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Safety: Antiseptic Air

By David Evans | June 1, 2003
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When passengers perceive the cabin of an airliner as an incubator of deadly disease, they won’t fly. Such has been the dire case facing Cathay Pacific. The Hong Kong-based airline has been at the epicenter of the outbreak of severe acute respiratory syndrome, or SARS, a deadly new form of pneumonia.

In early April, with plummeting bookings and an increasing number of flights cancelled, the airline issued a two-page listing of the steps it was taking to assure apprehensive passengers of the minimal risk of being infected with SARS on a Cathay Pacific flight. Persons with SARS symptoms will be denied boarding. If a suspected SARS passenger has flown, the sick person’s fabric seat cover will be removed and replaced. The same goes for seats in the entire row, and the row behind and in front.

With SARS, the folk wisdom that flying entails the risk of upper respiratory tract infection has taken a sinister twist–the flight could have deadly consequences. However, it may be possible to modify the design of the cabin air ventilation system so that it kills pathogens before they can kill people. Ideas that have lain dormant for years could be employed to virtually sterilize the air. Proponents say it can be done at a cost per passenger of a few pennies on a flight, maybe a dollar and change over longer journeys.

Disease-free air is an overdue safety advance. Sophisticated engine monitoring, navigation and communications systems are, from the passenger’s perspective, an abstraction. But every passenger breathes. And everyone who flies has a personal stake in cabin air quality and cabin sanitation.

After all, the cabin of a modern airliner is the most densely packed public space in early 21st century technology. To be sure, the air conditioning system is a wonder of that technology. Every day tens of thousands of people are carried in shirt-sleeves comfort above 80 percent of the Earth’s atmosphere. If they were exposed to the outside conditions, they would die in short order from the twin effects of hypoxia and hypothermia. Inside the cabin is life; death lurks outside.

With SARS, the prospect of death has crept inside. "We’d really like to go back to 100 percent fresh air to reduce the likelihood of cross-infection," declares Farrol Kahn, director of the UK-based Aviation Health Institute. Kahn’s organization has lobbied for cleaner cabin air, and his call for more fresh air referred to a major design change from first-generation jets. They provided a continuous flow of pressurized and heated air from the outside.

More recent designs have utilized about a 50:50 mix of fresh air and recirculated air, and less of it per unit of time. According to one estimate, the use of recirculated air can save about 60,000 gallons of fuel per year for a typical widebody.

Why is this? The saving comes from the basic design of the air ventilation system. A portion of the outside air compressed by the engine to mix with fuel and produce thrust is bled off before combustion to the environmental system, hence the term "bleed air." That air is used to pressurize the cabin and cockpit, and it is heated or cooled for occupant comfort. The incentive to recirculate pressurized air increased with the advent of engines with higher bypass ratios. These engines achieve their greater fuel efficiency by sending a smaller fraction of their compressed air to the combustion chamber. The bypass ratio is simply the proportion of fan-air flow to high-pressure engine core airflow. According to "The Airliner Cabin Environment," a 1986 study by the National Research Council (NRC), "The fuel and performance penalties associated with bleed-air extraction increase as the bypass ratio increases." On some airplanes, as much as 80 percent of the air is recirculated.

The air is used to cool avionics and other electrical equipment before being discharged overboard through outflow valves. In this respect, the pressurized hull of an airliner is like a huge leaky balloon–compressed air is pumped in, and it is metered out to maintain a denser artificial cabin altitude than the greater height at which the aircraft actually is flying.

On the ground, the cabin air may be less polluted than outside city air, but at altitude the outside air "always has a lower count of viable material than recycled air," in the delicate phrasing of Dr. Martin Hocking, a professor at Canada’s University of Vancouver and an expert on cabin air quality.

To be sure, filters placed in the air conditioning system can trap contaminants. Aircraft manufacturers have touted the use of high-efficiency particulate air (HEPA) filters to trap bacteria. But SARS is caused by a virus, which is a tiny fraction of the size of a bacterium. Unless the SARS viruses are carried in the droplets emitted by a passenger’s coughing or are clustered together, even a HEPA filter may not catch the virus. Moreover, the process of heating cabin air also dries it.

That dry air has a desiccating effect on bacteria. Again, to cite the NRC study, "Although the low relative humidities present in most aircraft during flight can be deadly for some bacteria, such conditions probably augment the viability of most viruses."

In response to the SARS threat, manufacturer Airbus advised its customers that "any virus retained in the cabin air recirculation filters is expected to become non-infective after just a short time."

But this may not quite be the case. In addition, organic matter collected on the filter could provide a medium for propagation rather than interdiction. Filters with antibacterial coatings might help, but less so with viruses.

Some Cathay Pacific pilots urged that the air conditioning packs should be switched to "high" during cruise. Packs operate automatically on "high" during climb but revert to "normal" during cruise. Switching them to "high" would increase airflow by about 25 percent, but at a slight increase in fuel consumption (reportedly 0.8 percent).

A Cathay Pacific official says, "The amount of fresh air cannot be adjusted through operational or procedural changes." In other words, setting airflow on "high" for cruise may pass the inside air through the filters more often but does not increase the inflow of ambient air with its nil bacterial/viral count.

There may, however, be a way to turn the fuel-saving virtue of cabin air recirculation into a health benefit. The air could be sterilized every time it passes through the air conditioning system. Two concepts have been articulated.

One features the use of ozone (O3). Introduced in a plenum, the ozone would kill any and all microorganisms passing through. The ozone could be brought in from outside air or introduced in the plenum by an ozone-generating light. By proprietary means, the ozone would decay to oxygen (O) at the outflow end of the plenum. To be lethal to microorganisms, it would have to be used in the chamber at a concentration higher than permitted for human exposure.

However, ultraviolet (UV) light might be a more viable concept. Generated by a low-pressure mercury arc lamp, UV light in the 253.7-nanometer wavelength would be lethal to bacteria and viruses. Hocking estimates that a 50-watt lamp would sanitize 1,000 cubic feet (28.3 cubic meters) of air per minute, or roughly the amount of air needed for 60 passengers. Given the lamp’s low wattage, the power for this air sterilizing technology would cost less than a cent per passenger for a 10-hour flight. Costs to modify the aircraft are not included. Four such lights would sanitize the air for 200 passengers. Duplicate light sources would provide redundancy, enabling pilots to switch to backup bulbs upon an alert in the cockpit.

Passengers could be given more outside air, too. Economy-class passengers may be getting as little as 7 to 10 cubic feet (0.2 to 0.28 cubic meter) per minute. They should be getting at least 15 cubic feet (0.42 cubic meter) per minute per person of outside air, whether or not some air is recycled, according to Hocking. That is the standard for buildings recommended by the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE).

Hocking suggests that an expander turbine operated on exhausted cabin air might be used to compress a part of the outside air intake, to help decrease the energy cost of providing more fresh air.

A greater flow rate of fresh air, and sterilization of all air, could help stop the spread of disease. Consider the following slogan–as opposed to the perception of airliners as flying petri dishes of infection: "Now flying with sanitized air, and more of it, too!"

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