Even a short stay for travellers in cities with high levels of air pollution leads to breathing problems that can take them at least a week to recover from, according to a study conducted in India, Pakistan and China.
Researchers at the New York University (NYU) School of Medicine in the US analysed pollution-related coughing and breathing difficulties, and recovery times upon returning home, in healthy, young adults travelling internationally.
The finding, published in the Journal of Travel Medicine, is timely given that the number of tourists travelling internationally is expected to grow to 1.8 billion by 2030, according to the World Tourism Organization.
"We had several reports that tourists were feeling sick when visiting polluted cities, so it became important for us to understand what was really happening to their health," said Terry Gordon, a professor at NYU School of Medicine.
Researchers analysed six measurements of lung and heart health in 34 men and women travelling abroad for at least a week from the metropolitan New York City area.
Most were visiting family in cities with consistently high levels of air pollution, including Ahmedabad and New Delhi in India; Rawalpindi, Pakistan; and Xian in China.
The team noted that New York City has relatively low levels of air pollution, in part because of strict regulations, its location on the coast, and weather patterns.
The study found that being in a polluted city reduced measures of lung function by an average of six per cent and by as much as 20 per cent in some people.
Participants also ranked their respiratory symptoms from one (mild) to five (requiring treatment), reporting a cumulative average symptom score of eight.
People who visited the highly polluted cities reported as many as five symptoms, while those who visited lower pollution cities had fewer or none.
Two patients sought medical attention because of their symptoms, researchers said.
The pollution levels of the cities studied did not make a significant difference in the blood pressure of visitors, they said.
All study participants had a normal body mass index (between 21 and 29 for men, and between 18 and 26 for women), and none had preexisting health conditions.
Before embarking on their travels, all were taught how to measure their lung function and heart rate daily using commercially available spirometers, wrist blood pressure monitors, and heart rate sensors.
Researchers then compared the health data against levels of air pollution collected from local government agencies.
They used international standards to categorise highly polluted cities as those having more than 100 microgrammes per cubic metre of particulate matter (PM), or air pollution dust.
Moderate pollution is anything between 35 and 100 microgrammes per cubic metre of PM, and low pollution levels are anything less than that.
"What travellers should know is that the potential effects of air pollution on their health are real and that they should take any necessary precautions they can," said MJ Ruzmyn Vilcassim, a postdoctoral fellow at NYU School of Medicine.
Gordon suggests that those visiting highly polluted cities should consider wearing masks or consult a doctor prior to travel if they have preexisting respiratory or cardiac health difficulties, and to consider avoiding travel during certain months.
For instance, farmers burn their fields during the winter months in New Delhi, raising levels of pollutants in the city.
Although participants gradually returned to normal health, researchers said there needs to be more follow-up research to know if there were long-term effects, or if longer stays would influence the pollution impact.