BioEd Online

A mottled glass tube bathed in ultraviolet light may prove a great help to hospitals by keeping dangerous bacteria out of their air. Scientists have developed a simple, reusable device that can knock out more than 99% of microbes in air conditioners.

The device relies on titanium dioxide, a compound used in white pigments and found in common household products such as toothpaste. When exposed to ultraviolet light, bacteria landing on a surface of titanium dioxide are converted to a vapour of carbon dioxide and water, along with harmless organics.

Scientists have previously exploited the antibacterial powers of this illuminated catalyst by putting powdered titanium dioxide in water systems and blasting it with ultraviolet light. But in air, it was hard to ensure that bacteria would come into contact with the material.

The new system passes air through a coated glass tube filled with fingers of glass. This hugely increases the surface area of the tube, says Valérie Keller, a materials scientist at the European Laboratory for Catalysis and Surface Sciences in Strasbourg, France, who helped to develop the system.

The team tested its device using Escherichia coli, which commonly cause food poisoning. Each cubic metre of contaminated air flowing into the reactor contained about 15,000 clumps of bacteria capable of forming a colony; the outlet air contained no viable colony-forming units, report the researchers in the journal Chemical Communications¹.

Killer device

Keller says the system should also work against Legionella pneumophila, which causes Legionnaires' disease and is about the same size as E. coli. This microbe is particularly good at breeding in the pools of water that can form inside air-conditioning ducts. "Legionella can be a problem wherever you have warm, recirculating water," says Colin Mathieson, an engineer who deals with disinfection equipment at Wedeco UV Systems, based in Sudbury, UK.

The US Centers for Disease Control and Prevention, based in Atlanta, Georgia, estimate that up to 18,000 people in the United States contract Legionnaires' disease each year. Up to 10,000 are thought to catch it annually in Europe. The disease is fatal in between 5% and 30% of cases, they add.

The French scientists plan to test their system on Legionella itself. "We're in contact with some industrial partners," Keller says. "We hope to develop a pilot process in the next two years."

But even if they make the system 100% effective, it won't guarantee freedom from bacteria notes Ian Smith, a consultant for the environmental consultancy STATS, based in St Albans, UK. No hospital system is airtight, he notes. So bacteria could still enter ducts or hospital rooms downstream from the cleaners. "Users should not be lulled into a false sense of security," he says.

Light work

Keller says that their system offers several advantages over conventional air-cleaning systems. Filters that remove bacteria are expensive and need to be changed regularly, she says.

Other devices already use direct ultraviolet light to 'sunburn' bacteria to death, but these require much higher energy levels. And the light can damage humans just as much as pathogens, so the lamps must be carefully enclosed.

Keller's system uses light with much lower energy, similar to that in nightclubs. "It could be used in open areas such as hospital rooms or tunnels without risk to people," she says. It also uses considerably less power.

Keller hopes that by adding certain materials to the catalyst, they could eventually run the system using sunlight.