of effective antibiotics and the increasing resistance of pathogens mean that it is better to try to prevent infection than to treat it. There has been a notable decline in the efficiency of anti-infective treatments.

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“There is no question that more efficient filters have been developed in the last five to eight years, due to huge effort from the devices industry. There are two fields where air filter technology is important. Firstly, in the OR, where filters in the ventilator have anti-microbial properties. This is where the biggest test for the filter industry lies. Secondly, in the ITU, where filters can be used for humidification. These also need to include anti-microbial properties.”


The use of air filters is widely accepted in anaesthesia, but their efficacy depends on the specific choice of filter product. Manufacturers point to research, such as the AAGBI report, that supports the use of air filters in reducing cross-infection, but this has been challenged on grounds of cost effectiveness and the lack of randomised tests.

“In medicine now there is a big controversy over how to deal with pharmaceutical companies and device manufacturers,” says Welte. “The question is one of independence. People want to know if research is sponsored by physicians or by industry. For air filters all of the randomised, controlled studies are sponsored by companies. There are no independent studies.

“This is partly because studies are expensive to undertake – many as much as €15m, a huge amount for a product that costs €3 per day. The result of this is that there are no convincing studies that show air filters to be better than older products, such as water cascades for humidification.”

Studies in ITU are also very difficult – much more so than pharmaceutical trials in primary care. The cost of air filters is lower, so most ITUs now use them. The quality of the filters, however, is different depending on the manufacturer. This is problematic because the purchase of air filters is often made based on price, not quality.

In the UK, British Standard EN 13328-1:2001 introduced a salt test method to benchmark filtration performance in breathing system filters for anaesthetic and respiratory use. “Until two or three years ago there was no international or European standard to measure the effectiveness of air filters,” says Duggan. In anaesthesia and respiratory care it was left to manufacturers to undertake independent studies and produce data to back up their claims. Now there is a standard and the majority of filters in the UK are tested to that standard.

“It is a step in the right direction, as there was no benchmarking for the performance of filters before,” he says. “It would be useful if it were more recognised among clinicians.There is a revision process in place already.”

Duggan also notes that research further tested the filters assessed for EN 1328 and found significant differences between products. “The study by A. R. Wilkes of the Department of Anaesthetics and Intensive Care Medicine at Cardiff University on the EN 13328 standard focused on 104 filters,” he adds. “There are a lot of manufacturers and many types of filter for different clinical needs. It is clear there is wide variance in performance, especially when it comes to manufacturers’ claims versus test results.”


The debate about the use of air filters should perhaps move on now to focus on the qualities of individual products in specific applications. “The choice should come down to how well the filter needs to perform,” says Duggan. “It may, for instance, require a humidifier to warm anaesthetic gases. The size of the filter also depends on the patient, resistance to flow also needs to be considered, as does its shape and weight.”

Cost, however, is also an increasingly important issue. Welter elaborates. “In Germany one major problem is that hospital administrators want the cheapest filters, which are not always the best. The knowledge among physicians about the properties of air filters is also relatively poor, so there is a need to educate them.

“Manufacturers need to explain to them why they need more expensive filters, because those physicians are responsible for choosing the product.” There are major questions on the topic of air filters that have not yet been solved. How long should they stay on a ventilator? Which filters are the best for each problem?

“The knowledge among physicians about the properties of air filters is relatively poor, so there is a need to educate them.”


Different filter media are suited to different applications. They have, for instance, varying reactions to humidity. One example is the debate about electrostatic synthetic fibre, which has a loose matrix and takes on a charge. This charge can dissipate quickly if it is exposed to moisture or some types of gases. That loss of charge reduces its performance.

Pleated paper, on the other hand, relies on mechanical filtration. So, ultimately it depends on what you need the filter to do and what lifespan you require. “Establish what you want to filter, such as surgical smoke or nebulised gases, consider the particle size and the reaction of the substance to humidity,” says Duggan.

“Then you are in a position to decide between the different media, as you will know, for instance, whether you need it to contain carbon to take out chemicals or smoke. There is usually some trade-off between resistance and efficiency, so you need to sit down and work through the different criteria.”

The message for clinicians is to improve their knowledge of air filters, make the right choice for their needs and ensure usage policies match the guidelines. Air filters are only one piece of the puzzle when it comes to preventing infection.

There are many areas where air filters can be recommended, but it is important to better define their role and get recommendations from an independent society. Filters are cheap and easy to use, so most hospitals do so, but they are often not sure if they are using the right filter in the right situation.

More work is needed by both clinicians and manufacturers to reap the benefits air filters seem to offer, but awareness of their potential is a sound base for further progress.