Clean air for high-risk patients: HEPA, cold plasma and protected environments

When the air is the risk

For most people, the microbes in indoor air are harmless. For patients with severely weakened immune systems, such as those in haematology, bone marrow transplant units or burns units, the same air can be dangerous. The mould Aspergillus fumigatus is a well-known example: its spores can cause invasive lung infections with a high death rate in these patients.

The risk rises during construction and renovation, which release large amounts of dust and spores. Outbreaks of aspergillosis have been linked to building work in and around hospitals. Resistant bacteria and airborne viruses add to the concern in intensive care and isolation areas.

How hospitals classify air risk

The French standard NF S90-351:2013 is one widely used framework. It asks hospitals to classify areas into risk levels and sets target air quality for each:

• Risk class 4, the highest, includes organ transplant and orthopaedic implant theatres, severe burns, protected haematology wards and cytotoxic drug preparation. The target is ISO 5 air cleanliness.
• Risk class 3 includes theatres for visceral, cardiovascular, ENT and eye surgery, haematology outside protected wards and interventional imaging. The target is ISO 7.
• Risk class 2 includes recovery rooms, multidisciplinary ICUs and neonatal resuscitation. The target is ISO 8.

ISO classes, defined in ISO 14644-1, set the maximum number of particles allowed per cubic metre of air. Lower numbers mean cleaner air.

Understanding HEPA filters

High-efficiency particulate air (HEPA) filters trap particles using several physical effects at once. Every filter has a particle size it captures least well, called the most penetrating particle size. A filter's rating is based on this worst case, so its performance is better for all other sizes.

Under the European standard EN 1822, H14 filters are about ten times more efficient than H13 filters. H14 filters must capture at least 99.995% of particles at the most penetrating size.

Capturing microbes is not the same as destroying them

A standard filter traps microorganisms but does not kill them. Some air decontamination systems add a stage that destroys microbes before or as they are captured, for example using low-energy cold plasma, so that viable organisms do not build up in the filter. Activated carbon stages can also remove volatile organic compounds and chemical vapours, which matters in laboratories, IVF units and rooms disinfected with chemicals.

Clinical research supports this approach in high-risk settings. A study by Gerlinger and colleagues, published in Infection Control and Hospital Epidemiology in 2015, associated mobile plasma-based air decontamination units with fewer invasive aspergillosis cases in haematology patients.

Protected environments without construction

Building a new clean room takes time and money. Mobile and deployable systems can create an ISO 5 zone around a single patient's bed, or turn a standard room into a positive pressure room for fragile patients or a negative pressure room for infectious ones, without major works.

Choosing a mobile air unit

• A flow rate matched to the size of the room.
• Technology suited to the contaminants: particles, microbes, chemicals or all three.
• No release of harmful by-products such as ozone or formaldehyde.
• Low noise, so patients can rest.
• Performance confirmed by independent, published studies.
• Simple maintenance and clear filter replacement schedules.