Older adults may be more likely to have bleeding stomach ulcers on days when the air has higher levels of nitrogen dioxide, a pollutant produced by car exhaust and power plants, a recent study in Hong Kong suggests.

The researchers focused on what’s known as peptic ulcers, or painful sores lining the stomach or small intestine, which are often caused by bacterial infections but have also been linked to drinking, smoking and certain medications. Untreated bleeding ulcers can lead to bloody vomit or stool, anemia and life-threatening blood loss that requires hospitalization.

Researchers examined whether short-term spikes in air pollution could also influence the risk of serious bleeds, and estimated a 7.6 percent increased risk of emergency admissions for bleeding peptic ulcers during five-day periods with higher average nitrogen dioxide levels.

“We already knew that air pollution exposure may alter intestinal immunity, increase gut permeability and influence intestinal microbial composition, which may contribute to the development of various intestinal diseases,” said senior study author Wai-Keung Leung of the University of Hong Kong.

“This is the first time that the association between air pollution and peptic ulcer bleeding, one of the most important complications of peptic ulcer, is being reported,” Leung said by email.

For the study, researchers examined data on air pollution levels in Hong Kong and 8,566 emergency admissions for bleeding peptic ulcer in adults 65 and older from 2005 to 2010.

In addition to nitrogen dioxide, they also looked at concentrations of ozone, an unstable form of oxygen produced when various types of traffic and industrial pollution react with sunlight; sulfur dioxide, a byproduct of fossil fuel combustion and refining minerals like copper, aluminum and iron; and so-called PM 2.5, a mixture of solid particles and liquid droplets smaller than 2.5 micrometers in diameter that can include dust, dirt, soot and smoke.

They also considered factors that can influence air quality like temperature, humidity and other weather conditions.

Out of all the air pollutants, only elevated nitrogen dioxide levels were independently associated with an increased risk of emergency admissions for bleeding peptic ulcer, the study team reports in The Lancet Planetary Health.

One limitation of the study is its use of air quality readings on the date of the emergency admission to examine the link between the pollutants and bleeding peptic ulcers because, the authors note, it’s possible some people might have developed symptoms triggered several days before they went to the hospital.

Researchers also didn’t have data on other risk factors for bleeding peptic ulcer such as infections with H. pylori bacteria or regular use of non-steroidal anti-inflammatory (NSAID) medications like aspirin and ibuprofen.

The results from Asian residents of Hong Kong also might differ from what would be found in other racial or ethnic groups because some people can have a genetic predisposition to this health problem.

An earlier study in Canada with a mostly white population didn’t find an association between nitrogen dioxide and bleeding peptic ulcer, the author of that study, Dr. Gilaad Kaplan of the University of Calgary, writes in an accompanying editorial.

However, residents of Hong Kong were exposed to much higher levels of nitrogen dioxide than people in the Canadian study, Kaplan notes in the editorial.

“Air pollution is a modifiable risk factor that is linked to diseases throughout the body from the respiratory system to the cardiovascular system, with growing evidence that it may influence the gastrointestinal tract,” Kaplan told Reuters Health by email.

“As a modifiable risk factor, people who are at risk for the adverse health effects of air pollution can take action to protect themselves on days when air pollution levels spike locally by avoiding larger sources of pollution when reasonably possible (e.g. avoiding jogging near heavy traffic, working earlier in the day to avoid the worst of rush hour traffic),” Kaplan added. “Moreover, society needs to prioritize actions to continue to improve the air quality of cities.”