Scientists have warned that air pollution in cities like New Delhi or Beijing may cause people to have year-round runny noses and chronic sinus problems.
Although human population studies have linked air pollution to chronic inflammation of nasal and sinus tissues, direct biological and molecular evidence for cause and effect has been scant.
Researchers found that mice continually exposed to dirty air have that direct biological effect. Scientists have long known that smog, ash and other particulates from industrial smokestacks and other sources that pollute air quality exacerbate and raise rates of asthma symptoms, but had little evidence of similar damage from those pollutants to the upper respiratory system.
The new findings have broad implications for the health and well-being of people who live in large cities and industrial areas with polluted air, particularly in the developing world.
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"In places like New Delhi, Cairo or Beijing, where people heat their houses with wood-burning stoves, and factories release pollutants into the air, our study suggests people are at higher risk of developing chronic sinus problems," said Murray Ramanathan, associate professor at the Johns Hopkins University School of Medicine in the US.
Chronic sinusitis can cause congestion, pain and pressure in the face, and a stuffy, drippy nose. Numerous studies have reported significant social implications of chronic sinonasal disease, including depression, lost productivity and chronic fatigue.
To see how pollution may directly affect the biology of the upper airways, the researchers exposed 38 eight-week-old male mice to either filtered air or concentrated Baltimore air with particles measuring 2.5 micrometers or less, which excludes most allergens, like dust and pollen.
The aerosolized particles, although concentrated, were 30 to 60 per cent lower than the average concentrations of particles of a similar size in cities like New Delhi, Cairo and Beijing.
Nineteen mice breathed in filtered air, and 19 breathed polluted air for 6 hours per day, 5 days a week for 16 weeks. The researchers used water to flush out the noses and sinuses of the mice, and then looked at the inflammatory and other cells in the flushed-out fluid under a microscope.
They saw many more white blood cells that signal inflammation, including macrophages, neutrophils and eosinophils, in the mice that breathed in the polluted air compared with those that breathed in filtered air.
For example, the mice that breathed in the polluted air had almost four times as many macrophages than mice that breathed filtered air. "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," said Ramanathan.
"Keeping this barrier intact is essential for protecting the cells in the tissues from irritation or infection from other sources, including pollen or germs," he said.