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Breathing causes chaos Gas mix leaves airborne particles no way out of lungs. 16 July 2002 PHILIP BALL Breathing difficulties: no way out for dust and pollution Gases may mix more thoroughly in our airways than had been thought, researchers have found. The discovery could overturn ideas about how airborne particles cause respiratory diseases, and how viruses and bacteria are transmitted in air. Particles of dust and pollution called aerosols trigger or exacerbate conditions such as asthma, emphysema, bronchitis, silicosis and lung cancer. Aerosols were thought to find their way out of a lung the same way they went in - like Theseus following Ariadne's thread out of the Minotaur's labyrinth. But this gassy thread becomes extremely tangled during an inhalation, calculate Akira Tsuda of Harvard University in Cambridge, Massachusetts, and colleagues. The flow cannot simply be reversed by exhaling1. This indicates that aerosols may become trapped in the lungs more easily than current theory would have it. Slice of life The fate of inhaled aerosols depends on how the inhaled gas interacts with the gas already present in the highly branched passages of the lungs that terminate in tiny pockets known as alveoli. When two rivers flow together, their waters rapidly swirl and mix. When fluids flow more slowly, or in narrower channels, they may act more like streams of treacle, remaining almost unadulterated. Air was thought to flow like this in the lungs. Tsuda and colleagues worked on rat lungs. Instead of gases, they used two silicone liquids, one blue, one white. They filled the lungs with white liquid, representing gasses initially in the alveoli ; then they pumped blue fluid in and out. The silicone liquids were much more viscous than air. The team allowed for this by pumping them in and out of the lungs very slowly - an 'inhalation' and 'exhalation' together lasted around five minutes. Adding a curing agent solidified these fluids quite suddenly an hour later. Once solidified, the fluids preserved a snapshot of how they were mixed in the lungs, which could be inspected in slices under a microscope. The liquids quickly became highly mixed. Slices of the polymerized lung revealed a turbulent, chaotic swirling pattern of blue and white, even in airways just a few tenths of a millimetre across. This means that each packet of fluid in the flow takes a complex path and rapidly loses track of where it started. Any tiny particles suspended in the flow also lose their way and cannot easily escape from the lungs during exhalation. References Tsuda, A., Rogers, R. A., Hydon, P. E. & Butler, J. P. Chaotic mixing deep in the lung. Proceedings of the National Academy of Science (2002). © Nature News Service / Macmillan Magazines Ltd 2002 With regards Lew
