(Reuters Health) - People are more likely to be hospitalized for respiratory disorders during the annual haze season in Southeast Asia, when densely polluted air hovers over the region, than at other times of year, a Malaysian study suggests.

Since 2005, Southeast Asia has experienced an annual haze due to a combination of human activities and climate factors that promote both drought and natural fires, researchers note in the journal Respirology. The haze has worsened in recent years with illegal agricultural and land-clearing fires in parts of Indonesia, they add.

“Although it’s been a known fact that haze and air pollution are related to breathing problems, there has not been any hard evidence showing the link between the two,” senior study author Tidi Hassan of the National University Malaysia Medical Center in Kuala Lumpur said by email.

For the current study, the researchers examined data on 1,968 respiratory admissions at one hospital in Malaysia from January 2014 through December 2015.

During a total of 16 weeks of haze periods over the two-year period, there were about 28 admissions for breathing problems each week, compared with about 16 cases per week during non-haze periods.

Once researchers accounted for other factors that can influence air pollution exposure such as wind speed and direction, temperature, rain and humidity, they found that people with heart disease were more than twice as likely to be admitted to the hospital during haze periods than other times of year.

Patients with asthma or chronic obstructive pulmonary disease (COPD) also had more than twice the risk of a hospital admission during haze periods than at other times of year.

In addition, haze was associated with about twice the risk of severe respiratory issues requiring admission to intensive care units. This happened in 4 percent of cases during the hazy periods compared with 2 percent of cases at other times.

Researchers used a government definition of haze as times when air quality led to atmospheric visibility of less than 10 kilometers (6.2 miles). On hazy days, levels of coarse particulate matter known as PM 10 exceeded 40 micrograms per cubic meter of air (ug/m3). This includes tiny particles of dust, soot, dirt, and smoke.

World Health Organization (WHO) standards limit PM 10 exposure outdoors to an average of 50 ug/m3 over 24 hours or a daily average of 20 ug/m3 over the course of a year.

Exposure to higher levels of air pollution is associated with an increased risk of respiratory infections, heart disease, COPD and lung cancer, according to WHO. Health risks tied to air pollution come from particulate matter as well as from exposure to ozone, nitrogen dioxide and sulfur dioxide gasses.

The researchers also observed elevated levels of white blood cells and C-reactive protein, both markers of inflammation, in patients admitted during haze periods compared to those admitted at other times.

The study wasn’t a controlled experiment designed to prove whether or how air pollution directly caused increased breathing problems.

Inhaling small particles in polluted air may irritate the airway and bronchial tubes causing inflammation in the lungs, said Dr. Denitza Blagev of the University of Utah in Salt Lake City and Intermountain Medical Center in Murray. The tiniest particles can enter the blood stream from the lungs and damage other organs.

“This is the reason that air pollution is associated not only with respiratory problems but also an increased risk in heart attacks and strokes,” Blagev, who wasn’t involved in the study, said by email.

Other limitations include drawing data from only one hospital and the potential for some respiratory admissions to be unrelated to air pollution, particularly in the elderly, the authors note.

It’s also possible that the health impacts of the haze might differ from medical issues related to smog because the traffic fumes can contain chemicals that aren’t present in smoke from fires, said Dr. Kian Fan Chung of Imperial College London in the UK.

Precautions are still necessary, however.

“It is important for patients, particularly those with chronic lung or heart disease, to stay indoors during the haze period (which can last for many weeks) or during periods of high pollution,” Chung, who wasn’t involved in the study, said by email.

SOURCE: bit.ly/2JhjP0y Respirology, online June 19, 2018.