There are not many things more irritating than having to walk around with a soggy tissue stuffed up your sleeve, constantly sniffing, because your nose won’t stop running. But now it seems that it might not be a weak immune system that is letting you down, but living in cities or areas with high levels of air pollution.

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Scientists have long known of the link between humans being exposed to dirty air and suffering from nasal and sinus issues, but the exact evidence of how environment affected the upper respiratory system was missing. Now the team from John Hopkins University have found the missing link by studying mice, which are continually exposed to concentrated levels of aerosolized particles, those that come from smog, wood burning or ash, that cause an “asthma of the nose”. After sixteen weeks of breathing in their contaminated surroundings, the mice had four times more macrophages – white blood cells that indicate inflammation – that those breathing in clean, filtered air. Murray Ramanathan, associate professor, said: “We’ve identified a lot of evidence that breathing in dirty air directly causes a breakdown in the integrity of the sinus and nasal air passages in mice. “Keeping this barrier intact is essential for protecting the cells in the tissues from irritation or infection from other sources, including pollen or germs.”