The Particles We Never Exhale ...

    29. November 2010, 14:54

    Millions of people die each year due to air pollution - a novel device has made it possible to measure the amount of airborne particles that are deposited in the lungs

    On the night of the 6 December 1952, a thick foul-smelling fog descended over London. There had been an unusual cold spell and to keep warm, the inhabitants of the city had diligently built fires, burning large quantities of coal. When the wind suddenly died down, the smoke spouting from the chimneys consolidated into an almost impenetrable haze. Cars were abandoned on the streets. Theatre performances were cancelled. Trains and planes were brought to a standstill. But worst of all, an estimated 12,000 people died and many more suffered health effects.


    The catastrophe in London, afterwards referred to as "The Great Smog", is sometimes considered the starting point for modern research on air pollution. We now know that millions of people die every year as a consequence of breathing polluted air. However, there is still much we do not understand. How much of the pollution surrounding us finds its way into our bodies?
    At Lund University, Sweden, we have developed an instrument that enables us to determine the number of airborne particles deposited in our lungs when we breathe. Although laboratories have been making similar measurements for more than a hundred years, the difference with our technique is that it can be used in real environments.

    Our first experiment, however, was carried out in the laboratory. We measured how many particles remained in the lungs after different degrees of exertion; thirty volunteers breathed into the instrument while sitting relaxed in a chair and while exercising on a training bike. The results demonstrated that about four times more particles remained in the lungs during exercise than when resting, due to the larger volume of air passing through the respiratory system - not an entirely unexpected result.

    More surprising was that the number of particles trapped in some individuals' lungs was much higher than in others. The difference was close to a factor three between the most and the least affected. This could be explained in part by variations in breathing patterns - the probability of a particle staying behind in the lungs is lower for a person who breathes rapidly and superficially than for a person who breathes slowly and deeply. But this theory did not apply to all our subjects. Most likely, the shapes and sizes of their airways also contributed to the differences.
    After the lab trials, we ventured out into the real world and examined how wood smoke and traffic exhaust enter the body. With wood smoke, fewer particles remained in the lungs than expected. The reason turned out to be the high humidity in the respiratory tract. Particles from wood combustion absorb water in a moist atmosphere, like that of the lungs. This alters deposition. Thus, for the wood smoke, this meant that only every fifth inhaled particle stayed in the respiratory tract, the rest returned to the atmosphere with the exhaled air.

    The situation was different for the traffic exhaust. Here two out of three particles inhaled remained in the body. The reason for the substantial difference compared with the wood smoke is that traffic exhaust particles are smaller and do not absorb water vapour.
    A side effect of the small size of the traffic particles is that a considerably larger numbers are needed to reach the same total particle mass as in an environment with wood smoke. Therefore, the complete surface area of the traffic particles is far greater, in the same way that the area of a kilo of grapes is greater than that of a kilo of apples.

    According to our measurements, the number of particles deposited in the lungs from traffic fumes was 16 times higher than for wood smoke particles, and their total surface area was three times greater, even though both sets of particles had the same total weight. Whether the adverse health effects result from the number particles, their surface area or their mass is debatable.

    What happens to the particles once they are inside the body is not entirely clear. We do know that some are removed by the small hairs, or cilia, that line the larger airways in the lungs. Some settle in the nose or mouth and are sneezed out or swallowed. While others are rendered harmless by macrophages, a type of white blood cell in the immune system. However, some particles appear to be able to pass unimpeded into the blood stream and are then transported to organs where they can give rise to inflammation and disease.

    At present, emissions are regulated by limits on the concentration of the particles in the air, regardless of their origin. Our studies demonstrate that different amounts of particles are retained in the body depending on their source, even when their concentrations in the air are the same.

    The noxious impenetrable "Great Smog" in London 1952 is history, but nonetheless air pollution still constitutes a major threat to our health. Because of improved combustion techniques the thick black smoke is now replaced by small invisible particles with a higher ability to penetrate into the deep lung. These particles are a silent disaster killing about half a million people in Europe each year.

    The casual connection is seldom obvious. It may for instance be a lung cancer that develops after many years of living in an industrial area or heart failure from an hour's visit in the traffic. Research is necessary to increase our understanding and thereby find solutions and appropriate measures for decreasing the effects. (Jakob Löndahl, Lund University,


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